Porting Guide
Application Note
Publication # 22621
Rev: B
Issue Date: August 1999
© 1999 Advanced Micro Devices, Inc. All rights reserved.
The contents of this document are provided in connection with Advanced
Micro Devices, Inc. (“AMD”) products. AMD makes no representations or
warranties with respect to the accuracy or completeness of the contents of
this publication and reserves the right to make changes to specifications and
product descriptions at any time without notice. No license, whether express,
implied, arising by estoppel or otherwise, to any intellectual property rights
is granted by this publication. Except as set forth in AMD’s Standard Terms
and Conditions of Sale, AMD assumes no liability whatsoever, and disclaims
any express or implied warranty, relating to its products including, but not
limited to, the implied warranty of merchantability, fitness for a particular
purpose, or infringement of any intellectual property right.
AMD’s products are not designed, intended, authorized or warranted for use
as components in systems intended for surgical implant into the body, or in
other applications intended to support or sustain life, or in any other application in which the failure of AMD’s product could create a situation where personal injury, death, or severe property or environmental damage may occur.
AMD reserves the right to discontinue or make changes to its products at any
time without notice.
AMD, the AMD logo, AMD Athlon, K6, 3DNow!, and combinations thereof, and K86 are trademarks, and AMD-K6
is a registered trademarks of Advanced Micro Devices, Inc.
Microsoft is a registered trademark of Microsoft Corporation.
MetroWerks and CodeWarrior are trademarks of Metrowerks, Inc.
MMX is a trademark and Pentium is a registered trademark of Intel Corporation.
Other product names used in this publication are for identification purposes only and may be trademarks of their
respective companies.
3DNow!™ Instruction Porting
22621B/0—August 1999
Revision History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii
3DNow!™ Instruction Porting Guide
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Detecting 3DNow!™ Technology Support . . . . . . . . . . . . . . . . . . . . . . 2
Related Documents. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3DNow!™ Instruction Porting
Code Support Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Separate Executables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Separate DLL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Different Optimized Versions . . . . . . . . . . . . . . . . . . . . . . . . . . .
Conditional Code Paths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3DNow! Porting Preparations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Perform High-Level Optimizations. . . . . . . . . . . . . . . . . . . . . . . 6
Profile Existing Code . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Port Major Hotspots. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Use Compiler Optimizations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Use MASM Code for Critical Code . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Port Code in Blocks. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3DNow! Code versus x87 FPU Code . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Optimize Register Allocation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Schedule Instructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
3DNow! Code Debugging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Decode Degradation Checking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
[ESI] Inhibits Short Decode . . . . . . . . . . . . . . . . . . . . . . . . . . .
Instructions Longer Than Seven Bytes . . . . . . . . . . . . . . . . . .
Crossing Cache Line Boundary . . . . . . . . . . . . . . . . . . . . . . . . .
Instruction Length Determination . . . . . . . . . . . . . . . . . . . . . .
Align Loops on 32-Byte Boundary. . . . . . . . . . . . . . . . . . . . . . .
3DNow!™ Instruction Porting
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Blended Code Guidelines
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Data Alignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Alignment of Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Alignment of Structure Components . . . . . . . . . . . . . . . . . . . .
Alignment of Dynamically Allocated Memory . . . . . . . . . . . .
Alignment of Stack Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Maximize SIMD Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Use PREFETCH and PREFETCHW Instructions . . . . . . . . . . . . . . . 18
Take Advantage of Write Combining . . . . . . . . . . . . . . . . . . . . . . . . . 19
Use FEMMS Instruction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Load-Execute Instruction Usage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Scheduling Instructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Instruction and Addressing Mode Selection . . . . . . . . . . . . . . . . . . . 21
General Porting Guidelines
Minimize AMD-K6®-2 Processor Switching Overhead . . . . . . . . . . . 23
Using PREFETCH. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
PREFETCH on the AMD-K6 Processor . . . . . . . . . . . . . . . . . .
PREFETCH on the AMD Athlon™ Processor . . . . . . . . . . . . .
PREFETCHW Usage . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Multiple Prefetches . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Determining Prefetch Distance . . . . . . . . . . . . . . . . . . .
Prefetch at Least 64 Bytes Away from Surrounding
Stores . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Use PFSUBR Instruction When Needed. . . . . . . . . . . . . . . . . . . . . . . 27
Using PAND and PXOR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Swapping MMX™ Registers Halves . . . . . . . . . . . . . . . . . . . . . . . . . . 28
PUNPCKL* and PUNPCKH* Instructions . . . . . . . . . . . . . . . . . . . . . 28
Storing the Upper 32 Bits of an MMX Register . . . . . . . . . . . . . . . . . 29
PFMIN and PFMAX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
22621B/0—August 1999
3DNow!™ Instruction Porting
Precision Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
Moving Data Between MMX and Integer Registers . . . . . . . . . . . . . 30
Store-to-Load Forwarding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
Block Copies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
Instruction Cache and Branch Prediction Effects . . . . . . . . . . . . . . . 33
Use the Linker . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
Code Alignment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
Software Write Combining. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
Addressing Modes on the AMD-K6-2 and AMD-K6-III
Processors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
3DNow!™ Instruction Porting
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3DNow!™ Instruction Porting
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Revision History
August 1999
Revision History
Initial public release.
3DNow!™ Instruction Porting
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Revision History
3DNow!™ Instruction Porting
22621B/0—August 1999
Application Note
3DNow!™ Instruction Porting
This document contains information to assist programmers in
creating optimized code for AMD processors with 3DNow!™
technology. Compiler and assembler designers and assembly
language programmers writing execution-sensitive code
sequences as well as high-level C programmers will also find the
guidelines useful. This document assumes that the reader
possesses in-depth knowledge of the x86 instruction set, the x86
architecture (registers, programming modes, etc.), and the IBM
PC-AT platform.
This document has three sections of guidelines for 3DNow!
3DNow!™ Instruction Porting
Blended Code Guidelines
General Porting Guidelines
The 3DNow! Instruction Porting section describes the actual
process of converting existing code to 3DNow! code. The
Blended Code Guidelines section deals specifically with the
creation of blended code—3DNow! code that provides high
performance on AMD-K6 ® processors as well as on the
AMD Athlon™ processor. New applications should use blended
code to ensure optimal performance on current and future
3DNow!™ Instruction Porting
22621B/0—August 1999
platforms. The General Porting Guidelines section describes a
number of important issues for 3DNow! code optimization
mainly for the family of AMD-K6 processors, but also
addressing the AMD Athlon processor.
Detecting 3DNow!™ Technology Support
3DNow! technology is an open standard that has been adopted
by multiple processor vendors. Therefore, checking for 3DNow!
technology capability should not be limited to AMD processors.
All 3DNow! technology licensees have agreed to indicate
3DNow! technology capability through bit 31 of the extended
feature flags. Checks for 3DNow! technology support can be
made without first checking for the processor vendor. This
allows current detection code to also detect future 3DNow!
technology licensees.
The basic steps of the 3DNow! technology capability detection
are as follows:
1. Test that the processor has the CPUID instruction.
2. Check that CPUID instruction also supports extended
function 8000_0001h.
3. Execute CPUID extended function 8000_0001h and retrieve
the EDX register.
