EE-375: Migrating Legacy SHARC to ADSP-SC58x/2158x SHARC+ Processors (Rev. 1) PDF

Engineer-to-Engineer Note
EE-375
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Migrating Legacy SHARC to ADSP-SC58x/2158x SHARC+ Processors
Contributed by Robert Hoffmann
Rev 1 – June 11, 2015
Introduction
Analog Devices recently introduced the next generation of floating-point SHARC® devices as the followon to the ADSP-214xx series. The ADSP-2158x model is the first dual-core SHARC product, and the
ADSP-SC58x has the dual-core SHARC architecture plus an integrated ARM Cortex A5 core. Combined,
these new products offer flexible System-on Chip (SoC) architectures to address a wide range of application
requirements. Notably, the SHARC cores resident on these products have been upgraded from the previous
generation SHARC products, now featuring an 11-stage pipeline and an ARM-friendly chip infrastructure
(system fabric) that allows flexibility in connections with various peripherals.
The new 11-stage pipeline been defined in such a way that it remains backwards-compatible at the assembly
code level with previous generations of SHARC devices. However, due to some phenomena like pipeline
stage splitting, stack dependencies, data hazards, and stall conditions, some corner case combinations of
code flow will be handled differently from the previous design, which may result in performance
degradation if not modified to work well with the new pipeline.
This EE-note will describe these conditions, provide benchmarks and definitions to the known pipeline
stalls, and provide examples of how to go about optimizing assembly code going from the 5-stage pipeline
of the ADSP-214xx (and older) SHARC processors (hereon referred to as SHARC) to the deeper 11-stage
pipeline of these ADSP-2158x/ADSP-SC58x processors (hereon referred to as SHARC+).
Legacy Core with 5-Stage Pipeline (SHARC)
The SHARC system is shown in Figure 1. It illustrates the core surrounded by its L1 memory blocks
(RAM/ROM) and its connectivity buses for core and I/O operations. The two master and two slave ports
communicate with the system infrastructure to exchange data with peripherals. This architecture applies to
almost all SHARC product models that include an external port (e.g., all ADSP-2136x and ADSP-214xx
SHARC products). One master/slave pair is assigned to a peripheral, and another is assigned to the external
port.
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Figure 1. Legacy SHARC Processor with SHARC Core SoC Block Diagram
New Core with 11-Stage Pipeline (SHARC+)
The SHARC+ system (Figure 2) illustrates the core surrounded by its L1 memory blocks (RAM, but no
ROM) and its connectivity buses for core and I/O operations. The two master and two slave ports
communicate with the system infrastructure to exchange data with peripherals. This architecture applies to
all ADSP-SC58x/ADSP2158x SHARC+ products. The master/slave pair is now assigned to any peripheral
via the system infrastructure. Similar to the SHARC implementation above, the system still supports two
memory-mapped slave ports, where one may be assigned to the external port while the other is assigned to
the 2nd SHARC+ coprocessor (refer to the product datasheet[1] for the address map of the slave ports).

The physical memory map of the L1 memory blocks has not changed from
the previous ADSP-214xx processors and can be directly derived from LDF
files for ADSP-214xx-based projects.
Cross SHARC+ Core Communication
The two master/slave ports allow high speed communication between the two SHARC+ cores. The slave
ports handle exchange of 64-bit single-instruction-multiple-data (SIMD) as serialized 2 x 32-bit transfers
before writing into L1 memory. For memory DMA (MDMA), the master drives 32-bit data at the core clock
speed over its master port into the system fabric, and the data is synchronized into 64-bit transfers at
SYSCLK speed (1/2 core clock). This data is then captured at any slave port by doing the inverse
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synchronization and writing into L1 destination memory. Both cores can exchange data in full-duplex mode
and can freely change between core and MDMA accesses.
Figure 2. New SHARC Processor with SHARC+ Core SoC Block Diagram
Core Register File Register Changes
Diving down into the cores, the first difference worth describing is with respect to the core registers
themselves. There are two groups of registers classified - register file registers accessed directly via an
instruction:
bit set mode1 PEYEN;
/* PEYEN bit set in MODE1 Register */
and the core memory-mapped registers accessed via addressing instructions:
dm(SYSCTL)=USTAT1;
/* USTAT1 is written via address to SYSCTL */
Table 1 provides the information regarding differences between the new SHARC+ implementation versus
the previous SHARC implementation.
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Register
Update
Reset Setting
Read Latency
Effect Latency
SIMD Pair
LW Pair
R/S0-15
No change
Undefined
No change
No change
Rn-Sn
R[n,n+1]
S[n,n+1]
MRF/B
MSF/B
No change
Undefined
No change
No change
MRF-MSF
MRB-MSB
N/A
DAG
I/B/M/L
No change
0x0
No change
No change
N/A
I/B/M/L
[n,n+1]
USTAT
No change
0x0
No change
No change
N/A
U [n,n+1]
PX
No change
Undefined
No change
No change
N/A
PX
PX1,PX2
No change
Undefined
No change
No change
PX1-PX2
N/A
ASTATx/y
Exclusive access response
ORed with EQ flag
?
