[PATCH] D87108: [ImplicitNullCheck] Handle instructions that do not modify null behaviour of null checked reg

[Anna Thomas via Phabricator via llvm-commits]

anna added inline comments.


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Comment at: llvm/lib/Target/X86/X86InstrInfo.cpp:3670
+  if (!MI->modifiesRegister(NullValueReg, TRI))
+    return true;
+  // Shift right/left of a null is still null.
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dantrushin wrote:
> Nit: Since you check specific opcodes anyway, that check is unnecessary, IMHO
We need this check because if you see the caller in ImplicitNullChecks, it was previously checking if `If MI re-defines the PointerReg (i.e. the NullValueReg`). We now have both checks within isNullBehaviourUnchanged. I originally thought of having the modifiesRegister in the caller itself and leaving an assert here instead of the check, but I felt this is better. 


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Comment at: llvm/lib/Target/X86/X86InstrInfo.cpp:3677
+      return true;
+  }
+  // Zero extend of a sub-reg of NullValueReg into itself does not change the
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dantrushin wrote:
> Again, since you're checking specific opcodes, you know that opnd 0 and 1 are registers,
> so you can do something like
> ```
> return MI->getOperand(0).getReg() == NullValueReg && MI->getOperand(1).getReg() == NullValueReg;
> ```
> I.e., no need to check `MO.isReg` and direct register comparision might be more readable that `isTied`
agreed. 


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  https://reviews.llvm.org/D87108/new/

https://reviews.llvm.org/D87108