4. If bit 31 of the EDX register is set, the processor supports
the 3DNow! instruction set.
The following assembly language code shows how this can be
;; check whether CPUID is supported
:: (bit 21 of Eflags can be toggled)
;save Eflags
;transfer Eflags into EAX
edx, eax
;save original Eflags
eax, 00200000h
;toggle bit 21
push eax
;put new value of stack
;transfer new value to Eflags
;save updated Eflags
;transfer Eflags to EAX
eax, edx
;updated Eflags and original differ?
;no diff, bit 21 can’t be toggled
Detecting 3DNow!™ Technology Support
3DNow!™ Instruction Porting
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;;test whether extended function 80000001h is supported
eax, 80000000h
;call extended function 80000000h
;reports back highest supported ext.
; function
eax, 80000000h
;supports functions > 80000000h?
;no 3DNow! support, either
;;test if function 80000001h indicates 3DNow! support
eax, 80000001h
;call extended function 80000001h
;reports back extended feature flags
test edx, 80000000h
;bit 31 in extended features
;if set, 3DNow! is supported
Related Documents
Related documents can be downloaded at the following URL:
Related Documents
AMD-K6® Processor Code Optimization Application Note,
order# 21924
3DNow!™ Technology Manual, order# 21928
AMD-K6® Processor Multimedia Technology, order# 20726
Implementation of Write Allocate Application Note, order#
AMD Athlon™ Processor x86 Code Optimization Guide, order#
AMD Extensions to the 3DNow!™ and MMX™ Instruction Sets
Manual, order# 22466
AMD Processor Recognition Application Note, order# 20734
3DNow!™ Instruction Porting
22621B/0—August 1999
Related Documents
3DNow!™ Instruction Porting
22621B/0—August 1999
3DNow!™ Instruction Porting
Code Support Considerations
Consider how your software can support several paths through
the different code optimized for various processors. Choices
include the following methods:
Separate Executables
Build separate executables optimized for each platform. This is
probably the highest performance opti on, but ca n be
impractical due to code distribution issues and other problems.
Separate DLL
Place all performance-sensitive code into a separate DLL,
providing several DLLs optimized for each target platform to be
supported. This is a high-performance solution as the overhead
is typically no more than selecting and loading the DLL version
most appropriate for the platform detected at run time. The
problem with this approach is that the performance-sensitive
code can come from different and unrelated parts of the source
tree, but becomes grouped together in a single DLL.
Code Support Considerations
3DNow!™ Instruction Porting
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Different Optimized Versions
Provide optimized versions of each performance-critical
function for each target platform, and call the functions
through pointers that are initialized at run time based on the
system processor the software is running on. This has a negative
performance impact on AMD-K6® processors because function
calls through pointers are slower than regular function calls.
Conditional Code Paths
Inside performance-critical parts of the code, conditionally
select code paths based on capability flags. On AMD-K6
processors, this can be faster than the approach using function
pointers, because the branches will be well predicted since the
capabilities do not change during run time. On the other hand,
this approach can make the code less clear and more difficult to
3DNow!™ Porting Preparations
Perform High-Level Optimizations
Before starting a 3DNow! porting effort, perform all high-level
optimizations that can be done at the source-code level. This
primarily affects loops, which can be transformed in a variety of
ways for better performance—loop unrolling, loop splitting,
loop merging, loop inversion, loop switching, and hoisting of
loop invariant expressions and conditionals. Function calls can
also be optimized by inlining. It is much more difficult to
perform high-level transformations once the code has been
ported to the assembly-language level.
Profile Existing Code
Before starting the actual porting process, profile the existing
code on the target platform to identify the hotspots that merit
manual porting work.
3DNow!™ Porting Preparations
3DNow!™ Instruction Porting
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Profilers come in various types. Some require source code, some
can instrument binaries, others use a sampling approach. All
profilers should work on AMD processors. VTUNE works too,
but doesn’t have event-based profiling or disassemble 3DNow!
code capabilities. Some profilers, like Metrowerks™ CATS,
have built-in support for 3DNow! instructions and can be easier
to use when reprofiling code during the porting process.
Port Major Hotspots
Candidates for 3DNow! porting are hotspots that frequently use
x87 floating-point unit (FPU) instructions. AMD-K6 processors
incur a penalty called switching overhead whenever the
instruction flow changes between the use of x87 instructions
and MMX™/3DNow! instructions (or vice versa). For full
3DNow! optimization, port all x87 code down to hotspots that
take up only a small percentage (approximately 2%) of the total
execution time. Due to switching overhead, porting a few small
functions to 3DNow! can often be detrimental to overall
performance. The goal is to keep the processor operating on
3DNow!/MMX code for long periods of time, with only
occasional use of x87 code.
Some manual porting work can be saved by compiling the code
which contains fewer hotspots with a compiler that can
g e n e ra t e n a t i ve 3 D N o w ! c o d e , s u c h a s M e t ro w e r k s
CodeWarrior™ Professional Release 4 and later. At this time,
m a j o r h o t s p o t s re q u i re m a nu a l p o r t i n g f o r o p t i m a l
Use Compiler Optimizations
To achieve the best performance from hotspots that are not
floating-point intensive and so do not lend themselves to
3DNow! porting, experiment with compiler flags to find which
flag settings provide the best code for AMD processors. Most
compilers allow processor-specific optimizations based on the
capabilities of Intel processors. Since AMD processors are
different from Intel processors, the available processor-specific
settings are not fully optimal for AMD processors. The
microarchitecture of AMD processors most closely resembles
Use Compiler Optimizations
3DNow!™ Instruction Porting
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the P6, Pentium® II, and Pentium III microarchitecture, and in
most cases selecting P6/PII/PIII-specific optimization results in
the highest performance for AMD processors (for example, -G6
for Microsoft ® Visual C/C++). The Metrowerks CodeWarrior
com piler has a specific optimizatio n setting fo r AMD
Use MASM Code for Critical Code
Use standalone MASM code for performance-critical parts of
code that are ported to 3DNow!. This gives the best control over
the code (for example, code alignment). To assemble 3DNow!
code, use MASM 6.13 or MASM 6.14. Upgrade from an existing
installation of MASM 6.11 to 6.13 by downloading ML613.EXE
from the following ftp site:
Apply this patch. To enable MMX instructions, use the .MMX
directive. To enable 3DNow! instructions, use the .K3D
directive after using the .MMX directive. It is order dependent.
MASM 6.14 supports most of the new 3DNow! and MMX
extensions introduced in the AMD Athlon processor. Use the
.XMM directive to enable the use of these new extensions. Note
that the new instructions, PFNACC and PFPNACC are not yet
accessible in MASM 6.14. Also, in order to use the new
PSWAPD instruction, users need to define the text macro as
pswapd TEXTEQU <pswapw>
For some big functions where only a small part of the C code is
replaced, use inline assembly. Since Microsoft Visual C 5.0 does
not have native inline assembly support for the 3DNow!
instruction set, download instruction macros from the AMD
web site. The macros are in the amd3d.h file in the 3DNow!