No change
No change
ASTATxASTATy
N/A
STKYx/y
No change
0x540_0000
No change
No change
ASTATxASTATy
N/A
MODE1
Bits [29,28,27]: new Parity
enable
Bit 30: Nested interrupt in
progress
Bit 31: “Exclusive PEx/PEy
in place of FLAG2
condition”
0x0
No change
No change
N/A
N/A
MODE1STK
New UREG
0x0
0
2
N/A
N/A
MODE2
Bit [0-2] IRQ-012 bits
removed
Bit 0: Reused for Selfnesting enable
Bit 3: EXTCADIS removed
Bit 7: New SLOWLOOP
0x540_0000
No change
No change
N/A
N/A
IRPTL
IVT changed
0x0
No change
No change
N/A
N/A
IMASK
IVT changed
0x3
No change
No change
N/A
N/A
IMASKP
IVT changed
0x0
No change
No change
N/A
N/A
MMASK
No change
0x20_0000
No change
No change
N/A
N/A
LIRPTL
Removed
PC
No change
Undefined
No change
No change
N/A
N/A
PCSTK
No change
0x7FFF_FFFF
No change
No change
N/A
N/A
PCSTKP
No change
0x0
No change
No change
N/A
N/A
LADDR
No change
0xFFFF_FFFF
No change
No change
N/A
N/A
LCNTR
No change
0x0
No change
No change
N/A
N/A
TPERIOD
No change
Undefined
No change
No change
N/A
N/A
TCOUNT
No change
Undefined
No change
No change
N/A
N/A
FLAG0-2
No change
Output selects 0x0
No change
No change
N/A
N/A
FADDR
Removed - now CMMR
DADDR
Removed - now CMMR
Table 1. SHARC+ Core Register Changes vs SHARC
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For specific details regarding these core MMRs, consult the ADSP-SC58x Programming Reference
Manual[2].
Memory-Mapped Register (MMR) Access Latencies
Core MMR (CMMR) accesses cause a variable number of stalls due to timing and effect latency reasons.
Stalls on reads and writes can be up to 4 cycles.
System MMR (SMMR) accesses (to various peripheral control/status registers) take between 38-60 core
cycles, depending on the system clock frequency (SYSCLK), the peripheral module clock, and the system
fabric arbitration.
Conflict Cache
The tiny (32-entry) conflict cache in the SHARC core continues to operate in the new SHARC+ core;
however, entries are cached only when internal bus conflicts are encountered. This is unlike the
implementation on ADSP-214xx products and earlier, where the conflict cache was also filled from external
ISA/VISA instruction and data memory spaces. As the new ADSP-SC58x/2158x devices feature dedicated
program and data cache memories, the conflict cache no longer needs to care for this.
SHARC+ Instruction Pipeline
The eleven stages of the SHARC+ pipeline were created by splitting each of the SHARC pipeline stages
into two. After splitting the floating point ALU/MAC unit into two stages, they were positioned at the 10th
stage of the pipe instead of the 9th, which created the 11th stage of the pipe. The benefit here is that this 1stage shift ensures no load-to-use stall in the SHARC+ core.
In the entire pipeline, memory is accessed two times (phases in red in Figure 3) - once for instructions and
once for data. There can be up to two data accesses, using either the PM or DM buses. These accesses can
happen simultaneously if they are to both blocks, otherwise they cause a one-cycle stall to resolve each
block conflict in exactly the same manner as with the SHARC core. Refer to the ADSP-SC58x Programming
Reference Manual for pipeline stage descriptions.
11
e2
10
m4
9
m3
8
m2
7
m1
6
d2
5
d1
4
f4
3
f3
2
f2
1
f1
Ex2
Mem4/Ex1
Mem3
Mem2
Mem1/COF
DAG
Dec/DAG
Fetch4
Fetch3
Fetch2
Fetch1
Target
Figure 3. Pipeline - Memory Access Phases
In the above, the pipeline stages can be defined further, as follows:
Stages 1-4 (Fetch1-4):
Instruction fetch
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Stage 5 (Dec):
Instruction decode
Stage 6 (DAG):
Instruction address + arithmetic (I+M)
Stages 7-10 (Mem1-4):
Data access
COF:
Change of flow (interrupts, jumps, calls, returns)
Target:
COF target
Stages 10-11 (Ex1-2):
Compute
Instruction Pipeline Flushes and Stalls
The processors use pipeline flushes and stalls to ensure correct and efficient program execution. Since the
instruction pipeline is fully interlocked, programmers need to be aware of different scenarios which result
in pipeline stages being either flushed or stalled.
Pipeline stalls are used in the following situations:




Structural hazards are incurred when different instructions at various stages of the instruction pipeline
attempt to use the same processor resources simultaneously. For example, when the processor issues a
data access on the PM bus, it conflicts with an instruction request being issued by the sequencer on the
same bus.
Data and control hazards are incurred when an instruction attempts to read a value from a register or
from a conditional flag that has been updated by an earlier instruction before the value becomes
available. For example, an index register is changed during data address generation, which happens in an
early stage of the pipeline, which causes a stall when the same index register is being loaded in a previous
instruction.
Performance execution for a certain sequence of instructions: For example, when both the input operands
are forwarded to the multiplier unit from a previous compute instruction, it causes stalls to accommodate
additional operation.
Additional stalls include those to retain effect latencies compatible with earlier SHARC processors and
stalls on double-precision computations.
Pipeline flushes are used when the processor branches to a new location, whether this is due to an interrupt
or execution of a branch instruction (jump, call, return). When these are encountered, the pipeline flushes
the instructions behind them in the instruction flow.
The following sections describe various scenarios of stalls and pipeline flushes in more detail.
Pipeline Stalls
Stalls are usually introduced into complex systems because data to be used in the next cycle is not available
because of crossing a clock domain, performing a change in instruction flow, or pending on a semaphore in
a multi-core system.
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Memory Access Stalls
This section describes stalls related to the sequencer making accesses to instruction and data memory. Table
2 summarizes the memory access stalls.
Details
Example
Conflict Cache Miss on PM Data Access
r0 = pm(Addr);
Structural
r0 = dm(Block0-addr1),
Structural
Two Accesses to Same Bank in Same Cycle
background DMA to Block0
If eq DM(A) = Fz;
Conditional Store to Any Load
Stall Type
SHARC
SHARC+
1
1
1
1
0
1
Timing
Fa = DM(A/B)
Table 2. Memory Access Stalls
As can be seen in Table 2, using different memory blocks will avoid the stall introduced when having two
accesses to the same bank. Similarly, the conditional store stall could be avoided by issuing any other
instruction between the conditional store and the subsequent load.
Compute Stalls
This section describes stalls related to computational instruction sequences that depend on conditions or
data forwarding. Table 3 summarizes the compute stalls.
