[PATCH] D29874: [X86] Generate VZEROUPPER for Skylake-avx512

[Craig Topper via Phabricator via llvm-commits]

craig.topper added inline comments.


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Comment at: lib/Target/X86/X86VZeroUpper.cpp:222
+    // explicitly specified.
+    if (IsCall && !callHasRegMask(MI))
       continue;
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aaboud wrote:
> craig.topper wrote:
> > Why did this change to not check specific register clobbers?
> We can get to this point only if ClobberAllTmmReg is true, in such case ClobberAnyYmmReg will also be true, and we will fall-through exactly as we do with this implementation.
> This was a kind of did code.
> 
> Also, can you explain what is unique with "cannot clobber any" and "cannot clobber all" regarding these lines?
This is probably the first time I've really looked at this code so I think explain what is unique without more thought.


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Comment at: lib/Target/X86/X86VZeroUpper.cpp:225
 
     // The VZEROUPPER instruction resets the upper 128 bits of all AVX
     // registers. In addition, the processor changes back to Clean state, after
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aaboud wrote:
> craig.topper wrote:
> > This comments is stale with ZMM. It's not upper 128 bits.
> Actually, this is what it does. even for the ZMM registers it reset the upper 128bits of the YMM corresponding register. Is not it?
> 
> The important behavior is the next sentence, which emphasize that processor changes back to Clean state.
Doesn't it clear the upper 384 bits of ZMM? The comment itself doesn't say YMM. It just says "AVX registers".


https://reviews.llvm.org/D29874