[PATCH] D25524: [DAGCombine] Preserve shuffles when one of the vector operands is constant

[Andrea Di Biagio via llvm-commits]

andreadb added a comment.

LGTM

Thanks Zvi!



================
Comment at: lib/CodeGen/SelectionDAG/DAGCombiner.cpp:13765-13773
+  bool N0AnyConst = isAnyConstantBuildVector(N0.getNode()),
+       N1AnyConst = isAnyConstantBuildVector(N1.getNode());
+  if (!(N0AnyConst && N1AnyConst) && N0.getOpcode() == ISD::BUILD_VECTOR &&
+      N1.getOpcode() == ISD::BUILD_VECTOR) {
+    if (N0AnyConst && !ISD::isBuildVectorAllZeros(N0.getNode()))
+      return {};
+    if (N1AnyConst && !ISD::isBuildVectorAllZeros(N1.getNode()))
----------------
zvi wrote:
> andreadb wrote:
> > If N1 is Undef, then we should never early exit. As I wrote in one of my previous comments, by construction N0 can't be Undef; only N1 can be Undef.
> > 
> > Therefore, you can skip the computation of N0AnyConst and N1AnyConst if N1 is known to be Undef.  What I am suggesting is to change that code into something like this:
> > 
> > ```
> > if (!N1.isUndef()) {
> >   bool N0AnyConst = isAnyConstantBuildVector(N0.getNode());
> >   bool N1AnyConst = isAnyConstantBuildVector(N1.getNode());
> >   if (N0AnyConst && !N1AnyConst && !ISD::isBuildVectorAllZeros(N0.getNode()))
> >     return SDValue();
> >   if (!N0AnyConst && N1AnyConst && !ISD::isBuildVectorAllZeros(N1.getNode()))
> >     return SDValue();
> > }
> > ```
> > 
> > The extra check for `N->isUndef()` in `isAnyConstantBuildVector` would become redundant if your code checks in advance if N1 is Undef.
> > 
> I humbly accept your improvement.
> 
> >The extra check for N->isUndef() in isAnyConstantBuildVector would become redundant if your code checks in advance if N1 is Undef.
> 
> Not sure if you are suggesting to change
> 
>     bool N1AnyConst = isAnyConstantBuildVector(N1.getNode());
> 
> to
> 
>     bool N1AnyConst = ISD::isBuildVectorOfConstantSDNodes(N) ||
>             ISD::isBuildVectorOfConstantFPSDNodes(N);
> 
> or that is just a side note.
> 
> I prefer the former for brevity.
> 
That's okay. Using isAnyConstantBuildVector is fine.
My point was that the check for isUndef() in that function is not longer needed because you check in advance if N1 is undef.




Repository:
  rL LLVM

https://reviews.llvm.org/D25524