[X86] When expanding a multiply by a negative of one less than a power of 2, like 31, don't generate a negate of a subtract that we'll never optimize.

We generated a subtract for the power of 2 minus one then negated the result. The negate can be optimized away by swapping the subtract operands, but DAG combine doesn't know how to do that and we don't add any of the new nodes to the worklist anyway.

This patch makes use explicitly emit the swapped subtract.

llvm-svn: 337858

1 files changed, 12 insertions, 10 deletions

diff --git a/llvm/lib/Target/X86/X86ISelLowering.cpp b/llvm/lib/Target/X86/X86ISelLowering.cpp
index ef8a6d68395..12f6d409a60 100644
--- a/llvm/lib/Target/X86/X86ISelLowering.cpp
+++ b/llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -33922,14 +33922,20 @@ static SDValue combineMul(SDNode *N, SelectionDAG &DAG,
             DAG.getNode(ISD::SHL, DL, VT, N->getOperand(0),
                         DAG.getConstant(Log2_64(NumSign * SignMulAmt - 1), DL,
                                         MVT::i8)));
+        // To negate, subtract the number from zero
+        if (SignMulAmt < 0)
+          NewMul = DAG.getNode(ISD::SUB, DL, VT,
+                               DAG.getConstant(0, DL, VT), NewMul);
       } else if (IsPowerOf2_64MinusOne) {
         // (mul x, 2^N - 1) => (sub (shl x, N), x)
-        NewMul = DAG.getNode(
-            ISD::SUB, DL, VT,
-            DAG.getNode(ISD::SHL, DL, VT, N->getOperand(0),
-                        DAG.getConstant(Log2_64(NumSign * SignMulAmt + 1), DL,
-                                        MVT::i8)),
-            N->getOperand(0));
+        NewMul = DAG.getNode(ISD::SHL, DL, VT, N->getOperand(0),
+                             DAG.getConstant(Log2_64(NumSign * SignMulAmt + 1),
+                                             DL, MVT::i8));
+        // To negate, reverse the operands of the subtract.
+        if (SignMulAmt < 0)
+          NewMul = DAG.getNode(ISD::SUB, DL, VT, N->getOperand(0), NewMul);
+        else
+          NewMul = DAG.getNode(ISD::SUB, DL, VT, NewMul, N->getOperand(0));
       } else if (IsPowerOf2_64MinusTwo && NumSign == 1) {
         // (mul x, 2^N - 1) => (sub (shl x, N), x)
         NewMul = DAG.getNode(ISD::SHL, DL, VT, N->getOperand(0),
@@ -33938,10 +33944,6 @@ static SDValue combineMul(SDNode *N, SelectionDAG &DAG,
         NewMul = DAG.getNode(ISD::SUB, DL, VT, NewMul, N->getOperand(0));
         NewMul = DAG.getNode(ISD::SUB, DL, VT, NewMul, N->getOperand(0));
       }
-      // To negate, subtract the number from zero
-      if (NewMul && NumSign == -1)
-        NewMul =
-            DAG.getNode(ISD::SUB, DL, VT, DAG.getConstant(0, DL, VT), NewMul);
     }
   }