Loop Unrolling & Predication

Loop Unrolling&

Predication

CSE 820

Michigan State UniversityComputer Science and Engineering

Software Pipelining

With software pipelining a reorganized loop contains instructions from different iterations of the original loop.

Sometimes called symbolic loop unrolling.


Software Pipelined Loop


Unrolled Loopselect subset of each iteration (bold)

Iteration 1: L.D F0,0 (R1) ADD.D F4, F0, F2 S.D F4, 0 (R1)




Software Pipelining

Loop: S.D F4, 16 (R1); stores into M[i] ADD.D F4, F0, F2 ; adds to M[i-1] L.D F0,0 (R1) ; loads M[i-2] DADDUI R1, R1, # -8 BNE R1, R2, Loop

Requires start-up and clean-up.


Symbolic Loop Unrolling

Software pipelining can be thought of as symbolic loop unrolling, but has the advantage of generating less code.


Software Pipelining has less overhead


Global Code Scheduling

allows moving instructions across branches

Most techniques concentrate on determining aStraight-line code segment representing

the most frequently executed code


Trace Scheduling

Concept1. Guess the likely path through branches

(called the trace)

2. Trace now contains long stretches of code without taken branches (predicted)

3. Schedule the trace allowing movement across branches

• Add code to off-the-trace to undo the effects of movement

• The increased ability to move across branches should improve scheduling


Movement + Undo

Considerif (cond)

then { x=x + 5; // likely }else // unlikely

After Movementx = x + 5;if (cond)

then { // likely}else { x = x – 5; // unlikely} // undo


Select a trace


Trace showing jumps off the trace


Superblocks

Avoid the multiple entry and exits of traces.

Superblock has one entry and multiple exits which makes scheduling easier.

The one-entry-multiple-exit is achieved by duplicating code where the unlikely path exits the trace so that no reentry is needed.


Superblock: one entry and multiple exits


Predicated Instructions

Requires– Hardware– ISA modification

Predicated instructions eliminate branches, converting a control dependence into a data dependence.

IA-64 has predicated instructions, but many existing ISA contain at least one(the conditional move).


Conditional Moveif (R1 == 0) R2 = R3;

Branch: BNEZ R1,L ADDU R2, R3, R0L:

Conditional Move: CMOVZ R2, R3, R1

In a pipeline, the control dependence at the beginning of the pipeline is transformed into a data dependence at the end of the pipeline.


Full Predication

Every instruction has a predicate:if the predicate is false, it becomes a NOP.

It is particularly useful for global scheduling since non-loop branches can be eliminated: the harder ones to schedule.


Exceptions & Predication

A predicated instruction must not be allowed to generate an exception,if the predicate is false.


Implementation

Although predicated instructions can be annulled early in the pipeline, annulling during commit delays annulment until later so data hazards have an opportunity to be resolved.

The disadvantage is that resources such as functional units and registers (rename or other) are used.


Predication is good for…

• Short alternative control flow

• Eliminating some unpredictable branches

• Reducing the overhead of global scheduling

But the precise rules for compilation are still being determined.


Limitations

• Annulled instructions waste resources: registers, functional units, cache & memory bandwidth

• If predicate condition cannot be separated from the instruction, a branch might have had better performance, if it could have been accurately predicted.


Limitations (con’t)

• Predication across multiple branches can complicate control and is undesirable unless hardware supports it (as in IA-64).

• Predicated instructions may have a speed penalty—not the case when all instructions are predicated.


Example

if (A==0) A=B; else A= A+4;

LD R1,0(R3) ;load ABNEZ R1,L1 ;test ALD R1,0(R2) ;then clauseJ L2 ;skip else

L1: DADDI R1,R1,#4 ;else clause

L2: SD R1,0(R3) ;store A


Hoist Load

if (A==0) A=B; else A= A+4;LD R1,0(R3) ;load ALD R14,0(R2) ;speculative load

B BEQZ R1,L3 ;other branch of ifDADDI R14,R1,#4 ;else clause

L3: SD R14,0(R3) ;store A

What if speculative load raises an exception?


Guardif (A==0) A=B; else A= A+4;

LD R1,0(R3) ;load AsLD R14,0(R2) ;speculative loadBNEZ R1,L1 ;test ASPECCK 0(R2) ;speculative

checkJ L2 ;skip else

L1: DADDI R14,R1,#4 ;else clauseL2: SD R14,0(R3) ;store A

sLD does not raise certain exceptions; leaves them for SPECCK (IA-64).


Other exception techniques

• Poison bit: – applied to destination register. – set upon exception– raise exception upon access to poisoned

register.


Hoist Load above Store

If memory addresses are known, a load can be hoisted above a store.

If not, …add a special instruction to check addresses before the loaded value is used.(It is similar to SPECCK shown earlier: IA-64)


Speculation: soft vs. hard

• must be able to disambiguate memory(to hoist loads past stores), but at compile time information is insufficient

• hardware works best when control flow is unpredictable and when hardware branch prediction is superior

• exception handling is easier in hardware• trace techniques require compensation code• compilers see further for better scheduling


IA-64

Loop Unrolling & Predication

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