- Teach SelectionDAG::isKnownNeverZero to return true (op x, c) when c is

non-zero.
- Teach X86 cmov optimization to eliminate the cmov from ctlz, cttz extension
  when the source of X86ISD::BSR / X86ISD::BSF is proven to be non-zero.

rdar://9490949


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@131948 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/SelectionDAG/SelectionDAG.cpp

@@ -2050,14 +2050,15 @@ void SelectionDAG::ComputeMaskedBits(SDValue Op, const APInt &Mask,
     break;
 
   default:
-    // Allow the target to implement this method for its nodes.
-    if (Op.getOpcode() >= ISD::BUILTIN_OP_END) {
+    if (Op.getOpcode() < ISD::BUILTIN_OP_END)
+      break;
+    // Fallthrough
   case ISD::INTRINSIC_WO_CHAIN:
   case ISD::INTRINSIC_W_CHAIN:
   case ISD::INTRINSIC_VOID:
-      TLI.computeMaskedBitsForTargetNode(Op, Mask, KnownZero, KnownOne, *this,
-                                         Depth);
-    }
+    // Allow the target to implement this method for its nodes.
+    TLI.computeMaskedBitsForTargetNode(Op, Mask, KnownZero, KnownOne, *this,
+                                       Depth);
     return;
   }
 }
@@ -2322,6 +2323,13 @@ bool SelectionDAG::isKnownNeverZero(SDValue Op) const {
     return !C->isZero();
 
   // TODO: Recognize more cases here.
+  switch (Op.getOpcode()) {
+  default: break;
+  case ISD::OR:
+    if (const ConstantSDNode *C = dyn_cast<ConstantSDNode>(Op.getOperand(1)))
+      return !C->isNullValue();
+    break;
+  }
 
   return false;
 }

lib/Target/X86/X86ISelLowering.cpp

@@ -11332,15 +11332,28 @@ static SDValue PerformCMOVCombine(SDNode *N, SelectionDAG &DAG,
   if (N->getNumValues() == 2 && !SDValue(N, 1).use_empty())
     return SDValue();
 
+  SDValue FalseOp = N->getOperand(0);
+  SDValue TrueOp = N->getOperand(1);
+  X86::CondCode CC = (X86::CondCode)N->getConstantOperandVal(2);
+  SDValue Cond = N->getOperand(3);
+  if (CC == X86::COND_E || CC == X86::COND_NE) {
+    switch (Cond.getOpcode()) {
+    default: break;
+    case X86ISD::BSR:
+    case X86ISD::BSF:
+      // If operand of BSR / BSF are proven never zero, then ZF cannot be set.
+      if (DAG.isKnownNeverZero(Cond.getOperand(0)))
+        return (CC == X86::COND_E) ? FalseOp : TrueOp;
+    }
+  }
+
   // If this is a select between two integer constants, try to do some
   // optimizations.  Note that the operands are ordered the opposite of SELECT
   // operands.
-  if (ConstantSDNode *TrueC = dyn_cast<ConstantSDNode>(N->getOperand(1))) {
-    if (ConstantSDNode *FalseC = dyn_cast<ConstantSDNode>(N->getOperand(0))) {
+  if (ConstantSDNode *TrueC = dyn_cast<ConstantSDNode>(TrueOp)) {
+    if (ConstantSDNode *FalseC = dyn_cast<ConstantSDNode>(FalseOp)) {
       // Canonicalize the TrueC/FalseC values so that TrueC (the true value) is
       // larger than FalseC (the false value).
-      X86::CondCode CC = (X86::CondCode)N->getConstantOperandVal(2);
-
       if (TrueC->getAPIntValue().ult(FalseC->getAPIntValue())) {
         CC = X86::GetOppositeBranchCondition(CC);
         std::swap(TrueC, FalseC);
@@ -11350,7 +11363,6 @@ static SDValue PerformCMOVCombine(SDNode *N, SelectionDAG &DAG,
       // This is efficient for any integer data type (including i8/i16) and
       // shift amount.
       if (FalseC->getAPIntValue() == 0 && TrueC->getAPIntValue().isPowerOf2()) {
-        SDValue Cond = N->getOperand(3);
         Cond = DAG.getNode(X86ISD::SETCC, DL, MVT::i8,
                            DAG.getConstant(CC, MVT::i8), Cond);
 
@@ -11368,7 +11380,6 @@ static SDValue PerformCMOVCombine(SDNode *N, SelectionDAG &DAG,
       // Optimize Cond ? cst+1 : cst -> zext(setcc(C)+cst.  This is efficient
       // for any integer data type, including i8/i16.
       if (FalseC->getAPIntValue()+1 == TrueC->getAPIntValue()) {
-        SDValue Cond = N->getOperand(3);
         Cond = DAG.getNode(X86ISD::SETCC, DL, MVT::i8,
                            DAG.getConstant(CC, MVT::i8), Cond);
 
@@ -11407,7 +11418,6 @@ static SDValue PerformCMOVCombine(SDNode *N, SelectionDAG &DAG,
 
         if (isFastMultiplier) {
           APInt Diff = TrueC->getAPIntValue()-FalseC->getAPIntValue();
-          SDValue Cond = N->getOperand(3);
           Cond = DAG.getNode(X86ISD::SETCC, DL, MVT::i8,
                              DAG.getConstant(CC, MVT::i8), Cond);
           // Zero extend the condition if needed.

test/CodeGen/X86/clz.ll

@@ -31,3 +31,18 @@ entry:
 }
 
 declare i16 @llvm.ctlz.i16(i16) nounwind readnone 
+
+; Don't generate the cmovne when the source is known non-zero (and bsr would
+; not set ZF).
+; rdar://9490949
+
+define i32 @t4(i32 %n) nounwind {
+entry:
+; CHECK: t4:
+; CHECK: bsrl
+; CHECK-NOT: cmov
+; CHECK: ret
+  %or = or i32 %n, 1
+  %tmp1 = tail call i32 @llvm.ctlz.i32(i32 %or)
+  ret i32 %tmp1
+}