[PATCH] D140208: [AMDGPU] Improved wide multiplies

[Jessica Del via Phabricator via llvm-commits]

OutOfCache added inline comments.


================
Comment at: llvm/lib/Target/AMDGPU/AMDGPULegalizerInfo.cpp:3007
             auto Mul = B.buildMul(S32, Src0[j0], Src1[j1]);
-            if (!LocalAccum[0]) {
+            if (!LocalAccum[0] || KB.getKnownBits(LocalAccum[0]).isZero()) {
               LocalAccum[0] = Mul.getReg(0);
----------------
arsenm wrote:
> OutOfCache wrote:
> > This check is required, when the accumulator is a zero register.
> > 
> > `!LocalAccum[0]` only checks for the existence of a Register. It is still true, if the Register is known to be all zeroes.
> > This particular case occurs when the lower bytes of an operand are masked. 
> > In that case, the check in line 3048 will fail and no `G_MAD` will be created. `LocalAccum[0]` will still be set to the result of the Unmerge of the `Tmp` register in line 3060. `Tmp` is set to a zero register in line 3041, so it is all zeroes at this point.
> > 
> > By stepping through the debugger, I confirmed that in that case the first condition, `!LocalAccum[0]` will be false, but the second condition will be correctly evaluated to true and therefore skip the addition to 0.
> If you're just looking for zero, just looking for the constant zero is cheaper than going through getKnownBits
Sounds like a good idea, but how do I do that?


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D140208/new/

https://reviews.llvm.org/D140208