user manual

vi Contents
AMD Athlon™ Processor x86 Code Optimization
22007E/0—November 1999
7 Scheduling Optimizations 67
Schedule Instructions According to their Latency . . . . . . . . . . . . . . 67
Unrolling Loops. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
Complete Loop Unrolling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
Partial Loop Unrolling. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
Use Function Inlining . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
Always Inline Functions if Called from One Site . . . . . . . . . . 72
Always Inline Functions with Fewer than 25 Machine
Instructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
Avoid Address Generation Interlocks. . . . . . . . . . . . . . . . . . . . . . . . . 72
Use MOVZX and MOVSX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
Minimize Pointer Arithmetic in Loops . . . . . . . . . . . . . . . . . . . . . . . . 73
Push Memory Data Carefully. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
8 Integer Optimizations 77
Replace Divides with Multiplies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
Multiplication by Reciprocal (Division) Utility . . . . . . . . . . . 77
Unsigned Division by Multiplication of Constant. . . . . . . . . . 78
Signed Division by Multiplication of Constant . . . . . . . . . . . . 79
Use Alternative Code When Multiplying by a Constant. . . . . . . . . . 81
Use MMX™ Instructions for Integer-Only Work . . . . . . . . . . . . . . . . 83
Repeated String Instruction Usage. . . . . . . . . . . . . . . . . . . . . . . . . . . 84
Latency of Repeated String Instructions. . . . . . . . . . . . . . . . . 84
Guidelines for Repeated String Instructions . . . . . . . . . . . . . 84
Use XOR Instruction to Clear Integer Registers . . . . . . . . . . . . . . . . 86
Efficient 64-Bit Integer Arithmetic . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
Efficient Implementation of Population Count Function. . . . . . . . . 91
Derivation of Multiplier Used for Integer Division
by Constants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
Unsigned Derivation for Algorithm, Multiplier, and
Shift Factor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93