AMD 250 manual Optimization, 206

Software Optimization Guide for AMD64 Processors 25112 Rev. 3.06 September 2005

9.6Avoid Moving Data Directly Between General-Purpose and MMX™ Registers

Optimization

Avoid moving data directly between general-purpose registers and MMX™ registers; this operation requires the use of the MOVD instruction. If it is absolutely necessary to move data between these two types of registers, use separate store and load instructions to move the data from the source register to a temporary location in memory and then from memory into the destination register, separating the store and the load by at least 10 instructions.

Application

This optimization applies to:

•32-bit software

•64-bit software

Rationale

The register-to-register forms of the MOVD instruction are either VectorPath or DirectPath Double instructions. When compared with DirectPath Single instructions, VectorPath and DirectPath Double instructions have comparatively longer execution latencies. In addition, VectorPath instructions prevent the processor from simultaneously decoding other insructions.

Example

Avoid code like this, which copies a value directly from an MMX register to a general-purpose register:

movd eax, mm2

If it is absolutely necessary to copy a value from an MMX register to a general-purpose register (or vice versa), use separate store and load instructions, separating them by at least 10 instructions:

movd DWORD PTR temp, mm2 ; Store the value in memory.

...

;At least 10 other instructions appear here.

...

mov eax, DWORD PTR temp ; Load the value from memory.

206 Optimizing with SIMD Instructions Chapter 9