[X86] Add a one use check on the setcc to the min/max canonicalization code in combineSelect.

This seems to improve std::midpoint code where we have a min and
a max with the same condition. If we split the setcc we can end
up with two compares if the one of the operands is a constant.
Since we aggressively canonicalize compares with constants.
For non-constants it can interfere with our ability to share
control flow if we need to expand cmovs into control flow.

I'm also not sure I understand this min/max canonicalization code.
The motivating case talks about comparing with 0. But we don't
check for 0 explicitly.

Removes one instruction from the codegen for PR43658.

llvm-svn: 374706

1 files changed, 1 insertions, 0 deletions

diff --git a/llvm/lib/Target/X86/X86ISelLowering.cpp b/llvm/lib/Target/X86/X86ISelLowering.cpp
index 8d538198aef..58cf9bd5174 100644
--- a/llvm/lib/Target/X86/X86ISelLowering.cpp
+++ b/llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -37009,6 +37009,7 @@ static SDValue combineSelect(SDNode *N, SelectionDAG &DAG,
   // subl   %esi, $edi
   // cmovsl %eax, %edi
   if (N->getOpcode() == ISD::SELECT && Cond.getOpcode() == ISD::SETCC &&
+      Cond.hasOneUse() &&
       DAG.isEqualTo(LHS, Cond.getOperand(0)) &&
       DAG.isEqualTo(RHS, Cond.getOperand(1))) {
     ISD::CondCode CC = cast<CondCodeSDNode>(Cond.getOperand(2))->get();