X86: replace SUB with CMP if possible

This patch will optimize the following
    movq    %rdi, %rax
    subq    %rsi, %rax
    cmovsq  %rsi, %rdi
    movq    %rdi, %rax
to
    cmpq    %rsi, %rdi
    cmovsq  %rsi, %rdi
    movq    %rdi, %rax

Perform this optimization if the actual result of SUB is not used.

rdar: 11540023
llvm-svn: 158126

llvm/lib/Target/X86/X86ISelLowering.cpp

@@ -8271,7 +8271,13 @@ SDValue X86TargetLowering::EmitTest(SDValue Op, unsigned X86CC,
     // Otherwise use a regular EFLAGS-setting instruction.
     switch (Op.getNode()->getOpcode()) {
     default: llvm_unreachable("unexpected operator!");
-    case ISD::SUB: Opcode = X86ISD::SUB; break;
+    case ISD::SUB:
+      // If the only use of SUB is EFLAGS, use CMP instead.
+      if (Op.hasOneUse())
+        Opcode = X86ISD::CMP;
+      else
+        Opcode = X86ISD::SUB;
+      break;
     case ISD::OR:  Opcode = X86ISD::OR;  break;
     case ISD::XOR: Opcode = X86ISD::XOR; break;
     case ISD::AND: Opcode = X86ISD::AND; break;
@@ -8297,6 +8303,13 @@ SDValue X86TargetLowering::EmitTest(SDValue Op, unsigned X86CC,
     return DAG.getNode(X86ISD::CMP, dl, MVT::i32, Op,
                        DAG.getConstant(0, Op.getValueType()));
 
+  if (Opcode == X86ISD::CMP) {
+    SDValue New = DAG.getNode(Opcode, dl, MVT::i32, Op.getOperand(0),
+                              Op.getOperand(1));
+    DAG.ReplaceAllUsesWith(Op, New);
+    return SDValue(New.getNode(), 0);
+  }
+
   SDVTList VTs = DAG.getVTList(Op.getValueType(), MVT::i32);
   SmallVector<SDValue, 4> Ops;
   for (unsigned i = 0; i != NumOperands; ++i)

llvm/test/CodeGen/X86/jump_sign.ll

@@ -83,3 +83,14 @@ entry:
   %cond = select i1 %cmp, i32 %sub, i32 0
   ret i32 %cond
 }
+; rdar://11540023
+define i32 @n(i32 %x, i32 %y) nounwind {
+entry:
+; CHECK: n:
+; CHECK-NOT: sub
+; CHECK: cmp
+  %sub = sub nsw i32 %x, %y
+  %cmp = icmp slt i32 %sub, 0
+  %y.x = select i1 %cmp, i32 %y, i32 %x
+  ret i32 %y.x
+}