Details
1
Example
Stall Type
SHARC
SHARC+
0
1
0
1
1
2
1
0
Data Forwarding to Compute Operation
Floating point compute/multiply operation –to- next
compute dependency
Fx = PASS Fy;
Data dependency
Fz = Fx + Fa;
Rx = PASS Ry;
If previous instruction is conditional fixed point
compute or conditional register read and condition set is
just before
IF eq Rz = Ra + Rb;
ASTATx/y register update –to- carry or overflow
dependent instruction
ASTATx = DM(..);
2
Fc = Fz + Fd;
Rx = Ry + Rz + CI;
Dual Forwarding to Multiplier
Fx = Fa+Fb, Fy = Fa–Fb;
Dual forwarding to multiply operation from N-2 to Nth
location.
Dual forwarding to multiply operation from N-1 to Nth
location
3
Floating point multiply operation to next fixed
point ALU
Timing
[unrelated instruction];
Fz = Fx * Fy;
Fx = Fa+Fb, Fy = Fa–Fb;
Fz = Fx * Fy;
Fz = Fx * Fy;
Ra = Rz + Rb;
Table 3. Compute Stalls
As was the case with the memory access stalls in the previous section, these compute stalls can be avoided
by inserting unrelated instructions that do not affect the registers of interest into the identified sequence.
Listing 1 illustrates some compute-to-compute hazards.
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.SECTION/SW seg_swco;
/* VISA instructions */
R4 = R1*R4(SSFR);
R14 = R4*R12;
nop;
nop;
R4 = R1*R4(SSFR), R12 = R8-R12;
nop;
R14 = R4*R12;
nop;
nop;
R4 = R1*R4(SSFR), R12 = R8-R12;
R14 = R4*R12;
nop;
nop;
/* single forwarding Fixed-Point */
/* SHARC = 0 stalls / SHARC+ = 1 stall */
F4 = F1*F4;
F14 = F4*F12;
nop;
nop;
F4 = F1*F4, F12 = F8-F12;
nop;
F14 = F4*F12;
nop;
nop;
/* single forwarding Floating-Point */
/* SHARC = 0 stalls / SHARC+ = 1 stall */
F4 = F1*F4, F12 = F8-F12;
F14 = F4*F12;
nop;
nop;
/* dual forwarding Floating-Point */
/* SHARC = 1 stall / SHARC+ = 2 stalls */
/* dual forwarding Fixed-Point */
/* SHARC = 0 stalls / SHARC+ = 1 stall */
/* dual forwarding Fixed-Point */
/* SHARC = 1 stall / SHARC+ = 2 stalls */
/* dual forwarding Floating-Point */
/* SHARC = 0 stalls / SHARC+ = 1 stall */
Listing 1. Compute-Compute Hazards
Data Address Generation (DAG) Stalls
This section describes stalls related to data address generation used to access on- and off-chip data memory.
Table 4 summarizes the DAG stalls.
Details
1
Unconditional DAG register load-touse
Stall Type
Example
Ix = DM(..);
SHARC
SHARC+*
Data Dependency
DM(Ix…) = …;
2
4
2
5
2
5
Rx = PASS Ry;
2
Conditional DAG register load (with
condition set just before) -to- use
3
Condition set -to- conditional post
modify DAG operation on Ix -to- any
DAG operation on same Ix
IF eq
DM(..);
Ix
=
DM(Ix…) = …;
Rx = PASS Ry;
IF eq DM(Ix,..);
DM(Ix…) = …;
Load of DAG register with immediate
value –to- use
Ix = [IMM VALUE];
2
0
4
DM(Ix..)..
* One stall cycle is added if the condition set happens through write to ASTATx and if the register load is used with sign extension modifier
Table 4. DAG Stalls
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Again, as was the case in the previous sections, these stalls can be mitigated by inserting unrelated
instructions that do not affect the registers of interest into the identified sequence. Listing 2 illustrates a
DAG register load hazard.
.SECTION/SW seg_swco;
m0 = L1_memory;
r0 = dm(i2,m0);
nop;
nop;
m0 = 2;
r0 = dm(i0,m1);
nop;
nop;
i4 = L1_memory;
r0 = dm(i4,m4);
nop;
nop;
b0 = L1_memory;
r0 = dm(i0,m1);
nop;
nop;
l0 = 16;
r0 = dm(i0,m4);
/* VISA instructions */
/* SHARC = 2 stalls / SHARC+ = 0 stalls */
/* SHARC = 0 stalls / SHARC+ = 2 stalls */
/* SHARC = 2 stalls / SHARC+ = 4 stalls */
/* SHARC = 2 stalls / SHARC+ = 2 stalls */
/* SHARC = 2 stalls / SHARC+ = 2 stalls */
Listing 2. DAG Register Load Hazards
Data Move Stalls
This section describes stalls that are introduced when data movement depends on instruction sequences.
Table 5 lists data move stalls.
Details
Example
Floating point compute or any multiplier
operation followed by move of the result to any
register outside the relevant PE
Condition set followed by a conditional load of a
DAG reg followed by move of that reg to any
other Ureg
F1 = F2 + F3;
USTAT1 = F1;
R0 = R1 + 1;
IF EQ I0 = PM(<Addr>);
USTAT1 = I0;
Stall Type
SHARC
SHARC+
0
1
0
1
Timing
Access of any Timer register
Read of these registers:
IRPTL, IMASKP, MODE1STK, LPSTK, CCNTR,
LCNTR, PCSTK, PCSTKP, MODE1, FLAGS,
ASTATx/y, STKYx/y, FADDR, DADDR
Write Followed by Read of these registers:
IMASK, USTAT, MMASK, MODE2
TCOUNT = USTAT1;
0
1
R0 = IRPTL;
USTAT1 = DM(<Addr>;
R0 = USTAT1;
0
1
0
1
Read and write of any core MMR
R0 = SYSCTL;
0-1
0-4
Table 5. Data Move Stalls
There is no way to avoid stalls associated with MMR accesses; however, the same concept as in previous
sections also applies here to stalls introduced by specific instruction sequences. Inserting unrelated
instructions that do not affect the registers of interest into the identified sequence will mitigate the stall
cycles that would otherwise be incurred.
