[x86] allow single source horizontal op matching (PR39195)

This is intended to restore horizontal codegen to what it looked like before IR demanded elements improved in:
rL343727

As noted in PR39195:
https://bugs.llvm.org/show_bug.cgi?id=39195
...horizontal ops can be worse for performance than a shuffle+regular binop, so I've added a TODO. Ideally, we'd 
solve that in a machine instruction pass, but a quicker solution will be adding a 'HasFastHorizontalOp' feature
bit to deal with it here in the DAG.

Differential Revision: https://reviews.llvm.org/D52997

llvm-svn: 344141

1 files changed, 6 insertions, 2 deletions

diff --git a/llvm/lib/Target/X86/X86ISelLowering.cpp b/llvm/lib/Target/X86/X86ISelLowering.cpp
index 4c18c5a84c2..67f98d8ee72 100644
--- a/llvm/lib/Target/X86/X86ISelLowering.cpp
+++ b/llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -37026,9 +37026,13 @@ static bool isHorizontalBinOp(SDValue &LHS, SDValue &RHS, bool IsCommutative) {
         continue;
 
       // The  low half of the 128-bit result must choose from A.
-      // The high half of the 128-bit result must choose from B.
+      // The high half of the 128-bit result must choose from B,
+      // unless B is undef. In that case, we are always choosing from A.
+      // TODO: Using a horizontal op on a single input is likely worse for
+      // performance on many CPUs, so this should be limited here or reversed
+      // in a later pass.
       unsigned NumEltsPer64BitChunk = NumEltsPer128BitChunk / 2;
-      unsigned Src = i >= NumEltsPer64BitChunk;
+      unsigned Src = B.getNode() ? i >= NumEltsPer64BitChunk : 0;
 
       // Check that successive elements are being operated on. If not, this is
       // not a horizontal operation.