SDK, which can be downloaded from the following URL:
To get started on assembly language code, have the C compiler
generate an assembly language listing and use that as the
initial assembly language version. Make sure to compile with
maximum optimizations to have the compiler perform all the
Use MASM Code for Critical Code
3DNow!™ Instruction Porting
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high-level optimizations up front. The compiler will convert
symbolic constants in the source code to “magic numbers”;
however, the programmer may need to set up a mechanism to
extract symbolic constants from C code and import them into
the assembly code to maintain the assembly code as well as the
C code.
Port Code in Blocks
Most functions contain several more-or-less self-contained
blocks. Port blocks one-by-one to 3DNow! and surround the
3DNow! code with FEMMS. This block-by-block approach
minimizes debug time. After each block is ported, run the code
to verify that it is still working. If the code is not working, it’s
usually easy to locate errors because they are isolated to a
block. With this approach a debugger is only rarely necessary in
3DNow! porting work. The commenting conventions for 3DNow!
code show the most significant half of the operand on the left
hand side, the least significant half of the operand on the right
hand side, with the halves separated by a vertical bar.
3DNow!™ Code versus x87 FPU Code
When porting, most programmers find that 3DNow! code is
much easier to write than x87 FPU code because the register
file is flat and because with the 3DNow! single instruction
multiple data (SIMD) capability twice as many operands can be
manipulated. It is often possible to remove local temporary
Maximize the use of SIMD—always try to do useful work on
both parts of the operands. It can be advantageous to add
overhead to pack and unpack operands in order to use the SIMD
arithmetic. Consider modifying existing data structures so the
data layout is more conducive to SIMD processing, thereby
eliminating the need for additional pack and unpack
Replace integer code with MMX code. Unroll small loops
completely. This can free up integer registers, and branches
Port Code in Blocks
3DNow!™ Instruction Porting
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that do not exist cannot be mispredicted. Due to the large
number of global history bits, the AMD-K6 processor does not
predict well on many short loops. If possible, use computations
to replace branches caused by “if...then...else” constructs
acting on 3DNow! data. Branching on 3DNow! data is a bit
slower since 3DNow! instructions don’t affect the integer flags.
Also, branching is disruptive to SIMD code as it is an inherently
scalar operation which diminishes the advantages of SIMD
Avoid moving data between the integer and the MMX registers
b e c a u s e t h i s i s t i m e - c o n s u m i n g o n t h e A M D -K 6 a n d
AMD Athlon™ processors. To move data between the integer
and the MMX registers, use the MOVD instruction. Write MMX
and 3DNow! code in a load/store construction—but do not use
load execute instructions such as PFADD MM0, [FOO]. Using a
load/store construct enables aggressive scheduling which is
essential for good performance. (See Schedule Instructions on
page 11.)
Maximize the use of instructions that guarantee high decode
bandwidth. These are called short-decode instructions on
AMD-K6 family processors and DirectPath for AMD Athlon
family processors. The optimization guides for both processors
list the short-decode or DirectPath instructions. Maintaining a
high decode bandwidth is essential for high performance code.
Using short-decoded instructions, the AMD-K6 family
processors can decode two instructions per cycle. Using
DirectPath instructions, the AMD Athlon family processors can
decode three instructions per cycle. On the AMD-K6 family
processors, the only 3DNow!/MMX instructions that are not
short-decoded are EMMS, FEMMS, and PREFETCH.
Avoid indirect calls and jumps, as the AMD-K6 processors do
not apply branch predict ion t o t he se control-t ransfe r
instructions. At the source code level, this affects functions
called through a function pointer (such as entry points into
DLLs). The latency of a JMP DWORD PTR is eight cycles, and
the latency of a CALL DWORD PTR is seven cycles. Note that
AMD-K6 processors use the return stack on indirect calls, so the
return from an indirectly called routine is still accelerated. The
AMD Athlon processor applies branch prediction to indirect
calls and jumps.
3DNow!™ Code versus x87 FPU Code
3DNow!™ Instruction Porting
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Optimize Register Allocation
After porting a complete function, optimize register allocation
across the function. Keep as much data as possible in registers
to reduce overall memory traffic. Make sure all data is aligned
to natural boundaries—QWORDs on QWORD boundaries,
DWORDs on DWORD boundaries. Note that data that is
accessed by 3DNow! code as QWORDs is not necessarily
declared as QWORDs in the program, and therefore can not be
properly aligned even if compiler switches are used to force
data alignment to natural boundaries. Ensuring alignment can
require slight changes or padding to data structures outside the
ported code, and can require manual QWORD alignment of
pointers returned by dynamic memory allocation routines such
as malloc(), calloc(), etc. Use the /zp8 switch on Microsoft Visual
C to pad and align structs to QWORD boundaries. Note however
that /zp8 doesn’t always do a perfect job, so a small amount of
manual padding may still be needed.
Schedule Instructions
Schedule the code according to instruction latencies.
Scheduling is important for AMD-K6-2 and AMD-K6-III
processors because their scheduler is six deep and four wide,
and it holds 24 OPs. OPs are pushed into the scheduler four OPs
(an op-quad) at a time. As new OPs come in at the top, the
previous lines shift down. When a line reaches the bottom of the
scheduler and the OPs haven’t all completed yet, the scheduler
stalls—no new OPs can be pushed in at the top. If all OPs have
completed and the line is at the bottom of the scheduler, the
results of the OPs are committed to architectural state (retired)
and the op-quad is discarded from the scheduler, allowing the
following lines to shift down.
In the best possible case, the decoders push in a new op-quad
every cycle. The OPs must complete after six cycles or else the
processor loses performance. The 24 OPs are equivalent to 12
short-decoded x86 instructions. So, the out-of-order window is
not very big, and an instruction that doesn’t get its source
operands right away can get to the bottom of the scheduler
without having completed, this prevents the scheduler from
Optimize Register Allocation
3DNow!™ Instruction Porting
22621B/0—August 1999
There are a few basic scheduling rules. All 3DNow! instructions
in the AMD-K6-2 and AMD-K6-III processors have two-cycle
latency. All MMX instructions have one-cycle latency, except
MMX multiplies which are two cycles. Loads have two-cycle
latency. To guarantee smooth flow of code through the machine,
group instructions into pairs that can decode together, issue
together, and retire together. To achieve this, observe the
following rules:
No dependencies between instructions in a decode pair
No resource conflicts between instructions in a decode pair
Per cycle, the AMD-K6-2 and AMD-K6-III processors can
perform the following:
One load
One store
Two integer ALU operations
One integer shift
Two MMX ALU operations
One MMX shift
One 3DNow! add pipe op
One 3DNow! mul pipe op
One branch
LEA counts as a store op. PUNPCK* instructions are MMX ALU
One scheduling method is to first group the code following the
above rules, marking the empty slots with <> and then move
instructions to fill the slots. For example:
mm1, [foo_var]
;0 | v[3],v[2],v[1],v[0]
mm1, mm0 ; 0,v[3],0,v[2] | 0,v[1],0,v[0]
; 0,v[3],0,v[2] | 0,v[1],0,v[0]
0,0,0,v[1] | 0,0,0,v[0]
0,0,0,v[3] | 0,0,0,v[2]
float(v[1]) | float(v[0])
float(v[3]) | float(v[2])
Schedule Instructions
3DNow!™ Instruction Porting
22621B/0—August 1999
3DNow!™ Code Debugging
To debug 3DNow! code, it is best to have a debugger that
supports both the disassembly of 3DNow! instructions, and
allows the MMX registers to be viewed as pairs of singleprecision floating-point values. NuMega SoftICE version 3.24
and later has both these capabilities. Microsoft Visual C/C++
6.0 can also disassemble 3DNow! instructions; however, it does
not provide a convenient way of viewing the MMX registers as
pairs of floating-point numbers.
