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Chapter 5 Cache and Memory Optimizations 123
Software Optimization Guide for AMD64 Processors
25112 Rev. 3.06 September 2005
5.15 Cache Issues when Writing Instruction Bytes to
Memory
Optimization
When writing data consisting of instructions for future execution to memory use streaming store
(write-combining) instructions such as MOVNTDQ and MOVNTI.
Application
This optimization applies to:
• 32-bit software
• 64-bit software
Rationale
This optimization pertains to software that writes executable instructions to memory for subsequent
execution, such as might be done by a just-in-time compiler. If normal store instructions are used to
write the code to memory, then the cache lines will be in a modified state (either in L1 data cache or in
L2). When the processor eventually tries to execute the code, it will miss in the instruction cache.
Because the instruction cache cannot contain cache lines that are in a modified state, the data must be
flushed to memory before it can be fetched into the instruction cache. This unneccesarily evicts
possibly useful information from the caches. By using write-combining instructions, the contents of
the cache is preserved with no performance penalty, and this possibly provides a performance
improvement.