[InstCombine] canonicalize select shuffles by commuting

In PR41304:
https://bugs.llvm.org/show_bug.cgi?id=41304
...we have a case where we want to fold a binop of select-shuffle (blended) values.

Rather than try to match commuted variants of the pattern, we can canonicalize the
shuffles and check for mask equality with commuted operands.

We don't produce arbitrary shuffle masks in instcombine, but select-shuffles are a
special case that the backend is required to handle because we already canonicalize
vector select to this shuffle form.

So there should be no codegen difference from this change. It's possible that this
improves CSE in IR though.

Differential Revision: https://reviews.llvm.org/D60016

llvm-svn: 357366

1 files changed, 9 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp b/llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
index cb5c22c69be..49c29fe651b 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
@@ -1343,6 +1343,15 @@ static Instruction *foldSelectShuffle(ShuffleVectorInst &Shuf,
   if (!Shuf.isSelect())
     return nullptr;
 
+  // Canonicalize to choose from operand 0 first.
+  unsigned NumElts = Shuf.getType()->getVectorNumElements();
+  if (Shuf.getMaskValue(0) >= (int)NumElts) {
+    assert(!isa<UndefValue>(Shuf.getOperand(1)) &&
+           "Not expecting undef shuffle operand with select mask");
+    Shuf.commute();
+    return &Shuf;
+  }
+
   if (Instruction *I = foldSelectShuffleWith1Binop(Shuf))
     return I;