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198 Optimizing with SIMD Instructions Chapter 9
25112 Rev. 3.06 September 2005
Software Optimization Guide for AMD64 Processors
9.3 Use MOVLPx/MOVHPx Instructions for Unaligned
Data Access
Optimization
When data alignment cannot be guaranteed, use MOVLPD/MOVHPD, MOVLPS/MOVHPS or
MOVLPD/MOVHPD pairs in lieu of MOVUPD, MOVUPS or MOVDQU, respectively.
Application
This optimization applies to:
• 32-bit software
• 64-bit software
Rationale
The MOVUPS, MOVUPD and MOVDQU instructions are VectorPath when one of the operands is a
memory location. It is better to use one of the MOVLPx/MOVHPx or MOVQ/MOVHPD pairs. It is
prefereable to load or store the 64-bit halves of an XMM register separately when the memory
location cannot be guaranteed to be aligned.