peephole optimization - rwth aachen...

Peephole Optimization

Simon Oehrl and Bao-Loc Nguyen Ngo

21. Juli 2014

Simon Oehrl and Bao-Loc Nguyen Ngo | 21. Juli 2014 0/31



1 IntroductionContext and Motivation

2 Peephole OptimizationGeneralReplacement RulesPattern Matching

3 Demo4 Replacement Rules Derivation

Finding replacement RulesAutomatic rules derivationValidation of EquivalenceCost Comparison

5 ConclusionReferencesPeephole Optimization in LLVMExecution Test on other Intermediate LanguageBoolean Test

Introduction - Context and Motivation

Table of Contents


I Improve code quality and efficiencyI Increase PerformanceI Reduce code size

I Optimization on different abstraction levelsI High-level programming languageI Intermediate codeI Machine code

I Optimization techniquesI Local optimizationI Global optimizationI Peephole optimizationI . . .

Introduction - Context and Motivation

Code Optimization







Peephole Optimization - General

Table of Contents


Usage Context

I Used within modern compiler (e.g. GCC, LLVM, ACK, . . . )I Typically applied on intermediate or machine code

Optimization precedure

I Optimize code in a small moving window (peephole)I Iterative Optimazation









Peephole Optimization - Replacement Rules

Table of Contents


I Replace Sequence of Instruction with faster and/or fewer InstructionsI Many different Replacement Techniques


Replacement Rules


Null Sequences

I Remove useless instructions

ExampleLOV ALOC 0ADD


Common Techniques


Null Sequences

I Remove useless instructions

ExampleLOV A

→LOV A

LOC 0ADD


Common Techniques


Constant folding

I Replace constant expressions with its result

ExampleLOC 8LOC 2ADD


Common Techniques


Constant folding

I Replace constant expressions with its result

ExampleLOC 8

→LOC 10

LOC 2ADD


Common Techniques


Algebraic Laws

I Use algebraic laws to simplify expressions

ExampleLOV ALOC 7ADDLOC 5SUB(A+ 7)− 5


Common Techniques


Algebraic Laws

I Use algebraic laws to simplify expressions

ExampleLOV A

→

LOV ALOC 7 LOC 7ADD LOC 5LOC 5 SUBSUB ADD(A+ 7)− 5 = A+ (7− 5)


Common Techniques


Strength reduction

I Replace stronger operations with weaker onesI Often applicable to multiplication or exponentiation in loops

ExampleLOV ALOC 2MUL


Common Techniques


Strength reduction


ExampleLOV A

→LOV A

LOC 2 DUP 2MUL ADD


Common Techniques


Strength reduction


ExampleLOV A

→LOV A

→LOV A

LOC 2 DUP 2 LOC 1MUL ADD SHL


Common Techniques


Special instructions

I Replace Instructions with specialized InstructionI Usually more relevant on machine code

ExampleLOV ALOC 1ADDSTV A


Common Techniques


Special instructions

I Replace Instructions with specialized InstructionI Usually more relevant on machine code

ExampleLOV A

→

INV ALOC 1ADDSTV A


Common Techniques







Peephole Optimization - Pattern Matching

Table of Contents


I Many instruction sequences have same semanticsI Generalize rules to avoid (semantically) identical rulesI Different kinds of matching strategies

ExampleLOC 7

→

LOC 7ADD LOC 5LOC 5 SUBSUB ADD


Pattern Matching



ExampleLOC 7

→

LOC 7 LOC 6

→

LOC 6ADD LOC 5 ADD LOC 5LOC 5 SUB LOC 5 SUBSUB ADD SUB ADD


Pattern Matching



ExampleLOC 7

→

LOC 7 LOC 6

→

LOC 6ADD LOC 5 ADD LOC 5LOC 5 SUB LOC 5 SUBSUB ADD SUB ADD . . .


Pattern Matching



ExampleLOC %0

→

LOC %0ADD LOC %1LOC %1 SUBSUB ADD


Pattern Matching


Demo

Demo -

Demo







Replacement Rules Derivation - Finding replacement Rules

Table of Contents


Manual rule creation

I Replacement rules written by handI Using common techniques

Automatic rules derivation

I Deriving rules using superoptimization technique

Replacement Rules Derivation - Finding replacement Rules

How to find replacement rules?







