X86: Lower a select directly to a setcc_carry if possible.

  int test(unsigned long a, unsigned long b) { return -(a < b); }
compiles to
  _test:                              ## @test
    cmpq  %rsi, %rdi                  ## encoding: [0x48,0x39,0xf7]
    sbbl  %eax, %eax                  ## encoding: [0x19,0xc0]
    ret                               ## encoding: [0xc3]
instead of
  _test:                              ## @test
    xorl  %ecx, %ecx                  ## encoding: [0x31,0xc9]
    cmpq  %rsi, %rdi                  ## encoding: [0x48,0x39,0xf7]
    movl  $-1, %eax                   ## encoding: [0xb8,0xff,0xff,0xff,0xff]
    cmovael %ecx, %eax                ## encoding: [0x0f,0x43,0xc1]
    ret                               ## encoding: [0xc3]


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@122451 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 17 insertions, 0 deletions

diff --git a/lib/Target/X86/X86ISelLowering.cpp b/lib/Target/X86/X86ISelLowering.cpp
index 5290453..fca7b02 100644
--- a/lib/Target/X86/X86ISelLowering.cpp
+++ b/lib/Target/X86/X86ISelLowering.cpp
@@ -7313,6 +7313,23 @@ SDValue X86TargetLowering::LowerSELECT(SDValue Op, SelectionDAG &DAG) const {
     Cond = EmitTest(Cond, X86::COND_NE, DAG);
   }
 
+  // a <  b ? -1 :  0 -> RES = ~setcc_carry
+  // a <  b ?  0 : -1 -> RES = setcc_carry
+  // a >= b ? -1 :  0 -> RES = setcc_carry
+  // a >= b ?  0 : -1 -> RES = ~setcc_carry
+  if (Cond.getOpcode() == X86ISD::CMP) {
+    unsigned CondCode = cast<ConstantSDNode>(CC)->getZExtValue();
+
+    if ((CondCode == X86::COND_AE || CondCode == X86::COND_B) &&
+        (isAllOnes(Op1) || isAllOnes(Op2)) && (isZero(Op1) || isZero(Op2))) {
+      SDValue Res = DAG.getNode(X86ISD::SETCC_CARRY, DL, Op.getValueType(),
+                                DAG.getConstant(X86::COND_B, MVT::i8), Cond);
+      if (isAllOnes(Op1) != (CondCode == X86::COND_B))
+        return DAG.getNOT(DL, Res, Res.getValueType());
+      return Res;
+    }
+  }
+
   // X86ISD::CMOV means set the result (which is operand 1) to the RHS if
   // condition is true.
   SDVTList VTs = DAG.getVTList(Op.getValueType(), MVT::Glue);