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Miscellaneous Stalls
Table 6 lists miscellaneous stalls not covered in the previous sections.
Details
Example
Stall Type
SHARC
SHARC+
0
4-N
1-N
4-N
1
4
Target/next-to-target of CALL/RTS/RTI itself
being an RTS/RTI
0
3
Target of CALL/RTS/RTI itself being a Jump
0
1
0
5
1
5
1
0
During the execution of first four instructions of
an unrolled loop, when COF is at Nth position in
loop from top, where N = 0-3
Loop
machine
If RTS/RTI is returning to a loop at “Last-Addr”N, where N = 0-3
Data dependency
Jump with loop abort
Loop-stack
modification
RTS/RTI/Jump
Jump <Target> (LA);
followed
state
by
Sreg or SYSCTL update to N+2 instruction
Bit set MODE1 CBUFEN;
[Instr];
DM(I0…);
Bit set/clear MODE1 PEYEN to N+2 instruction
Bit set MODE1 PEYEN;
Control
dependency
Table 6. Miscellaneous Stalls/Flushes
Stalls and Pipeline Flushes Related to Instruction Flow
In most cases, any branch instruction (jumps, calls, returns, rframe, and cjump) flushes the pipeline, and
some cycles are lost. A new feature of the ADSP-2158x/ADSP-SC58x devices is the branch predictor,
which attempts to minimize the loss of cycles incurred as a result of branch instructions that cause a change
in instruction flow.
Branch Target Buffer (BTB)
With the deeper pipeline, changes of flow (COF) become more costly to the processor’s performance due
to the increased number of pipeline flushes and stalls. However, these stall and flush cycles can be mitigated
by employing the branch target buffer (BTB) condition predictor. COF works differently with and without
BTB, and this section provides details regarding BTB-enabled COF in a SHARC+ system.

The BTB is enabled after reset (similar to conflict cache). BTB content can
be disabled and frozen to test efficiency and to inspect for debug purposes.
BTB Condition Prediction
The BTB adds value to the SHARC+ pipeline, as all branch-related latencies and stalls are significantly
reduced with correct BTB prediction. A listing of the incurred stalls/flushes is shown in Table 7.
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Branch
Prediction
Condition
Prediction
Target Prediction
Stall/Flush Cycles
for Non-DB/DB
Conditional
Taken
HIT
HIT
2/0
Conditional
Not Taken
HIT
—
0
Conditional
Taken
MISS
—
11
Conditional
Not Taken
MISS
—
11/9
Conditional
Taken
HIT
MISS
6/4
Unconditional
—
—
HIT
2/0
Unconditional
—
—
MISS
6/4
Table 7. Branch Target Buffer Flushes/Stalls
BTB Prediction Masking
While the branch predictor provides some execution gains when it is enabled, BTB prediction can be masked
off in two ways. First, options like indirect branch prediction are configurable, so the user has the ability to
disable various aspects of the prediction logic via writes to configuration registers, and this is obviously at
the full discretion of the user. However, architectural limitations also influence the behavior of the hardware.
For example, hardware loops and BTB are incompatible because they interfere with each other’s state
machines. As such, when hardware loops are executing, the processor automatically masks BTB prediction
to ensure proper sequencing.
Table 8 lists places where BTB prediction is masked due to architectural reasons.
Cases Where BTB Prediction Is Masked
Reason
Two instructions after a branch is predicted
Basic functional requirement
Return to last 8 instructions of an F1 active loop
To prevent counter decrement
Return to last 3 instructions of an E2 active and arithmetic loop
Stack dependency
DO_UNTIL instruction from D1 to E2 stage
Stack dependency
RTI
Status/mode stack dependency
Branches which are placed within 5 instructions after loop-stack modification
Loop stack dependency
Table 8. Branch Target Buffer Disable Scenarios
Table 9 describes the number of lost cycles when BTB is disabled or the branch entry is not present in the
BTB (i.e., a BTB miss occurs). If the branch is the one with a delay slot of two instructions, the number of
flushed instructions is reduced by 2.
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Stall Type
#
Details
Example
1
Unconditional branch
Condition set for conditional
branch
Jump(My,Ix);
Rx = PASS Ry;
IF eq Jump(My,Ix);
2
SHARC
SHARC+
(Non-Delayed/Delayed)
(Non-Delayed/Delayed)
3/1
6/4**
4/2***
11/9*
Pipeline flush
Control dependency
* One additional stall cycle if condition set happens through write to ASTATx or ASTATy register
** As an exception, RTI (DB) and RTI cause a 7-cycle pipeline flush
*** Two-cycle stall for multiplier-generated conditions
Table 9. Stalls/Flushes for COF
When the BTB is enabled and there is a hit, the pipeline stalls and flushes are possible. These stalls are
described for various situations in Table 10.
SHARC+
#
Branch Type
Prediction
Condition
Prediction
1
Conditional
Taken
Correct
Correct
2
Conditional
Not Taken
Correct
—
0
3
Conditional
Taken
Incorrect
—
11
4
Conditional
Not Taken
Incorrect
—
11/9
5
Conditional
Taken
Correct
Incorrect
6/4
6
Unconditional
—
—
Correct
2/0
7
Unconditional
—
—
Incorrect
6/4
Target Prediction
(Non-Delayed/Delayed)
2/0
Table 10. Stalls/Flushes for COF with BTB
In addition to the above stalls, there are other data and control dependency stalls relative to branch
instructions. The cycles in Table 11 are in addition to the cycles incurred due to the reasons described in the
above tables in this section.
