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If your assembler instructions access memory in an unpredictable fashion, add `memory' to the list of clobbered registers. This will cause GCC to not keep memory values cached in registers across the assembler instruction and not optimize stores or loads to that memory. You will also want to add the volatile keyword if the memory affected is not listed in the inputs or outputs of the asm, as the `memory' clobber does not count as a side-effect of the asm.
In a 1996 mailing list post Re: Is clobber "memory" in include/asm-i386/system.h necessary?, Linus said:
Essentially, think of the "memory" thing as a serializing flag rather than as a "this instruction changes memory" flag. It is extremely important that gcc not re-order memory instructions across these serializing instructions, because otherwise you might find that gcc moves a memory load over the serializing instruction and then you lose...
Volatile keyword elsewhere
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