compiler optimization

Compiler Optimization

Speaker：呂宗螢Adviser：梁文耀　老師Date ： 2006/10/11

Embedded and Parallel Systems Lab

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The Structure of Recent

Front end per language

High-level optimizations

Global optimizer

Code generator

Intermediate representation

Dependencies FunctionLanguage Dependent machine independent

Transform language to common intermediate form

Largely machine independent For example, procedure inlineing (also called procedure intergration)

Small language dependencies machine dependencies slight (e.g., register count/types)

Including global and local optimizations + register allocation

Highly machine dependent

Language independent

Detailed instruction selection and machine-dependent optimizations

May include or be followed by assembler

Embedded and Parallel Systems Lab

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Major types of optimizations

Embedded and Parallel Systems Lab

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Procedure integration

Replace procedure call by procedure body

Int a;

void up(){

a=a+1;

}

Void main(){

a=10;

up();

b=a+5;

}

Int a;

Void main(){

a=10;

a=a+1;

b=a+5;

}

Embedded and Parallel Systems Lab

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Common subexpression elimination

Replace two instances of same computation by single copy

a = b * c + g;

d = b * c * d;

tmp = b * c;

a = tmp + g;

d = tmp * d;

Embedded and Parallel Systems Lab

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Constant propagation

Replace all instances of a variable that is assigned a constant with the constant

int x = 14;

int y = 7 - x / 2;

return y * (28 / x + 2);

int x = 14;

int y = 7 - 14 / 2;

return y * (28 / 14 + 2);

int x = 14;

int y = 0;

return y * 4;

Embedded and Parallel Systems Lab

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Stack height reduction

Rearrange expression tree to minimize resources needed for expression evaluation

ADD R6,R2,R3ADD R7,R6,R4ADD R8,R7,R5

ADD R6,R2,R3ADD R7,R4,R5ADD R8,R7,R6

I1

I2

I3

I1 I2

I3

R8=((R2+R3)+R4)+R5 R8=(R2+R3)+(R4+R5)

Embedded and Parallel Systems Lab

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Copy propagation

Replace all instances of a variable A that has been assigned X (i.e., A = X) with X

y = x ;

z = 3 + y; z = 3 + x

Embedded and Parallel Systems Lab

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Code motion

Remove code from a loop that computes same value each iteration of the loop

Loop-invariant code

while (j < maximum - 1) {

x=1;

j = j + 4 * a;

}

int maxval = maximum - 1;

int calcval = 4 * a;

x=1;

while (j < maxval) {

j = j + calcval;

}

Embedded and Parallel Systems Lab

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Induction variable elimination

Simplify / eliminate array addressing calculations within loops

Int i=0;

while( i<10){

i=i+1;

p = 4*i ;

do some things;

}

Int p=0;

while( p<40){

p=p+4;

do some things;

}

Embedded and Parallel Systems Lab

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Strength reduction

Such as , replace multiply by constant with adds and shifts

for (i=0 ; i<n ; i++){

z = i * x;

do some things;

}

for (i=0 ; i<n ; i++){

z = z + x;

do some things;

}

Embedded and Parallel Systems Lab

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Branch offset optimization

Choose the shortest branch displacement that reaches target

if( a ){             statement 1} else {             goto L1;}statement 2L1:statement 3

if( !a ){                  goto L1;}statement 1statement 2L1:statement 3

Embedded and Parallel Systems Lab

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dead (unreachable) code elimination

Remove instructions that will not affect the behavior of the program.

Int func( int a){

int b, c;

b=a*2;

c=a*3;

return b;

}

Int func( int a){

int b, c;

b=a*2;

return b;

}

Embedded and Parallel Systems Lab

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Boolean Expression

If( a=0 || b=0 || c=0) {

 statement 1;

}

If( a=0) {

 statement 1;

}else if( b=0){

 statement 1;

}else if( c=0){

 statement 1;

}

Embedded and Parallel Systems Lab

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Loop unrolling

int i;

for (i = 0; i < 5; i++){

function(i);

}

function(0);

function(1);

function(2);

function(3);

function(4);

Duplicates the body of the loop multiple times, in order to decrease the number of times the loop condition is tested and the number of jumps, which hurt performance by impairing the instruction pipeline

Embedded and Parallel Systems Lab

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Thanks