User's Manual

22007E/0—November 1999 AMD Athlon™ Processor x86 Code Optimization

Short Forms 127

General x86 Optimization

Guidelines

This chapter describes general code optimization techniques

specific to superscalar processors (that is, techniques common

to the AMD-K6

processor, AMD Athlon™ processor, and

Pentium

family processors). In general, all optimization

techniques used for the AMD-K6 processor, Pentium, and

Pentium Pro processors either improve the performance of the

AMD Athlon processor or are not required and have a neutral

effect (usually due to fewer coding restrictions with the

AMD Athlon processor).

Short Forms

Use shorter forms of instructions to increase the effective

number of instructions that can be examined for decoding at

any one time. Use 8-bit displacements and jump offsets where

possible.

Example 1 (Avoid):

CMP REG, 0

Example 2 (Preferred):

TEST REG, REG

Although both of these instructions have an execute latency of

one, fewer opcode bytes need to be examined by the decoders

for the TEST instruction.