DAGCombine add (sext i1), X into sub X, (zext i1) if sext from i1 is illegal. The latter usually compiles into smaller code.

example code:
unsigned foo(unsigned x, unsigned y) {
  if (x != 0) y--;
  return y;
}

before:
  _foo:                           ## @foo
    cmpl  $1, 4(%esp)             ## encoding: [0x83,0x7c,0x24,0x04,0x01]
    sbbl  %eax, %eax              ## encoding: [0x19,0xc0]
    notl  %eax                    ## encoding: [0xf7,0xd0]
    addl  8(%esp), %eax           ## encoding: [0x03,0x44,0x24,0x08]
    ret                           ## encoding: [0xc3]

after:
  _foo:                           ## @foo
    cmpl  $1, 4(%esp)             ## encoding: [0x83,0x7c,0x24,0x04,0x01]
    movl  8(%esp), %eax           ## encoding: [0x8b,0x44,0x24,0x08]
    adcl  $-1, %eax               ## encoding: [0x83,0xd0,0xff]
    ret                           ## encoding: [0xc3]

llvm-svn: 122455

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

@@ -1443,6 +1443,15 @@ SDValue DAGCombiner::visitADD(SDNode *N) {
     }
   }
 
+  // add (sext i1), X -> sub X, (zext i1)
+  if (N0.getOpcode() == ISD::SIGN_EXTEND &&
+      N0.getOperand(0).getValueType() == MVT::i1 &&
+      !TLI.isOperationLegal(ISD::SIGN_EXTEND, MVT::i1)) {
+    DebugLoc DL = N->getDebugLoc();
+    SDValue ZExt = DAG.getNode(ISD::ZERO_EXTEND, DL, VT, N0.getOperand(0));
+    return DAG.getNode(ISD::SUB, DL, VT, N1, ZExt);
+  }
+
   return SDValue();
 }
 

llvm/test/CodeGen/X86/add.ll

@@ -120,3 +120,15 @@ entry:
 ; X64: addq
 ; X64-NEXT: sbbq
 ; X64-NEXT: testb
+
+define i32 @test9(i32 %x, i32 %y) nounwind readnone {
+  %cmp = icmp eq i32 %x, 10
+  %sub = sext i1 %cmp to i32
+  %cond = add i32 %sub, %y
+  ret i32 %cond
+; X64: test9:
+; X64: cmpl $10
+; X64: sete
+; X64: subl
+; X64: ret
+}