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

[Andrea Di Biagio via llvm-commits]

andreadb added inline comments.


================
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()))
----------------
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.



Repository:
  rL LLVM

https://reviews.llvm.org/D25524