Peephole OptimizationPeephole Optimization

Improve code by examining and Improve code by examining and changing a small sequence changing a small sequence (peephole) of code at a time.(peephole) of code at a time.

Does not require “expensive” Does not require “expensive” dataflow analysis.dataflow analysis.

Pattern match for improvementsPattern match for improvements

Possible PatternsPossible Patterns

Store r0, aStore r0, a

Load r0, aLoad r0, a

r5 = r5r5 = r5

r7 = r5r7 = r5

r6 = r7r6 = r7

r7 = whateverr7 = whatever

Others?Others?

Example – C CodeExample – C CodeX = A + B * C – (D * A)X = A + B * C – (D * A)

Example – Intermediate Example – Intermediate CodeCode

t1 = D * At1 = D * A

t2 = B * Ct2 = B * C

t3 = A + t2t3 = A + t2

t4 = t3 – t1t4 = t3 – t1

X = t4 X = t4

Example – “Assembly” Example – “Assembly” CodeCode

r5 = *(fp + Dr5 = *(fp + Doffset)offset)

r6 = *(fp + Ar6 = *(fp + Aoffset)offset)

r7 = r5 * r6r7 = r5 * r6

*(fp + t1*(fp + t1offset) = r7offset) = r7

r5 = *(fp + Br5 = *(fp + Boffset)offset)

r6 = *(fp + Cr6 = *(fp + Coffset)offset)

r7 = r5 * r6r7 = r5 * r6

*(fp + t2*(fp + t2offset) = r7offset) = r7

“ “Assembly” Code (cont.)Assembly” Code (cont.)

r5 = *(fp + Ar5 = *(fp + Aoffset)offset)r6 = *(fp + t2r6 = *(fp + t2offset)offset)r7 = r5 + r6r7 = r5 + r6*(fp + t3*(fp + t3offset) = r7offset) = r7r5 = *(fp + t3r5 = *(fp + t3offset)offset)r6 = *(fp + t1r6 = *(fp + t1offset)offset)r7 = r6 – r5r7 = r6 – r5*(fp + t4*(fp + t4offset) = r7offset) = r7r5 = *(fp + t4r5 = *(fp + t4offset) offset) *(fp + X*(fp + Xoffset) = r5offset) = r5

Assembler After PeepholeAssembler After Peephole

r5 = *(fp + Dr5 = *(fp + Doffset)offset)

r6 = *(fp + Ar6 = *(fp + Aoffset)offset)

r7 = r5 * r6r7 = r5 * r6

*(fp + t1*(fp + t1offset) = r7offset) = r7

r5 = *(fp + Br5 = *(fp + Boffset)offset)

r6 = *(fp + Cr6 = *(fp + Coffset)offset)

r7 = r5 * r6r7 = r5 * r6

*(fp + t2*(fp + t2offset) = r7offset) = r7

After Peephole (cont.)After Peephole (cont.)

r5 = *(fp + Ar5 = *(fp + Aoffset)offset)r6 = *(fp + t2r6 = *(fp + t2offset)offset)r7 = r5 + rr7 = r5 + r77*(fp + t3*(fp + t3offset) = r7offset) = r7r5 = *(fp + t3r5 = *(fp + t3offset)offset)r6 = *(fp + t1r6 = *(fp + t1offset)offset)r7 = r6 – rr7 = r6 – r77*(fp + t4*(fp + t4offset) = r7offset) = r7r5 = *(fp + t4r5 = *(fp + t4offset)offset) *(fp + X*(fp + Xoffset) = roffset) = r77

SoSo

How big a peephole do we need?How big a peephole do we need? How could you represent code to How could you represent code to

easeease Re-write your code to generate Re-write your code to generate

intermediateintermediate Do peephole optimizationDo peephole optimization Generate CMachine code from Generate CMachine code from

intermediateintermediate

Instruction Selection by Instruction Selection by Peephole OptimizationPeephole Optimization

Represent each instruction by RTLRepresent each instruction by RTL Look for pairs of instructions whose Look for pairs of instructions whose

combined RTL == that of a single combined RTL == that of a single instructioninstruction

PO, Hop, Chop (late 1970s – early PO, Hop, Chop (late 1970s – early 1980s)1980s)

Used for code selection by GNU Used for code selection by GNU compilerscompilers