Decode Degradation Checking
After code has been scheduled and thoroughly tested, the last
s t ag e o f t we a k in g is t o m a ke s u re t h e re i s n o de c o d e
degradation. All AMD-K6 processors use a technique called predecode to speed up decoding. In certain instances, the predecode information can be degraded, resulting in decode of
only one instruction per cycle (long decode) or even one
instruction per two cycles (vector decode), even though the
instruction itself is listed as short decoded. Use the following
guidelines for AMD-K6 family processors:
[ESI] Inhibits Short Decode
Use of [ESI] addressing mode inhibits short decode. Note that
[ESI+disp], [ESI+reg] etc. is acceptable. Also, note that
specifying [ESI+0] is optimized by most assemblers to [ESI].
Instructions Longer Than Seven Bytes
If the length of an instruction exceeds seven bytes, short
decode is inhibited, and the instruction can never be short
3DNow!™ Code Debugging
3DNow!™ Instruction Porting
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Crossing Cache Line Boundary
If an instruction crosses a cache line boundary and the opcode
byte and modR/M byte are not in the same cache line, short
decode is inhibited. Instruction cache lines are 32-bytes long in
the AMD-K6 family processor. If the code segment is only
paragraph (16-byte) aligned, check all 16-byte boundaries for
the occurrence of this case. Bad cases can be remedied as
Swap instructions in a decode pair.
Choose alternative instructions to move code.
(For example, use CMP EAX, 0 instead of TEST EAX, EAX)
Insert filler instructions like NOPs. Since an instruction
degraded to vector decode takes up two cycles, it’s better to
add an additional instruction and have both be short
Hand code an instruction to add a zero displacement or to
make a displacement 32 bits instead of 8 bits.
Instruction Length Determination
Short-decode is inhibited if more than three instruction bytes
are required to determine the length of an instruction. This
happens for certain SIB addressing modes where the decoder
needs to look at the SIB byte to determine instruction length,
but 0Fh, opcode, and modR/M already make up the maximum of
three bytes. Avoid these SIB addressing modes. For more
information, see the AMD-K6 Processor Code Optimization
Application Note, order# 21924. AMD-K6-2 processors with the
CXT core (CPUIDs of 588h to 58Fh) and AMD-K6-III processors
eliminate this particular form of degraded predecode.
Align Loops on 32-Byte Boundary
Align important loops on a 32-byte cache line boundary. At the
minimum, make sure that after the start of the loop there are at
least two instructions before the next 32-byte boundary.
Decode Degradation Checking
22621B/0—August 1999
3DNow!™ Instruction Porting
Blended Code Guidelines
Blended code is 3DNow!™ optimized code that runs well on
both the AMD-K6® and AMD Athlon™ processor platforms. The
basic approach to blended code optimization is to address the
AMD-K6 processor requirements first, and then to look for
specific AMD Athlon processor improvements and issues which
do not adversely affect AMD-K6 processor performance.
With much larger buffers and a much larger out-of-order
instruction window than other x86 processors, the AMD Athlon
processor is good at automatically extracting performance out
of existing executables, even if they are specifically optimized
for a different processor. Of course, the best AMD Athlon
performance can be achieved by optimizing code to exploit the
specific strengths of the AMD Athlon processor.
To learn more about AMD Athlon code optimization, refer to
the AMD Athlon™ Processor x86 Code Optimization Guide, order#
3DNow!™ Instruction Porting
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Data Alignment
Data alignment is very important for both AMD-K6 and
AMD Athlon processor performance. Standard processor
designs will work to their full potential if data is aligned.
Alignment is specially important for data that is written by one
instruction and subsequently read by another instruction.
Three typical areas to watch for data alignment are:
Alignment of structures and structure components
Alignment of dynamically allocated memory
Alignment of stack data
Alignment of
With regard to alignment of structures, many compilers offer
switches to automatically pad and align structures. These
switches do not always work perfectly. It is best to check the
alignment and to pad manually if necessary.
Alignment of
Arranging structure components in order of decreasing size
may help. For example, declare components with larger base
type (e.g., DWORD) ahead of components with smaller base
types (e.g., BYTE).
Alignment of
Allocated Memory
With regard to alignment of dynamically allocated memory, if
your programming environment does not guarantee pointers
returned by dynamic memory allocators, such as malloc(), to be
suitably aligned, allocate a slightly larger chunk of memory and
align the pointer manually. For example, a QWORD alignment
should be:
p=(QWORD *)malloc(sizeof(QWORD)*number_of_qwords)+7L);
np=(QWORD *)((((long)(p))+7L) & (-8L));
Alignment of Stack
Alignment of stack data is hard to control unless the complete
functions are written in assembly language. In this case, use
code like the following example to keep local 3DNow! data
QWORD aligned.
ESP, -8
ESP, size_of_local_variable
Data Alignment
3DNow!™ Instruction Porting
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Note: Use EBP to access arguments, use ESP to access local
Maximize SIMD Processing
Maximize the amount of SIMD processing in your code. If the
p rog ram m er ex p l oit s t h e S I M D na t u re o f t h e 3 DN ow !
instructions aggressively, using 3DNow! instructions in the code
can provide significant performance benefits as compared to
x87 code.
Using PUNPCK instructions to combine scalar data for SIMD
processing can create significant overhead and should be
avoided where possible. It is best to rearrange computations
and data structures in the source such that the amount of SIMD
computation can be maximized.
Example 1 (Avoid):
float Xscale, Xoffset, Yscale, Yoffset;
xnew = x*Xscale+Xoffset;
ynew = y*Yscale+Yoffset;
Example 2 (Better):
float Xscale, Yscale, Xoffset, Yoffset;
xnew = x*Xscale+Xoffset;
ynew = y*Yscale+Yoffset;
The second example can now be efficiently implemented using
3DNow! instructions:
mm0, x
mm1, Xscale
mm2, Xoffset
mm0, mm1
mm0, mm2
xnew, mm0
;y | x
;Yscale | Xscale
;Yoffset | Xoffset
;y*Yscale | x*Xscale
;y*Yscale+Yoffset | x*Xscale+Xoffset
;store ynew | xnew
As a rough goal, strive to use 90% or more of the available
computational slots provided by the SIMD instructions.
Maximize SIMD Processing
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Use PREFETCH and PREFETCHW Instructions
Use PREFETCH and PREFETCHW as aggressively as possible.
On the AMD-K6-2 processor, use of PREFETCH results in only a
small performance improvements, because the prefetches share
the frontside bus (FSB) bandwidth. However, due to the high
FSB utilization at high core-clock multipliers, the prefetches
often get bumped because they are a low priority memory
access. This situation improves with the AMD-K6-III processor,
where L2 traffic is redirected to a separate backside bus, which
frees up FSB bandwidth.
The A MD At hlo n pro ces sor has large am ount s o f FSB
bandwidth available, and application-level improvements of up
to 20% have been observed using PREFETCH(W) aggressively.