#
Details
Example
1
CJUMP/RFRAME –to- use of I6
CJUMP; DM(I6,..) = …;
2
CJUMP/RFRAME –to- read of I6/7
Unconditional DAG register load -to- use
in indirect branch
Conditional DAG register load (with
condition set just before) -to- use in
indirect jump
RFRAME; R0 = I6;
Ix = DM(..);
Jump(Ix…) = …;
Rx = PASS Ry;
IF eq Ix = DM(..);
Jump(Ix…) = …;
3
4
Stall Type
Data dependency
SHARC
SHARC+
2
6
2
6
2
4†
2
5*†
Data and control
dependencies
† One additional stall cycle if register load is used with sign extension modifier
* One additional stall cycle if condition set happens through write to ASTATx or ASTATy register
Table 11. Stalls/Flushes for COF Data/Control Dependency
Hardware Loop Stalls/Flushes
With SHARC+, the LCE condition governing loop lengths of 10 and under will not give the same result as
its SHARC predecessor. This is because of pipeline depth and because termination of loop lengths of 1 to
10 happens when the CCNTR value is 2 rather than 1. Restrictions for call and jump instructions inside a
loop are identical to those on SHARC.
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New restrictions due to the increased pipeline depth include:





The last five instructions of an arithmetic loop cannot be a delayed branch (dB).
The last three instructions of an arithmetic loop cannot be a branch.
Short loops of 2-4 instructions cannot contain two branches.
Short loops of 1-4 instructions carrying a nested loop cannot contain branches.
In short loops carrying a nested loop, 1/2 instructions before the inner do until instruction cannot be a
branch.
Table 12 shows the hardware loop stalls.
Details
Example
On termination of E2 active and short loops
On termination of arithmetic condition based
loops
LCNTR = 4, DO (PC,2) UNTIL LCE;
Write to CCNTR to LCE based instruction
DO (PC,2) UNTIL EQ;
CCNTR = 4;
If not LCE R0 = R1;
Start of 1,2,4 instruction loop
LCNTR = 4, DO (PC,2) UNTIL LCE;
Stall Type
Pipeline flush
SHARC
SHARC+
4
11
2
11
2
1
1
0
Timing
Table 12. Hardware Loop Stalls/Flushes
Assembly Code Optimization Example (FIR Filter)
With the stall conditions defined above, it is useful to describe methods that can be applied when taking
existing SHARC code and moving to the SHARC+ core. While the code itself will yield the same correct
results using the SHARC+ core as it did using the SHARC core, architectural differences between the two
cores may cause performance degradation when executed on the SHARC+ core. This section uses the FIR
filter library code that ADI provides with the development tools as an example to apply some pipelinespecific optimizations to.
Code/Data Buffer Placement
The linker description file (LDF) plays a significant role in throughput optimization when using
multifunction instructions with dual-data moves. The application should try to employ conflict-free
placement of the source and destination data buffers to avoid additional block arbitration latencies. The
SHARC’s Super Harvard architecture drives an internal conflict cache during loop iterations while DAG1
and DAG2 drive both data buses.
All the known rules for the SHARC memory architecture continue to apply to the SHARC+ architecture.
Consider the case of a dual data move instruction. During cycle a of the SHARC pipeline (Figure 4), the
pipeline posts addresses to instruction-assigned memory and data memory (assuming a hit in the conflict
cache). Figure 5 shows the address and data phases in the SHARC+ pipeline for the same dual data move
instruction. Note that there is another stage between the address and the data, which allows for higher-speed
accesses.
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Cycles
1
2
3
4
5
E
N (Data)
A
N (Addr)
N+1
N
N+1
N+2
N
N+1
N+2
N+3 (Data)
N+1
N+2
N+3 (Addr)
N+4
D
f2
f1
N
Figure 4: Dual Data Move Instruction in SHARC
Cycles
1
2
3
4
5
6
7
8
9
10
11
e2
N
m4
N
N+1
N
N+1
N+2
N (Data)
N+1
N+2
N+3
N
N+1
N+2
N+3
N+4
N (Addr)
N+1
N+2
N+3
N+4
N+5
N
N+1
N+2
N+3
N+4
N+5
N+6
N+1
N+2
N+3
N+4
N+5
N+6
N+7
m3
m2
m1
d2
d1
f4
N
N+5
f3
N
N+1
N+2
N+3
N+4
(Addr)
N+6
N+7
N+8
N
N+1
N+2
N+3
N+4
N+5
N+6
N+7
N+8
N+9
N+1
N+2
N+3
N+4
(Addr)
N+6
N+7
N+8
N+9
N+10
f2
N+5
f1
N
Figure 5: Dual Data Move Instruction in SHARC+
In the case of an application like an FIR filter, the code/data might be placed as follows:
Block 0:
VISA/ISA code and FIR coefficients
Block 1:
State memory (delay line)
Blocks 2/3:
Ping-Pong DMA data buffers
This layout guarantees conflict-free block accesses (core PM/DM bus vs. slave port1-2 DMA buses).

Core vs DMA block conflicts are more expensive based on the 2::1
CCLK::SYSCLK clock speed ratio. Refer to the ADSP-SC58x
Programming Reference Manual for the block arbitration priorities.
SHARC FIR Filter
When attempting to run SHARC FIR filter source code on a SHARC+ core target, it will function correctly.
However, as stated above, the new SHARC+ pipeline will add a mixture of static and dynamic stalls that
might negatively impact the performance. As described earlier in this EE-note, there are various reasons for
the sequencer to stall the instruction flow, with three of the more notable being:
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-
floating-point compute-to-compute hazards
loop sequencer stalls
conflict cache stalls
In both the single channel (SISD, Listing 3) and dual-channel (SIMD, Listing 4) listings below, the
SHARC+ core will stall in the loop because the sequencer identifies identical source and destination data
registers in consecutive 32-bit floating-point compute operations. As this stall is taken in the inner loop of
the FIR, it is extremely costly from a performance perspective (highlighted in red).

In all the source code provided below, a common header is used to differentiate
among the various implementations being shown. Please refer to it when comparing
between the SHARC core and SHARC+ core targets.
