[PATCH] D17573: [X86] PR26554: Enable using of true long nops for x86-64 for every CPU

[Andrey Turetskiy via llvm-commits]

aturetsk added a comment.

Hi Bruno,


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Comment at: lib/Target/X86/MCTargetDesc/X86AsmBackend.cpp:353
@@ -345,3 +352,3 @@
   // Select the right NOP table.
   // FIXME: Can we get if CPU supports long nops from the subtarget somehow?
   const uint8_t (*Nops)[10] = HasNopl ? TrueNops : AltNops;
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bruno wrote:
> Any reason why this shouldn't be done as a target feature as suggested in the FIXME above?
Target feature can be used as indicator of true long nop support, but that won't fix the issue.

The alternative nop instructions I introduced in http://reviews.llvm.org/D14178 are not actually nops for x86-64 arch. But all CPUs which don't support true long nops are 32-bit-only. So I propose to use true long nops if we compile for x86-64 arch for any CPU even if it's marked as 'doesn't support true long nops'.
Well, we do generate 64-bit instructions for 32-bit-only CPUs when x86-64 arch is used, why not generate true long nops as well?
And I think 'generic' should use true long nops in 64-bit mode (since every real 64 bit CPU seems to support them) and use alternative long nops in 32 bit mode.


http://reviews.llvm.org/D17573