Examine code carefully to find opportunities for using
PREFETCH(W). Good use of PREFETCH requires that
essentially all of the prefetched data is actually used, and it
therefore works best if data is accessed with unit stride and in
ascending order. Sometimes algorithms can be rewritten to
create such a data access pattern. On the AMD-K6 processor,
PREFETCH creates a small overhead, since it is a vector
decode instruction. On the AMD Athlon processor, PREFETCH
is DirectPath.
Use PREFETCH as aggressively as possible without decreasing
AMD-K6 processor performance due to the overhead of the
PREFETCH instruction. This is possible in almost all cases.
PREFETCH on the AMD Athlon processor brings in 64 bytes
per PREFETCH due to the cache line length having doubled
over the AMD-K6 processor (32 bytes versus 64 bytes), but it is
acceptable to have overlapping (on the AMD Athlon processor)
prefetches to account for the shorter 32-byte cache lines of the
AMD-K6 processor. Make sure to prefetch to addresses at least
64 bytes apart from the target address of any stores in the
vicinity of a PREFETCH(W) instruction. Also, for best
AMD Athlon performance, prefetch about three cache lines
(192 bytes) ahead of current loads. For a more detailed formula,
see the PREFETCH usage guideline in the AMD Athlon™
Processor Code Optimization Guide, order# 22007.
Use PREFETCH and PREFETCHW Instructions
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Take Advantage of Write Combining
M a ke s u re t o t a ke a dva n t a g e o f t he w r i t e - c o m bi n i n g
mechanisms provided by the hardware. For the AMD-K6
processor, the best performance is achieved by using software
write combining. (See “Software Write Combining” on
page 34.) Also enable the write-combining features provided by
the hardware on the AMD-K6-2 processor with the CXT core
and on the AMD-K6-III processor. Aggressive software write
combining can often do a better job than the AMD-K6
processor’s hardware write-combining mechanism, but enabling
the hardware write-combining mechanism provides the
additional benefit of shorter latency writes to non-cacheable
memory areas.
The AMD Athlon processor has a very powerful writecombining mechanism that achieves even better acceleration of
writes to non-cacheable space than is possible with write
combining on the AMD-K6 processor. Specifically, the
AMD Athlon write-combining buffer is 64 bytes and can
combine writes of any size. The programming of the writecombining hardware is through model-specific registers
(MSRs), which have been implemented compatibly with the
Intel Pentium II processor. In addition to accelerating writes to
write-combining (WC) regions, the AMD Athlon write
combining can also accelerate writes to write-through (WT)
memory areas if they occur in strictly ascending order. (Writes to
WC areas can be combined regardless of the order of the
writes.) See the Write Combining chapter for the AMD Athlon™
Processor Code Optimization Guide, order# 22007 for more
Use FEMMS Instruction
The AMD Athlon processor does not have any switching
overhead when switching between 3DNow!/MMX instructions
and x87 instructions. Also, the FEMMS and EMMS instructions
are essentially free because they execute with apparent zerocycle latency. However, for blended code it is important to avoid
frequent switching between 3DNow!/MMX and x87 code blocks
and to use FEMMS before entering and after leaving a block of
Take Advantage of Write Combining
3DNow!™ Instruction Porting
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3 D N o w ! / M M X c o d e . O t h e r w i s e , A M D -K 6 p ro c e s s o r
performance can suffer significantly.
Load-Execute Instruction Usage
The AMD Athlon processor performs well when load-execute
instructions (i.e., instructions that have a register and a
memory source, where the result goes to a register) are used. In
fact, use of load-execute instructions is recommended for the
AMD Athlon processor because they improve code density.
However, for blended code, do not use load-execute instructions
in 3DNow!/MMX code to enable proper scheduling of loads and
to avoid potential problems with load-execute instructions
(degradation to vector decode due to instruction length) on
AMD-K6 family processors. The AMD Athlon processor has a
built-in mechanism that enables a sequence of a load and a
dependent 3DNow!/MMX instruction to execute just as quickly
a s a lo a d - ex e c u t e in st r u c t i on . Avoi d i n g l o ad - ex ec ut e
instructions does not cause performance degradation on the
AMD Athlon processor but can help the AMD-K6 processor.
Scheduling Instructions
Schedule instructions for the AMD-K6 processor. (See
“Schedule Instructions” on page 11.) Due to the relatively small
inst ruction re- order buff er in t he AMD-K6 processor,
i n s t r u c t i o n s ch e d u l i n g i s i m p o r t a n t f o r m a x i m i z i n g
performance on AMD-K6 processors. However, the AMD Athlon
processor is a very aggressive out-of-order machine with a huge
instruction re-order buffer. Therefore, instruction scheduling
on the AMD Athlon processor is of minor importance, because
the CPU can extract the available parallelism automatically.
Scheduling code for the AMD-K6 processor has no adverse side
effects on AMD Athlon performance.
Load-Execute Instruction Usage
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Instruction and Addressing Mode Selection
As far as instruction selection is concerned, only a few issues
require attention. On the AMD Athlon processor, transferring
data between integer and MMX registers is somewhat slower
than on the AMD-K6 processor. Therefore, such transfers should
be minimized. Usually, this is not difficult to do.
Among the integer instructions, avoid the LOOP instruction.
While very fast on the AMD-K6 processor, it is somewhat slower
on the AMD Athlon processor. It should be replaced with the
sequence DEC ECX;JNZ. This will, in most cases, not reduce
AMD-K6 performance, and if so, only to a very limited amount.
The AMD Athlon processor uses a different instruction predecode scheme than the AMD-K6 processor. It therefore has no
sub-optimal addressing modes. However, since this is a real
performance issue on the AMD-K6 processor, addressing modes
considered sub-optimal for the AMD-K6 processor should be
avoided in blended code. Sub-optimal addressing modes are
described in “Addressing Modes on the AMD-K6 ® -2 and
AMD-K6®-III Processors” on page 36.
Instruction and Addressing Mode Selection
3DNow!™ Instruction Porting
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Instruction and Addressing Mode Selection
3DNow!™ Instruction Porting
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General Porting Guidelines
Minimize AMD-K6®-2 Processor Switching Overhead
Minimize the FPU to 3DNow!™ and MMX™ switching overhead
by porting all hotspots containing x87 code to 3DNow! code.
Even if FEMMS is used, switching incurs about 25 cycles in each
direction—50 cycles round-trip. Always use FEMMS, and not
EMMS, as the switching overhead with EMMS is about 100
cycles round-trip.
Always bracket 3DNow! code with FEMMS to ensure proper
operation and minimize switching overhead. If there are
function calls to functions that can contain FPU code, bracket
the function call with FEMMS.
It is also beneficial to simply minimize the number of FEMMS.
One technique to use if there are multiple calls to a DLL (where
the functions are _stdcall), is to perform the following in order:
Push all the arguments first
Execute a FEMMS
Call all the functions (which unload the stack)
Execute another FEMMS
Since FEMMS is a three-cycle vector path instruction, functions
should not be made very small to avoid adding significant
Minimize AMD-K6®-2 Processor Switching Overhead
3DNow!™ Instruction Porting
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overhead (functions have been observed in OpenGL that consist
of just five x86 instructions).
