[PATCH] D93092: [PowerPC] KnownBits should be constant when performing non-sign comparison

[Kai Luo via Phabricator via llvm-commits]

lkail added inline comments.


================
Comment at: llvm/lib/Target/PowerPC/PPCISelLowering.cpp:13192
 
-      if (Op1Known.Zero != Op2Known.Zero || Op1Known.One != Op2Known.One)
+      if (!Op1Known.isConstant() || !Op2Known.isConstant() ||
+          Op1Known.getConstant() != Op2Known.getConstant())
----------------
nemanjai wrote:
> Hmm, but we've cleared bit zero from both known zero and known ones for both. So they will both be non-constant, won't they? I think that we would have to rewrite the above lines to something like this:
> ```
>       // We don't really care about what is known about the first bit (if
>       // anything), so pretend that it is known zero for both to ensure
>       // they can be compared as constants.
>       Op1Known.Zero.setBit(0); Op1Known.One.clearBit(0);
>       Op2Known.Zero.setBit(0); Op2Known.One.clearBit(0);
> ```
Good catch! This check is over kill, since we are requiring Op1[1..BitWidth) == Op2[1..BitWidth). I tend to extract bits in [1..BitWidth) from `Op1` and `Op2` and do same constant check as above.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D93092/new/

https://reviews.llvm.org/D93092