Compiler optimization
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The document discusses various compiler optimizations including: 1. Procedure integration replaces procedure calls with the procedure body to eliminate function call overhead. 2. Common subexpression elimination replaces repeated computations of the same expression with a single variable to store the result. 3. Constant propagation replaces variables assigned a constant value with the constant throughout the code. 4. The document provides examples of these and other optimizations like copy propagation, code motion, induction variable elimination, and loop unrolling which aims to improve performance by reducing instructions and improving pipeline utilization.
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Compiler optimization
- 1. Compiler Optimization Speaker ：呂宗螢 Adviser ：梁文耀 老師 Date ： 2006/10/11
- 2. Embedded and Parallel Systems Lab2 The Structure of Recent Front end per language High-level optimizations Global optimizer Code generator Intermediate representation Dependencies Function Language 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
- 3. Embedded and Parallel Systems Lab3 Major types of optimizations
- 4. Embedded and Parallel Systems Lab4 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; }
- 5. Embedded and Parallel Systems Lab5 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;
- 6. Embedded and Parallel Systems Lab6 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;
- 7. Embedded and Parallel Systems Lab7 Stack height reduction Rearrange expression tree to minimize resources needed for expression evaluation ADD R6,R2,R3 ADD R7,R6,R4 ADD R8,R7,R5 ADD R6,R2,R3 ADD R7,R4,R5 ADD R8,R7,R6 I1 I2 I3 I1 I2 I3 R8=((R2+R3)+R4)+R5 R8=(R2+R3)+(R4+R5)
- 8. Embedded and Parallel Systems Lab8 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
- 9. Embedded and Parallel Systems Lab9 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; }
- 10. Embedded and Parallel Systems Lab10 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; }
- 11. Embedded and Parallel Systems Lab11 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; }
- 12. Embedded and Parallel Systems Lab12 Branch offset optimization Choose the shortest branch displacement that reaches target if( a ){ statement 1 } else { goto L1; } statement 2 L1: statement 3 if( !a ){ goto L1; } statement 1 statement 2 L1: statement 3
- 13. Embedded and Parallel Systems Lab13 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; }
- 14. Embedded and Parallel Systems Lab14 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; }
- 15. Embedded and Parallel Systems Lab15 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
- 16. Embedded and Parallel Systems Lab16 Thanks