Single-Channel 40-Bit SISD FIR
.SECTION/SW seg_swco;
/* SHARC Pipelined 40-bit SISD Version */
bit set MODE1 CBUFEN;
bit clr MODE1 PEYEN;
b5 = Input_Buf;
m5 = 1;
l5 = 0;
b6 = Output_Buf;
l6 = 0;
b9 = FIR_Coeff_Buf;
m9 = 1;
l9 = TAPS;
b2 = FIR_State_Buf;
m2 = 1;
l2 = TAPS;
lcntr = BP_COUNT, do Block until lce;
r8 = r8-r8, f0 = dm(i5,m5);
r9 = r9-r9, dm(i2,m2) = f0;
MAC:
Block:
// VISA instructions
// FIR Input Buffer
// FIR Output Buffer
// i9 allows broadcast mode
// Delay Line Buf
// block processing
// clr acc, read new sample
// clr acc, write into DL
f0 = dm(i2,m2), f4 = pm(i9,m9);
f12 = f0*f4,
f0 = dm(i2,m2), f4 = pm(i9,m9);
lcntr = TAPS-2, do MAC until lce;
f12 = f0*f4, f8 = f8+f12, f0 = dm(i2,m2), f4 = pm(i9,m9); // 1-cycle SHARC+ stall
f12 = f0*f4, f8 = f8+f12;
f8 = f8+f12;
dm(i6,m5) = f8;
// write to output
Listing 3. SHARC SISD FIR
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Dual-Channel 32-Bit SIMD FIR
.SECTION/SW seg_swco;
//
/* SHARC Pipelined 32-Bit SIMD Version */
bit set MODE1 PEYEN|CBUFEN|BDCST9;
b5 = Input_Buf;
//
m5 = 2;
l5 = 0;
b6 = Output_Buf;
//
l6 = 0;
b9 = FIR_Coeff_Buf;
//
m9 = 1;
l9 = TAPS;
b2 = FIR_State_Buf;
//
m2 = 2;
l2 = 2*TAPS;
lcntr = BP_COUNT/2, do Block until lce;
//
r8 = r8-r8, f0 = dm(i5,m5);
//
r9 = r9-r9, dm(i2,m2) = f0;
//
f0 = dm(i2,m2), f4 = pm(i9,m9);
f12 = f0*f4,
f0 = dm(i2,m2), f4 = pm(i9,m9);
lcntr = TAPS-2, do MAC until lce;
MAC:
f12 = f0*f4, f8 = f8+f12, f0 = dm(i2,m2), f4 = pm(i9,m9); //
f12 = f0*f4, f8 = f8+f12;
f8 = f8+f12;
Block:
dm(i6,m5) = f8;
//
VISA instructions
FIR Input Buffer
FIR Output Buffer
i9 allows broadcast mode
Delay Line Buffer
block processing
clr acc, read new sample
clr acc, write into DL
1-cycle SHARC+ stall
write to output
Listing 4. SHARC SIMD FIR
Hand-Optimizing the Single-Channel FIR Filter in SHARC+
The basic idea for optimizing the code is to use the floating-point compute-to-compute hazard stall cycle
identified in Listing 3 to interleave an independent MAC operation. In other words, split the MAC into two
MAC operations with different result registers (MAC F8 and MAC F9) for accumulation, as shown in
Listing 5.
.SECTION/SW seg_swco;
/* SHARC Unpipelined 40-Bit SISD Version */
lcntr = BP_COUNT, do Block until lce;
r8 = r8-r8, f0 = dm(i5,m5);
r9 = r9-r9, dm(i2,m2) = f0;
MAC:
Block:
f0 = dm(i2,m2), f4
f12 = f0*f4;
f8 = f8+f12;
lcntr = TAPS/2, do
f0 = dm(i2,m2), f4
f12 = f0*f4;
f8 = f8+f12;
f0 = dm(i2,m2), f4
f13 = f0*f4;
f9 = f9+f13;
f10= f8+f9;
dm(i6,m5) = f10;
// VISA instructions
// block processing
// clr acc, read new sample
// clr acc, write into DL
= pm(i9,m9);
// MAC only for odd TAP count
MAC until lce;
= pm(i9,m9);
// MAC F8
= pm(i9,m9);
// MAC F9
// add MAC1-0
// write to output
Listing 5. SISD FIR Optimization 1
From a performance perspective, this is actually a step in the wrong direction, as cycles are being added to
split the MAC operations; however, doing so lends itself to further possible optimizations targeting the
SHARC+ pipeline performance. In Listing 6, the code is reordered to take full advantage of the parallel
Migrating Legacy SHARC to ADSP-SC58x/2158x SHARC+ Processors (EE-375)
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operation instructions, where data accesses are performed first, multiplication second, and accumulation
third; hence, three instruction cycles are required to fill the pipeline with data. When filled, the instruction
is executed in the HW loop body until the loop exits, at which point the pipeline needs to be flushed (which
takes another 3 cycles until finished). Note that this code sequence illustrates separate pipelining for the F8
and F9 result registers, controlled by two separate inner loops, which is the next opportunity for further
optimization.
.SECTION/SW seg_swco;
/* SHARC Pipelined 40-Bit SISD Version with Separate MAC */
lcntr = BP_COUNT, do Block until lce;
r8 = r8-r8, f0 = dm(i5,m5);
r9 = r9-r9, dm(i2,m2) = f0;
MAC0:
MAC1:
f12 =
lcntr
f12 =
f12 =
f13 =
lcntr
f13 =
f13 =
Block:
f0 = dm(i2,m2),
f0*f4,
f0 = dm(i2,m2),
= (TAPS-4)/2, do MAC0 until lce;
f0*f4, f8 = f8+f12, f0 = dm(i2,m2),
f0*f4, f8 = f8+f12;
f8 = f8+f12;
f0 = dm(i2,m2),
f0*f4,
f0 = dm(i2,m2),
= (TAPS-4)/2, do MAC1 until lce;
f0*f4, f9 = f9+f13, f0 = dm(i2,m2),
f0*f4, f9 = f9+f13;
f9 = f9+f13;
f10= f8+f9;
dm(i6,m5) = f10;
// VISA instructions
// block processing
// clr acc, read new sample
// clr acc, write into DL
f4 = pm(i9,m9);
f4 = pm(i9,m9);
f4 = pm(i9,m9);
f4 = pm(i9,m9);
f4 = pm(i9,m9);
f4 = pm(i9,m9);
// MAC f8+f9
// write to output
Listing 6. SISD Optimization 2
As a final step, the MAC operations can be interleaved such that the now singular inner loop body contains
just two instructions. Keeping the same red/green color encoding to differentiate between the F8 and F9
MACs, Listing 7 depicts this modification.
