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72 Instruction-Decoding Optimizations Chapter 4
25112 Rev. 3.06 September 2005
Software Optimization Guide for AMD64 Processors
4.1 DirectPath Instructions
Optimization
❖ Use DirectPath instructions rather than VectorPath instructions. (To determine the type of an
instruction—either DirectPath or VectorPath—see Appendix C, “Instruction Latencies.”)
Application
This optimization applies to:
• 32-bit software
• 64-bit software
Rationale
DirectPath instructions minimize the number of operations per AMD64 instruction, thus providing
for optimally efficient decode and execution. Up to three DirectPath Single instructions, or one and a
half DirectPath Double instructions, can be decoded per cycle. VectorPath instructions block the
decoding of DirectPath instructions.
The AMD Athlon 64 and AMD Opteron processors implement the majority of instructions used by a
compiler as DirectPath Single and DirectPath Double instructions. However, assembly writers must
still take into consideration the use of DirectPath versus VectorPath instructions.