Compiler Optimizations: Classical, Machine Specific, and ILP Enhancing - Prof. Nathan Clar, Assignments of Computer Science

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CS 6241 – Class 9Classic Optimization

Georgia Tech.February 5, 2008

• 1 -

Class stuff

HW1 grading appt

HW2 no yacc, code fixes

HW

HW

• 3 -

Classical Optimizations

Operation-level – 1 operation in isolation

Constant folding, strength reduction

Dead code elimination (global, but 1 op at a time)

Local/Global – Pairs of operations

Constant propagation

Forward copy propagation

Backward copy propagation

CSE

Constant combining

Operation folding

Loop – Body of a loop

Invariant code removal

Global variable migration

Induction variable strength reduction

Induction variable elimination

• 4 -

Caveat

Traditional compiler class

Fancy implementations of optimizations, efficient algorithms

Entire papers written on how to do 1 optimization

Spend entire class on 1 optimization

For this class – Go over concepts of each optimization

What it is, why its useful

When can it be applied (set of conditions that must be satisfied)

Challenges

How do predicates affect things?

Register pressure?

ILP verses operation count

• 6 -

Strength Reduction

Replace expensive ops with cheaper ones

Constant propagation creates opportunities for this

Power of 2 constants

Multiply by power of 2, replace with left shift

r1 = r2 * 8

r1 = r2 << 3

Divide by power of 2, replace with right shift

r1 = r2 / 4

r1 = r2 >> 2

Remainder by power of 2, replace with logical and

r1 = r2 REM 16

r1 = r2 & 15

More exotic

Replace multiply by constant by sequence of shift and adds/subs

r1 = r2 * 6



r100 = r2 << 2; r101 = r2 << 1; r1 = r100 + r

r1 = r2 * 7



r100 = r2 << 3; r1 = r100 – r

• 7 -

Way more exotic

// ntclark: need to flip the low order bit.

if(tmp_sym & 1) {

tmp_sym = ((tmp_sym >> 1) << 1);

} else {

tmp_sym = tmp_sym | 1;

• 9 -

Class Problem

r1 = 0

r4 = r1 | -1r7 = r1 * 4

r6 = r

r3 = 8 / r

r3 = 8 * r r3 = r3 + r

r2 = r2 + r1r6 = r7 * r

r1 = r1 + 1 store (r1, r3)

Optimize this applying 1. constant folding2. strength reduction3. dead code elimination

• 10 -

Constant Propagation

Forward propagation of movesof the form

rx = L (where L is a literal)

Maximally propagate

Assume no instructionencoding restrictions

When is it legal?

SRC: Literal is a hard codedconstant, so never a problem

DEST: Must be available



Guaranteed to reach



May reach not good enough

r1 = 5r2 = r1 + r

r1 = r1 + r

r7 = r1 + r

r8 = r1 + 3

r9 = r1 + r

• 12 -

Global Constant Propagation

Consider 2 ops, X and Y indifferent BBs

1. X is a move

2. src1(X) is a literal

3. Y consumes dest(X)

4. X is in a_in(BB(Y))

5. Dest(x) is not modified betweenthe top of BB(Y) and Y

6. No danger betw X and Y



When dest(X) is a Macro reg,BRL destroys the value

r1 = 5r2 = ‘_x’

r1 = r1 + r

r7 = r1 – r

r8 = r1 * r

r9 = r1 + r

• 13 -

Class Problem

r1 = 0r2 = 10 r4 = 1r7 = r1 * 4r6 = 8p1,p2 = cmppUNUC r3 > 0r2 = 0 if p1r3 = r4 * r6 if p2r3 = r3 + r2 if p2r6 = r6 * r7 if Tr3 = r2 / r6 if p1r2 = r2 + 1 if Tr1 = r1 + 1 if T store (r1, r3)

Optimize this applying 1. constant propagation2. constant folding

• 15 -

Backward Copy Propagation

Backward propagation of the LHSof moves

»

r1 = r2 + r



r4 = r2 + r

»

…

»

r5 = r1 + r



r5 = r4 + r

»

…

»

r4 = r



noop

Rules (ops X and Y in same BB)

»

dest(X) is a register

»

dest(X) not live out of BB(X)

»

Y is a move

»

dest(Y) is a register

»

Y consumes dest(X)

»

dest(Y) not consumed in (X…Y)

»

dest(Y) not defined in (X…Y)

»

There are no uses of dest(X) afterthe first redefinition of dest(Y)

r1 = r8 + r9r2 = r9 + r1r4 = r2r6 = r2 + 1r9 = r1r10 = r6r5 = r6 + 1r4 = 0r8 = r2 + r

• 16 -

CSE – Common Subexpression Elimination

Eliminate recomputation of anexpression by reusing the previousresult

»

r1 = r2 * r

»



r100 = r

»

…

»

r4 = r2 * r



r4 = r

Benefits

»

Reduce work

»

Moves can get copy propagated

Rules (ops X and Y)

»

X and Y have the same opcode

»

src(X) = src(Y), for all srcs

»

expr(X) is available at Y

»

if X is a load, then there is no storethat may write to address(X) alongany path between X and Y

r1 = r2 * r r3 = r4 / r

r2 = r2 + 1

r6 = r3 * 7

r5 = r2 * r r8 = r4 / r7r9 = r3 * 7

if op is a load, call it redundantload elimination rather than CSE

• 18 -

Loop Optimizations

The most important set of optimizations

Because programs spend so much time in loops

Optimize given that you know a sequence of code will berepeatedly executed

Optis

Invariant code removal

Global variable migration

Induction variable strength reduction

Induction variable elimination

• 19 -

Recall Loop Terminology

r1 = 3 r2 = 10

r4 = r4 + 1r7 = r4 * 3

r2 = 0

r3 = r2 + 1

r1 = r1 + 2 store (r1, r3)

loop preheader loop header

backedge BB

exit BB

• r1, r4 are basicinduction variables- r7 is a derivedinduction variable