Replacement Rules Derivation - Automatic rules derivation

Table of Contents


Superoptimization

I Given a set of instruction sequences to optimizeI Calculate every possible instruction sequence out of a set of

instructionsI Compare outcome of calculated sequences to original sequencesI Check if new found equivalent sequence has lower cost

Replacement Rules Derivation - Automatic rules derivation

Automatic rules derivation







Replacement Rules Derivation - Validation of Equivalence

Table of Contents


Execution Test: Process

I Run both instruction sequences on machineI Use multiple vectors as inputI Compare outcomeI Discard sequences with not matching results

Execution Test: Property

I Stochastical testI Accuracy depends on test effort


Equivalence Test


ExampleSequence

Original ReplacementLOV X LOV XLOV Y LOV YADD SUB

Input Vector Stack(X,Y) Original Replacement(0,0) [0] [0] 3

(1,1) [2] [0] 7


Execution Test


ExampleSequence

Original ReplacementLOV X LOV XLOC 2 SHRDIV SHLLOC 2MUL

Input Vector Stack(X) Original Replacement(0) [0] [0] 3

(1) [0] [0] 3

(65535) [65534] [65534] 3


Execution Test







Replacement Rules Derivation - Cost Comparison

Table of Contents


Cost Functions

I RuntimeI Code sizeI Memory usage

Replacement Rules Derivation - Cost Comparison

Cost Comparison


I Optimization technique used in modern compilersI Improve code quality and efficiencyI Using replacement rules for optimizationI Generalize with patternsI Automatic generation of rules possible

I Generated optimizer speedup of 1.7 to factor of 10 over conventionaloptimizer

Conclusion -

Conclusion


I Tanenbaum, Andrew S. and van Staveren, Hans and Stevenson,Johan W. Using Peephole Optimization on Intermediate Code. ACMTrans. Program. Lang. Syst., pp. 21-36

I Bansal, Sorav and Aiken, Alex. Automatic Generation of PeepholeSuperoptimizers. IGARCH Comput. Archit. News.

I Elif Aktolga and Supervisor Dr. Des Watson. Pattern MatchingStrategies for Peephole Optimisation

I Henry Massalin. Department of Computer Science ColumbiaUniversity New York, NY 10027. Superoptimizer - A Look at theSmallest Program

Appendix - References

References


Example

int vadd(int a){

return a*2}

llvm IR code%a.addr = alloca i32, align 4store i32 %a, i32* %a.addr, align 4%0 = load i32* %a.addr, align 4%mul = mul nsw i32 %0, 2 → %mul = shl i32 %0, 1ret i32 %mul

Appendix - Peephole Optimization in LLVM

Peephole Optimization in LLVM


ExampleOriginal Sequence Replacement Candidate

ld r0, x ld r0, xld r1, y ld r1, y

add r0, r1vector (x,y) Original Sequence Replacement Candidate

(0,0) (0,0) (0,0) 3

(1,1) (2,1) (1,1) 7

Appendix - Execution Test on other Intermediate Language

Execution Test


I Generate boolean formula for machine statesI RegisterI MemoryI Stack

I Express each instruction as formula that changes machine stateI Express equivalence relation of final machine states as constraintI Use SAT solver to check if constraint holds

Appendix - Boolean Test

Boolean Test


I Remove multiplication from a loopI Occurs often during array access

Example

f o r ( I = 0 , A = 0; I < 10; ++ I ) {A = I ∗ 4;. . .

}. . .

Appendix - Strength Reduction in Loops

Strength Reduction in Loops


I Remove multiplication from a loopI Occurs often during array access

Example

f o r ( I = 0 , A = 0; I < 10; ++ I ) {. . .A += 4;

}. . .




ExampleZRV I

→

ZRV IZRV A ZRV ALOV I LOV ILOC 10 LOC 10BGE 7 BGE 7LOC 4 . . .LOV I LOC 4MUL LOV ASTV A ADD. . . STV AINV I INV IBRA -8 BRA -8. . . . . .




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