Note: For the AMD Athlon processor, it is important that CALLs
not be spaced too closely together. No more than two CALLs
for every 16 bytes of code are recommended.
The switching overhead occurs on the first floating-point unit
instruction after a piece of 3DNow!/MMX code, and it occurs on
the first MMX or 3DNow! instruction after a piece of x87 code.
FEMMS and EMMS are 3DNow!/MMX instructions. Thus,
looking at the following sample code:
<FPU instructions>
<MMX/3DNow! instructions>
<1st FPU instruction>
3 + switching overhead
x + switching overhead
Note that PREFETCH(W), although introduced as part of the
3DNow! instruction set extension, is treated like an ordinary
integer instruction and therefore never incurs switching
overhead. PREFETCH(W) can be used to accelerate integer,
x87, MMX, or 3DNow! code.
Use PREFETCH judiciously. PREFETCH on the AMD-K6® -2
and AMD-K6 ® -III processors is microcoded, so it adds some
overhead. Also on the AMD-K6-2 processor, all cache and
memory accesses have to flow through the same frontside bus.
Do not waste bandwidth on the frontside bus by executing
useless prefetching.
Opportunities for using PREFETCH are typically inside loops
that process large amounts of data. If the loop goes through less
than a cache line of data per iteration, partially unroll the loop.
Make sure that close to 100% of the prefetched data is actually
being used. This usually requires unit stride access—all
accesses are to contiguous memory locations.
3DNow!™ Instruction Porting
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PREFETCH on the AMD-K6® Processor
The usefulness of PREFETCH on AMD-K6-III processors is
limited by hardware constraints, the most important is that the
AMD-K6-III processor allows only one load miss to be
outstanding at any time.
The cases where PREFETCH can most likely provide benefits
are characterized as follows:
The bandwidth requirements of the code are moderate—
there is a relatively large amount of computation and
relatively few memory accesses. An example of moderate
bandwidth requirements would be code that consumes
about 250 Mbytes per second worth of data when running
out of the L1 cache on a 400-MHz processor.
Stores in the code that access cacheable memory write to a
small area of memory only—the working sets for stores is
small or empty. Due to the write-allocate feature of the
AMD-K6-2 and AMD-K6-III processors, stores bring lines
into the cache which are subsequently dirtied and must be
written back from the cache when the cache line is replaced
with data brought in by PREFETCH. Cache writebacks use
up bandwidth on the front-side bus.
PREFETCHes do not overlap—no two PREFETCH
instructions try to bring in the same data.
The number of distinct memory regions being prefetched is
small, preferably only one region — if there are multiple
memory regions being prefetched (like multiple source
arrays), the density of the loads must be low compared to the
amount of computation, such that the computation can be
overlapped with each PREFETCH. The PREFETCH
instructions should be scheduled separately in such cases to
allow each to overlap with computation, and to avoid the
first PREFETCH blocking subsequent PREFETCHes due to
the limit of one load miss in the machine at any time.
PREFETCH on the AMD Athlon™ Processor
PREFETCH on the AMD Athlon™ processor is a very powerful
tool both because of the much larger available bandwidth that
it can exploit and because of the ability to have multiple
outstanding load misses.
3DNow!™ Instruction Porting
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Code that intends to modify the cache line brought in through
prefetching should use the PREFETCHW instruction. While
PREFETCHW works the same as a PREFETCH on the
AMD-K6-2 and AMD-K6-III processors, PREFETCHW gives a
hint to the AMD Athlon processor of an intent to modify the
cache line. The AMD Athlon processor will mark the cache line
being brought in by PREFET CHW as modif ied. Using
PREFETCHW can save an additional 15-25 cycles compared to
a PREFETCH and the subsequent cache state change caused by
a write to the prefetched cache line.
Multiple Prefetches
Programmers can initiate multiple outstanding prefetches on
t h e A M D A t hl o n p ro c e s s o r. Wh il e t h e A M D -K 6 -2 a n d
AMD-K6-III processors can have only one outstanding prefetch,
the AMD Athlon processor can have up to six outstanding
prefetches. For example, when traversing more than one array,
the programmer should initiate multiple prefetches.
Example (Multiple Prefetches):
for (i=0; i<A_REALLY_LARGE_NUMBER/4; i++) {
prefetchw (a[i*4+64]); // will be modifying a
prefetch (b[i*4+64]);
prefetch (c[i*4+64]);
= b[i*4]
* c[i*4];
a[i*4+1] = b[i*4+1] * c[i*4+1];
a[i*4+2] = b[i*4+2] * c[i*4+2];
a[i*4+3] = b[i*4+3] * c[i*4+3];
Determining Prefetch
To make sure code with PREFETCH works well on the
AMD Athlon processor, prefetch several cache lines ahead of
t h e c u r re n t l o a d s . A g o o d h e u r i s t i c i s t o f e t ch t h re e
AMD Athlon cache lines (at 64 bytes each), or 192 bytes ahead
of current loads. That is, if the code is currently operating on
data at address X, prefetch at X+192.
Given the latency of a typical AMD Athlon processor system
and expected processor speeds, the following formula should be
used to determine the prefetch distance in bytes:
Prefetch Distance = 200 (DS/C) bytes
Round up to the nearest 64-byte cache line.
3DNow!™ Instruction Porting
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Prefetch at Least 64
Bytes Away from
Surrounding Stores
The number 200 is a constant that is based upon expected
AMD Athlon processor clock frequencies and typical system
memory latencies.
DS is the data stride in bytes per loop iteration.
C is the number of cycles for one loop to execute entirely
from the L1 cache.
The PREFETCH and PREFETCHW instructions can suffer
from false dependencies on stores. If there is a store to an
address that matches a request on bits 14–6, that request (the
PREFETCH or PREFETCHW instruction) is blocked until the
store is written to the cache. Therefore, code should prefetch
data that is located at least 64 bytes away from any surrounding
store’s data address.
If PREFETCH helps on a piece of code, but doesn’t affect the
AMD-K6-III processors, keep the PREFETCH code anyway.
There is a good chance that it will help on the AMD Athlon
p r o c e s s o r, b e c a u s e t h e A M D A t h l o n p ro c e s s o r ’s
implementation of PREFETCH is very aggressive. If an
AMD Athlon processor is available, check that it benefits from
the PREFETCH, and then make sure that the PREFETCH
doesn’t hurt the AMD-K6-III processor.
Use PFSUBR Instruction When Needed
Note that there is a PFSUBR instruction, so in a subtraction the
programmer can choose which operand to destroy.
Using PAND and PXOR
Use PAND and PXOR to perform FABS and FCHS work on
3DNow! operands. For example:
DQ 07fffffff7fffffffh
DQ 08000000080000000h
mm0, [mabs]
mm1, [sgn]
mm2, mm1
;change sign
mm2, mm0
;absolute value
Use PFSUBR Instruction When Needed
3DNow!™ Instruction Porting
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Use a PXOR MMreg, MMreg instruction to clear the bits in an MMX
Use a PCMPEQD MMreg, MMreg instruction to set the bits in an
MMX register.