.SECTION/SW seg_swco;
// VISA instructions
/* SHARC+ Pipelined 40-Bit SISD Version with Interleaved MAC */
lcntr = BP_COUNT, do Block until lce;
// block processing
r8 = r8-r8, f0 = dm(i5,m5);
// clr acc, read new sample
r9 = r9-r9, dm(i2,m2) = f0;
// clr acc, write into DL
MAC:
Block:
f12 =
f13 =
lcntr
f12 =
f13 =
f12 =
f13 =
f0 = dm(i2,m2),
f0*f4,
f0 = dm(i2,m2),
f0*f4,
f0 = dm(i2,m2),
= (TAPS-4)/2, do MAC until lce;
f0*f4, f8 = f8+f12, f0 = dm(i2,m2),
f0*f4, f9 = f9+f13, f0 = dm(i2,m2),
f0*f4, f8 = f8+f12, f0 = dm(i2,m2),
f0*f4, f9 = f9+f13;
f8 = f8+f12;
f9 = f9+f13;
f10= f8+f9;
dm(i6,m5) = f10;
f4 = pm(i9,m9);
f4 = pm(i9,m9);
f4 = pm(i9,m9);
f4 = pm(i9,m9);
f4 = pm(i9,m9);
f4 = pm(i9,m9);
// MAC f8+f9
// write to output
Listing 7. SISD Optimization 3
The loop execution ensures the desired source and destination registers are sequenced such that consecutive
reads and writes of the F12 and F13 registers never occur on consecutive cycles, which results in the
compute-to-compute stall penalty being removed. In the loop epilogue, one instruction must be added to
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sum the two MAC result registers to get the correct result stored to the F10 register to be written to the
output buffer, and the FIR is now fully optimized for the SHARC+ pipeline!

As an addendum to this exercise, the FIR filter design criteria may require that the
TAP count be odd. In that case, one additional multi-function instruction must be
added into the loop prologue to account for it, as shown in red in Listing 8.
.SECTION/SW seg_swco;
/* Pipelined SISD Version with Interleaved MAC */
lcntr = BP_COUNT, do Block until lce;
r8 = r8-r8, f0 = dm(i5,m5);
r9 = r9-r9, dm(i2,m2) = f0;
MAC:
Block:
f12 =
f13 =
f12 =
lcntr
f12 =
f13 =
f12 =
f13 =
f0 = dm(i2,m2),
f0*f4,
f0 = dm(i2,m2),
f0*f4,
f0 = dm(i2,m2),
f0*f4, f8 = f8+f12, f0 = dm(i2,m2),
= (TAPS-4)/2, do MAC until lce;
f0*f4, f8 = f8+f12, f0 = dm(i2,m2),
f0*f4, f9 = f9+f13, f0 = dm(i2,m2),
f0*f4, f8 = f8+f12, f0 = dm(i2,m2),
f0*f4, f9 = f9+f13;
f8 = f8+f12;
f9 = f9+f13;
f10= f8+f9;
dm(i6,m5) = f10;
// VISA instructions
// block processing
// clr acc, read new sample
// clr acc, write into DL
f4
f4
f4
f4
=
=
=
=
pm(i9,m9);
pm(i9,m9);
pm(i9,m9);
pm(i9,m9); // only odd TAP count
/* for odd loop (TAPS-5)/2 */
f4 = pm(i9,m9);
f4 = pm(i9,m9);
f4 = pm(i9,m9);
// MAC f8+f9
// write to output
Listing 8. SISD Optimization 3 with Odd Loop Count
Dual-Channel SHARC+ SIMD FIR Filter
Given the SISD example above, moving from single-channel to dual-channel SIMD source code is fairly
straightforward, though there are a few differences that need to be discussed.
First, the total loop count is half that of the SISD implementation, as the workload in the SIMD model is
distributed between the PEX and PEY processing units. Second, as a result of the data being fetched and
forwarded to the two processing units in parallel, the data access modifier (DAG stride) is set to two in the
SIMD version rather than 1, as was the case in the SISD version (Listing 9).
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.SECTION/SW seg_swco;
// VISA instructions
/* SHARC+ Pipelined 32-Bit SIMD Version with Interleaved MAC */
bit set MODE1 PEYEN|CBUFEN|BDCST9;
b5 = Input_Buf;
// FIR Input Buffer
m5 = 2;
// SIMD Stride is 2
l5 = 0;
b6 = Output_Buf;
// FIR Output Buffer
l6 = 0;
b9 = FIR_Coeff_Buf;
// i9 broadcast mode
m9 = 1;
l9 = TAPS;
b2 = FIR_State_Buf;
// Delay Line Buffer
m2 = 2;
// SIMD Stride is 2
l2 = 2*TAPS;
MAC:
Block:
lcntr = BP_COUNT/2, do Block until lce;
// SIMD block processing
r8 = r8-r8, f0 = dm(i5,m5);
// clr acc, read new sample
r9 = r9-r9, dm(i2,m2) = f0;
// clr acc, write into DL
f0 = dm(i2,m2), f4 = pm(i9,m9);
f12 = f0*f4,
f0 = dm(i2,m2), f4 = pm(i9,m9);
f13 = f0*f4,
f0 = dm(i2,m2), f4 = pm(i9,m9);
f12 = f0*f4, f8 = f8+f12, f0 = dm(i2,m2), f4 = pm(i9,m9);
// only odd loop count
lcntr = (TAPS-4)/2, do MAC until lce;
/* for odd loop (TAPS-5)/2 */
f12 = f0*f4, f8 = f8+f12, f0 = dm(i2,m2), f4 = pm(i9,m9);
f13 = f0*f4, f9 = f9+f13, f0 = dm(i2,m2), f4 = pm(i9,m9);
f12 = f0*f4, f8 = f8+f12, f0 = dm(i2,m2), f4 = pm(i9,m9);
f13 = f0*f4, f9 = f9+f13;
f8 = f8+f12;
f9 = f9+f13;
f10= f8+f9;
// add MAC1-0
dm(i6,m5) = f10;
// write to output
Listing 9. SIMD Optimization (Including Odd Tap Count)
As was the case in the SISD implementation, note also that explicit and implicit DAG addressing is
interleaved within the multi-function instructions (the I2/M2 register pair is used on the DM bus access, and
the I9/M9 pair is used on the PM bus access). Finally, keep in mind that dual-channel processing depends
on the broadcast mode.
