Optimizing Compilers Nai-Wei Lin Department of Computer Science and Information Engineering National Chung Cheng University

Content Introduction Control flow analysis Data flow analysis Program transformations Case studies

Introduction

Optimizing Compilers Compilers that apply code-improving transformations Must preserve the meaning of programs Must improve programs by a measurable amount Must be worth the effort

Levels of Code Optimization Source code level –profile programs, change algorithms, transform loops Intermediate code level –analyze programs, inline procedure calls, improve loops, eliminate common subexpressions Target code level –use machine characteristics, do peephole optimizations

Organization of an Optimizing Compiler Front end Code optimization –control flow analysis –data flow analysis –code transformation Code generation

Program Transformations Statement level –common-subexpression elimination, copy propagation, dead-code elimination, constant folding, constant propagation Loop level –code motion, reduction in strength, induction variable elimination Procedure level –inline expansion, tail recursion optimization

Common Subexpression Elimination a = b + c b = a - d c = b + c d = a - d a = a + c c = a - d a = b + c b = a - d c = b + c d = b a = a + c c = a - d

Copy Propagation a = b + c b = a - d c = b + c d = b a = a + c c = a - d a = b + c b = a - d c = b + c d = b a = a + c c = a - b

Dead-Code Elimination a = b + c b = a - d c = b + c d = b a = a + c c = a - b a = b + c b = a - d c = b + c a = a + c c = a - b

Constant Folding a = 4 * 2 b = a + 3 a = 8 b = a + 3

Constant Propagation a = 8 b = a + 3 a = 8 b = 8 + 3

Code Motion t = 4 * i b = a[t] i = i + 1 if i < n- 1 goto L i = 0 L: t = 4 * i b = a[t] i = i + 1 if i < f goto L i = 0 f = n - 1 L:

Reduction in Strength t = 4 * i b = a[t] i = i + 1 if i < f goto L i = 0 f = n - 1 t = t + 4 b = a[t] i = i + 1 if i < f goto L i = 0 f = n - 1 t = -4

Induction Variable Elimination t = t + 4 b = a[t] if t < 4 * f goto L i = 0 f = n - 1 t = -4 t = t + 4 b = a[t] i = i + 1 if i < f goto L i = 0 f = n - 1 t = -4

Inline Expansion p: a = c + d b = a + c param a param b call q, 2 c = d + e return q: d = sp[0] + sp[1] return p: a = c + d b = a + c d = a + b c = d + e return q: d = sp[0] + sp[1] return

Tail Recursion Optimization p: a = sp[0] + c b = sp[1] + d param a param b call p, 2 return p: a = sp[0] + c b = sp[1] + d sp[0] = a sp[1] = b goto p return