Swapping MMX™ Registers Halves
To swap the two register halves of an MMX register (which
should be avoided) use the following:
;mm1 = swapd (mm0), mm0 destroyed
mm1, mm0
;y | x
punpckldq mm0, mm0
;x | x
punpckhdq mm1, mm0
;x | y
;mm1 - swapd (mm0), mm0 preserved
mm1, mm0
;y | x
punpckhdq mm1, mm1
;y | y
punpckldq mm1, mm0
;x | y
For code being used only on AMD Athlon family processors, use
the new PSWAPD instructions. See the AMD Extensions to the
3DNow!™ and MMX Instruction Sets Manual, order# 22466 for
the instruction usage.
PUNPCKL* and PUNPCKH* Instructions
PUNPCKL* and PUNPCKH* are essential facilities for
m a n i p u l a t i n g M M X a n d 3 D N o w ! o p e ra n d s . B e s i d e s
MOVQ/MOVD, these are the most frequently used MMX
instructions in 3DNow! code. For example, converting a stream
of unsigned bytes into 3DNow! floating-point operands:
; outside loop:
mm0, mm0
;inside loop:
punpcklbw mm1,
punpcklwd mm1,
punpckhwd mm2,
;0 | v[3],v[2],v[1],v[0]
;0,v[3],0,v[2] | 0,v[1],0,v[0]
;0,v[3],0,v[2] | 0,v[1],0,v[0]
;0,0,0,v[1] | 0,0,0,v[0]
;0,0,0,v[3] | 0,0,0,v[2]
;float(v[1]) | float(v[0])
; float(v[3]) | float(v[2])
Swapping MMX™ Registers Halves
3DNow!™ Instruction Porting
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Storing the Upper 32 Bits of an MMX™ Register
To store the upper 32 bits of an MMX register using MOVD, one
can use either a PSRLQ or a PUNPCKHDQ instruction to move
the high-order 32 bits of the register to the low-order 32 bits of
the register. In this situat ion, it is optimal to use the
PUNPCKHDQ instruction. The AMD-K6-III processor has only
one MMX shifter (which can execute a PSRLQ), but two MMX
ALUs (which can execute a PUNPCKHDQ). Using PUNPCHDQ
therefore maximizes the likelihood of an execution unit being
Use PFMIN and PFMAX where possible. They are much faster
than the equivalent code using MMX and 3DNow! instructions.
PFMIN and PFMAX can be used for clamping. They can also be
used in SIMD code that avoids branching by replacing it with
computation. For example:
float x,z;
z = abs(x);
if (z >= 1) {
z = 1/z;
can be coded using branchless SIMD code as follows:
;;in: mm0 = x
;;out: mm0 = z
pfrcpit1 mm0,
pfrcpit2 mm0,
Storing the Upper 32 Bits of an MMX™ Register
;z < 1 ? 0xffffffff : 0
;1/z approx
;save z
;1/z step
;1/z final
;z = z < 1 ? z : 1/z
3DNow!™ Instruction Porting
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Another example. The following code:
#define PI 3.14159265358979323f
float x,z,r,res;
z = abs(x)
if (z < 1) {
res = r;
else {
res = PI/2-r;
This can be code as branchless SIMD code as follows:
mm0 = x
mm1 = r
;;out: mm1 = res
mm5, mabs
mm6, one
mm0, mm5
mm6, mm0
mm4, pio2
mm4, mm1
mm6, mm4
mm1, mm6
;z < 1 ? 0xffffffff : 0
;z < 1 ? 0 : pi/2-r
;res = z < 1 ? r : pi/2-r
Precision Considerations
Carefully consider whether to use reciprocals, divides, square
roots, and reciprocal square roots to full precision. If full
precision is not required, accelerate code by using just the
approximations returned by PFRCP (14 bits accuracy), and
PFRSQRT (15 bits accuracy) instead of coding the reciprocal or
reciprocal square root sequence with the Newton-Raphson step
instructions. For lighting computations, the accuracy of the
approximation instructions often suffices, but geometry
transforms typically require full precision.
Moving Data Between MMX™ and Integer Registers
For the AMD Athlon processor, avoid moving data between
MMX and integer registers or vice versa. If this cannot be
avoided, use the MOVD instruction to accomplish the transfer,
and do not pass the data manually through memory (except
Precision Considerations
3DNow!™ Instruction Porting
22621B/0—August 1999
where the store can be scheduled at least 15 instructions ahead
of the load).
Store-to-Load Forwarding
Avoid any store-to-load forwarding (store feeding into load) that
does not have address and size matches. The only exception is a
wide store feeding into a small load where the addresses match:
[foo], mm0
eax, [foo]
Here are some cases to avoid:
[foo], eax
[foo+4], edx
mm0, [foo]
[foo], mm0
eax, [foo+4]
[foo], mm0
[foo+8], mm1
mm2, [foo+4]
Block Copies
For memory block copies on the AMD-K6-III processor, most
code will have very similar performance for large blocks,
because it is limited by the bus interface. For the AMD-K6-2
processor, this was verified by creating multiple block copy
t y p e s a n d d i s c ove r i n g t h a t t h e re we re i n s i g n i f i c a n t
performance differences. This is also true for block copies
inside L2 (for off-chip L2). However, in L1-to-L1 block copies
there can be a big difference.
Th e fo l l ow i n g a re m e a s u re m e n t s p e r fo r m e d w i t h a n
AMD-K6-2/300 on an Epox motherboard with VIA MVP3
chipset and PC100 DRAM. Data blocks are QWORD aligned.
MSV 5.0
Store-to-Load Forwarding
985 MB/s
122 MB/s
71 MB/s
aggressive MOVQ loop
1718 MB/s
124 MB/s
72 MB/s
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The L2-to-L2 and mem-to-mem throughput increases with the
A M D -K 6 -I I I p ro c e s s o r a n d f u r t h e r i n c re a s e s o n t h e
AMD Athlon processor.
The aggressive MOVQ loop performs at a minimum as well as a
memcpy(), and does much better for L1-to-L1 transfers. It is also
preferable for copies to non-cacheable areas on the AMD-K6-III
processor due to the doubled chunk size over the REP MOVSD
inside the memcpy() function. For this reason, consider using it
for all block copies. The code is as follows:
_asm { mov
eax, [src]
edx, [dst]
ecx, (SIZE >> 6)
movq mm0, [eax]
edx, 64
mm1, [eax+8]
eax, 64
mm2, [eax-48]
[edx-64], mm0
mm3, [eax-40]
[edx-56], mm1
mm4, [eax-32]
[edx-48], mm2
mm5, [eax-24]
[edx-40], mm3
mm6, [eax-16]
[edx-32], mm4
mm7, [eax-8]
[edx-24], mm5
[edx-16], mm6
[edx-8], mm7
Care should be taken to make the label xfer: 32-byte-aligned for
maximum performance. As a side note, the Microsoft Visual C
5.0 without Service Pack 3 appears to ignores align directives in
Block Copies
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inline assembly. This problem may not occur after applying
Service Pack 3 to Microsoft Visual C 5.0.
Instruction Cache and Branch Prediction Effects
Try d i f f e re n t f u n c t i o n o rd e r i n g t o s e e h ow i t a f f e c t s
performance, there are sometimes interesting differences of
several F PS (frames per s econd— as a me as u re o f the
pe rformance of graphics applicatio ns) based on that.
Instruction cache thrashing is one suspect in this. The other one
is the branch prediction which has a global history component
where branches can influence the prediction of other branches.