Filter Broadcast Mode
Broadcast mode is useful if the same filter kernel is processed by both channels (PEX and PEY). In this
case, the length of the coefficient buffer is exactly the number of taps (identical to SISD mode), and the
circular pointer increment stride is 1 regardless of SIMD mode.
Filter No Broadcast Mode
With broadcast mode disabled (MODE1.BDCST9 bit is cleared), the length of the coefficient buffer is twice
the number of taps (one coefficient for each channel), and the PEX and PEY coefficients are interleaved in
a single buffer in memory. If the tap count for each channel is odd, then the total count becomes even. In
this case, the circular pointer increment is 2 (SIMD mode).
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FIR Benchmark Summary - SHARC vs SHARC+
The following section summarizes floating-point benchmarks, measured in core clock (CCLK) cycles,
obtained when running various versions of the FIR source code referenced throughout this EE-note on target
devices representing each of the SHARC (ADSP-21469) and SHARC+ (ADSP-SC58x) core technologies.
The SHARC FIR Code is the DSP library code furnished by Analog Devices, which was developed and
optimized for the legacy SHARC processor architecture and has been a staple offering in the VisualDSP++®
and CrossCore® Embedded Studio development tools platforms for a number of years. The SHARC+ FIR
Code is the fully optimized example from this note, which is part of the new DSP libraries supporting the
new SHARC+ architecture.
In each of the tables provided, a 255-tap FIR was executed for a sample size of 256. Each algorithm was
run on both architectures, such that a comprehensive analysis between the two architectures can be made.
32-bit Standard Precision (SIMD Implementation)
For the SIMD implementation, Table 13 shows the relevant benchmark data.
Dual-Channel (SIMD)
32-Bit Precision
SHARC Target
(ADSP-21469)
SHARC+ Target
(ADSP-SC589)
SHARC FIR Code
33,560
65,950
SHARC+ FIR Code
33,820
34,090
Table 13. FIR Code SIMD Mode
As can be seen, the legacy ADSP-21469 SHARC code performs admirably with a baseline benchmark of
33,560 CCLK cycles; however, when the very same code is run on the SHARC+ core, it takes nearly twice
as many cycles (96.5% performance degradation). By fixing the consecutive compute-compute stalls
described in the previous section, the cycle count drops by 48.5% and is very nearly on par with the optimal
performance of the original FIR code running on the SHARC target (34,090 CCLK cycles vs 33,560 CCLK
cycles). If this optimized SHARC+ code is then run on a SHARC core, the benchmark is even closer by
comparison (now 34,090 CCLK cycles vs 33,820 CCLK cycles). This minor delta can be explained by the
added loop prologue/epilogue instructions and by effects of the new pipeline/loop controller.
40-bit Extended Precision (SISD Implementation)
By comparison, the 40-bit extended precision SISD implementation data is shown in Table 14 (SIMD mode
is not supported).
Single-Channel (SISD)
40-Bit Precision
SHARC Target
(ADSP-21469)
SHARC+ Target
(ADSP-SC589)
SHARC FIR Code
67,100
131,870
SHARC+ FIR Code
67,610
68,140
Table 14. FIR Code SISD Mode
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The data follows that depicted in the previous section for the 32-bit SIMD implementation in terms of
performance degradation of the original code between the two targets, the performance improvement on the
SHARC+ core after the recommended optimizations were made, and the performance of the SHARC+ code
when run on the original SHARC target.
Conclusion
There are many architectural differences between the legacy SHARC and the new SHARC+ core
technologies, which cause different behavior in terms of stall conditions that are transparent to the user.
Simply taking code from the old architecture to the new will work, but the performance might suffer due to
some of these differences relative to the longer pipeline. As an example, the FIR filter is a good reference
because it introduces one such stall (compute-to-compute) in the oft-executed inner loop of the algorithm.
Convolution is based on a two-deep nested loop, where the outer loop does the block accesses to/from the
system buffers (block processing vs. sample processing, system decisions with regards to latency, ISR
context savings, etc.) while the inner loop performs the filter convolution (tap filter kernel, filter design
requirements, etc.). Due to this structure, cycles can be traded between the loops, and the inner loop is
iterated with interleaved MAC operations to mitigate the compute-to-compute stall.
The overall performance highly depends on the total loop count (inner loop count x outer loop count). For
example, a system with a large filter kernel (e.g., 1024) does sample processing much faster than a 512sample block processing algorithm, even though both use the same optimized inner loop count.
References
[1] SHARC+ Core Dual Processor with ARM Cortex-A5 Data Sheet. Rev PrC, May 2015. Analog Devices, Inc.
[2] SHARC+ Core Programming Reference. Preliminary Revision 0.1. May 2015. Analog Devices, Inc.
[3] ADSP-SC58x Processor Hardware Reference. Preliminary Revision 0.2, June 2015. Analog Devices, Inc.
Document History
Revision
Description
Rev 1 – June 11, 2015
by Robert Hoffmann
Initial Release
Migrating Legacy SHARC to ADSP-SC58x/2158x SHARC+ Processors (EE-375)
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