[X86] Improve mul combine for negative multiplayer (2^c - 1)

This patch improves the mul instruction combine function (combineMul) 
by adding new layer of logic. 
In this patch, we are adding the ability to fold (mul x, -((1 << c) -1)) 
or (mul x, -((1 << c) +1)) into (neg(X << c) -x) or (neg((x << c) + x) respective.

Differential Revision: https://reviews.llvm.org/D28232

llvm-svn: 292358

1 files changed, 31 insertions, 16 deletions

diff --git a/llvm/lib/Target/X86/X86ISelLowering.cpp b/llvm/lib/Target/X86/X86ISelLowering.cpp
index 8c531f42179..ee6a14cdb01 100644
--- a/llvm/lib/Target/X86/X86ISelLowering.cpp
+++ b/llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -30256,22 +30256,37 @@ static SDValue combineMul(SDNode *N, SelectionDAG &DAG,
   }
 
   if (!NewMul) {
-    assert(MulAmt != 0 && MulAmt != (VT == MVT::i64 ? UINT64_MAX : UINT32_MAX)
-           && "Both cases that could cause potential overflows should have "
-              "already been handled.");
-    if (isPowerOf2_64(MulAmt - 1))
-      // (mul x, 2^N + 1) => (add (shl x, N), x)
-      NewMul = DAG.getNode(ISD::ADD, DL, VT, N->getOperand(0),
-                                DAG.getNode(ISD::SHL, DL, VT, N->getOperand(0),
-                                DAG.getConstant(Log2_64(MulAmt - 1), DL,
-                                MVT::i8)));
-
-    else if (isPowerOf2_64(MulAmt + 1))
-      // (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(MulAmt + 1),
-                                DL, MVT::i8)), N->getOperand(0));
+    assert(MulAmt != 0 &&
+           MulAmt != (VT == MVT::i64 ? UINT64_MAX : UINT32_MAX) &&
+           "Both cases that could cause potential overflows should have "
+           "already been handled.");
+    int64_t SignMulAmt = C->getSExtValue();
+    if ((SignMulAmt != INT64_MIN) && (SignMulAmt != INT64_MAX) &&
+        (SignMulAmt != -INT64_MAX)) {
+      int NumSign = SignMulAmt > 0 ? 1 : -1;
+      bool IsPowerOf2_64PlusOne = isPowerOf2_64(NumSign * SignMulAmt - 1);
+      bool IsPowerOf2_64MinusOne = isPowerOf2_64(NumSign * SignMulAmt + 1);
+      if (IsPowerOf2_64PlusOne) {
+        // (mul x, 2^N + 1) => (add (shl x, N), x)
+        NewMul = DAG.getNode(
+            ISD::ADD, DL, VT, N->getOperand(0),
+            DAG.getNode(ISD::SHL, DL, VT, N->getOperand(0),
+                        DAG.getConstant(Log2_64(NumSign * SignMulAmt - 1), DL,
+                                        MVT::i8)));
+      } 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));
+      }
+      // To negate, subtract the number from zero
+      if ((IsPowerOf2_64PlusOne || IsPowerOf2_64MinusOne) && NumSign == -1)
+        NewMul =
+            DAG.getNode(ISD::SUB, DL, VT, DAG.getConstant(0, DL, VT), NewMul);
+    }
   }
 
   if (NewMul)