Most of the time this helps. (Two branches might be closely
correlated—if one is taken the other one is always not taken.)
But it can also hurt, like all heuristic algorithms.
In order to reduce the potential for instruction cache thrashing,
group all the program’s hotspots close together. For example
extract all the performance-critical functions into a single file.
Use the Linker
There is another way to affect function ordering that may be
more desirable. The linker allows the programmer to specify
the exact order of every function in a DLL/executable as
1. All source code must be compiled with the /Gy switch. This
creates packaged functions—a COMDAT record is emitted
into the object file for each function.
2. At link time, use the /ORDER:@filename switch to fix the
order of functions in the DLL/executable. The term
filename, refers to a file that lists all function names in the
order to be emitted, one function name per line. For C code
it’s simply the function name as it appears in the source (no
pre-pended underscore, no @xx suffix for Pascal calling
3. This does not work for object files produced by MASM.
MASM doesn’t have a switch to create packaged functions,
and it does not allow the user to create a COMDAT entry
manually by putting COMDAT func into your source.
To reduce potential problems due to branch prediction,
eliminate as many branches as possible. The AMD-K6-III and
Instruction Cache and Branch Prediction Effects
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AMD Athlon processors have large instruction caches, and
aggressive loop unrolling (which increases the code size) helps.
It is also worthwhile to eliminate branches which have small
am o u n t s of c o d e, re p la c ing t h e b ra n ch e s w it h in- line
Code Alignment
To get 32-byte alignment in MASM 6.13, forgo the convenience
of new-style segment declarations, and use something like the
MASM may not allow ALIGN to be more restrictive than the
SEGMENT alignment. If .CODE is used, the result is a PARA
aligned segment—a 16-byte aligned segment.
For inline assembly in Microsoft® Visual C, the best alignment is
16-byte alignment by using align 16 in the inline assembly code.
Microsoft Visual C 5.0 without SP3 ignores this directive, so
check whether the alignment is actually there. Microsoft Visual
C 4.2 seems to work in this regard. At present, correct operation
of align under Microsoft Visual C 5.0 with SP3 has not been
For inline assembly in Metrowerks™ CodeWarrior Pro 4, align
32 is accepted and works. See the specific vendor for more
Software Write Combining
The writes-to-non-cacheable space is an important issue for lowlevel drivers. Processors communicate with graphics chips
through a command buffer on the graphics card which is
mapped to non-cacheable PCI (or AGP) space. On a Pentium® II,
this can be made high-performance by setting up that space as
Code Alignment
22621B/0—August 1999
3DNow!™ Instruction Porting
UCWC (non-cacheable write-combining), in which case the
Pentium II does write-combining and even bursting to that
AMD-K6-2 processors that predate the CXT core (CPUID less
than 588h) do not support a UCWC memory type, and they
neither perform write combining, nor do they burst to UC
memory. AMD-K6-2 processors with the CXT core (CPUID 588h
to 58Fh) and AMD-K6-III processors support write combining to
non-cacheable space, but they are not able to use burst
transfers when writing to non-cacheable memory areas.
Also, AMD-K6-2 processors predating the CXT core do not
pipeline writes to non-cacheable space well. This can create a
bottleneck when a lot of data needs to be transferred to the
graphic s card, which for 3D g raphics drivers happens
predominantly in texture download and triangle download
code. (These two can cover about 99% of all writes.) Therefore,
for good performance with the millions of existing AMD-K6-2
processors and even for AMD-K6-2 processors with the CXT
core and AMD-K6-III processors, software needs to organize the
PCI or AGP wr it es ca ref ully t o achieve around a 2 0%
performance gain in the process.
This technique is called software write combining. The basic
technique is to collect all writes to non-cacheable space into
aligned QWORDs as much as possible. This is accomplished by
using an MMX register as a write buffer and collecting DWORD
writes using PUNPCK. Then store data out using aligned MOVQ
stores. The following two basic approaches can align the
QWORD writes:
1. If there is a NOP command consisting of a single DWORD,
which takes no processing time in the graphics chip, issue
the NOP command if the buffer pointer is not QWORD
aligned, then continue writing out QWORDs. This works if
the DWORDs in the command buffer are at least DWORD
aligned. It has the drawback of wasting some bandwidth for
the NOP commands.
2. We can split the code into two code streams. If the buffer
pointer is not QWORD aligned, take path one and write the
first chunk as a DWORD, then continue writing QWORD. If
the buffer pointer is aligned, take path two and start
writing out QWORDs immediately.
Software Write Combining
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In both cases there is the end case where we need to flush the
write buffer (MMX register) at the end of the write loop.
Option 2 is recommended for highest possible performance, but
option 1 is often easier to implement and often provides similar
Fo r A M D -K 6 -2 p ro c e s s o rs w i t h t h e C X T c o re a n d fo r
AMD-K6-III processors, use both software write combining and
enable the hardware write-combining features of these
Addressing Modes on the AMD-K6®-2 and AMD-K6®-III
The addressing modes listed below are sub-optimal for all
instructions. They degrade short-decoded instructions to vector
decode (degrade to long-decode in the case of 3DNow!
instructions). This is due to the lack of on-the-fly corrections to
the instruction length that is computed during predecode.
16-bit addressing: [SI], [SI+disp8], [SI+disp16], [DI]
32-bit addressing: [ESI]
The following addressing modes are sub-optimal for all
instructions with 0Fh prefix (including all MMX/3DNow!
instructions). Again, it degrades short-decoded instructions to
vector (long decode in the case of the 3DNow! instruction set).
This is due to the inability to determine the instruction length
from the first three bytes (0F-prefix, opcode, ModR/M). Note:
This category has been eliminated in AMD-K6-2 processors with
the CXT core and AMD-K6-III processor. However millions of
existing AMD-K6-2 processors are affected by this issue, so it is
highly recommended to avoid these addressing modes.
1. ModR/M = 00_xxx_100b is the only ModR/M encoding that
requires the SIB value to determine instruction length. For
this ModR/M, the processor doesn’t know whether there is a
disp32 or not until it looks at the SIB (which predecode
cannot do in the case of MMX/3DNow!). For ModR/M =
01_xxx_100b there is always a disp8, and for ModR/M =
Addressing Modes on the AMD-K6®-2 and AMD-K6®-III Processors
3DNow!™ Instruction Porting
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10_xxx_100b there is always a disp32, so the length can be
determined from looking at ModR/M without looking at the
SIB byte.
2. This ModR/M encoding is encountered with the following
source-level addressing modes:
■ [base+scale×index]
■ [scale×index+disp]
■ [scale×index]
■ [base+index]
The following example demonstrates the ModR/M byte and SIB
byte resulting from several addressing modes; note that the
MOV instruction is not affected by the issue described here.
sib disp
D5 00000000
Note that the third mode is actually identical to the second as
far as the actual encoding is concerned (basically it’s encoded
as [scale×index+0]).
Also, there is a length restriction. Any instruction longer than
seven bytes cannot be short decoded. For MMX instructions,
avoid addressing modes with SIB and 32-bit displacement. For
3DNow! instructions, avoid all addressing modes with 32-bit
Addressing Modes on the AMD-K6®-2 and AMD-K6®-III Processors