[DAGCombine] Improve (sra (sra x, c1), c2) -> (sra x, (add c1, c2)) folding

Add support for cases where only some c1+c2 results exceed the max bitshift, clamping accordingly.

Differential Revision: https://reviews.llvm.org/D35722

llvm-svn: 340010

1 files changed, 16 insertions, 17 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index c0af91c965a..4d9fd18d71e 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -6591,31 +6591,30 @@ SDValue DAGCombiner::visitSRA(SDNode *N) {
   }
 
   // fold (sra (sra x, c1), c2) -> (sra x, (add c1, c2))
+  // clamp (add c1, c2) to max shift.
   if (N0.getOpcode() == ISD::SRA) {
     SDLoc DL(N);
     EVT ShiftVT = N1.getValueType();
+    EVT ShiftSVT = ShiftVT.getScalarType();
+    SmallVector<SDValue, 16> ShiftValues;
 
-    auto MatchOutOfRange = [OpSizeInBits](ConstantSDNode *LHS,
-                                          ConstantSDNode *RHS) {
+    auto SumOfShifts = [&](ConstantSDNode *LHS, ConstantSDNode *RHS) {
       APInt c1 = LHS->getAPIntValue();
       APInt c2 = RHS->getAPIntValue();
       zeroExtendToMatch(c1, c2, 1 /* Overflow Bit */);
-      return (c1 + c2).uge(OpSizeInBits);
-    };
-    if (ISD::matchBinaryPredicate(N1, N0.getOperand(1), MatchOutOfRange))
-      return DAG.getNode(ISD::SRA, DL, VT, N0.getOperand(0),
-                         DAG.getConstant(OpSizeInBits - 1, DL, ShiftVT));
-
-    auto MatchInRange = [OpSizeInBits](ConstantSDNode *LHS,
-                                       ConstantSDNode *RHS) {
-      APInt c1 = LHS->getAPIntValue();
-      APInt c2 = RHS->getAPIntValue();
-      zeroExtendToMatch(c1, c2, 1 /* Overflow Bit */);
-      return (c1 + c2).ult(OpSizeInBits);
+      APInt Sum = c1 + c2;
+      unsigned ShiftSum =
+          Sum.uge(OpSizeInBits) ? (OpSizeInBits - 1) : Sum.getZExtValue();
+      ShiftValues.push_back(DAG.getConstant(ShiftSum, DL, ShiftSVT));
+      return true;
     };
-    if (ISD::matchBinaryPredicate(N1, N0.getOperand(1), MatchInRange)) {
-      SDValue Sum = DAG.getNode(ISD::ADD, DL, ShiftVT, N1, N0.getOperand(1));
-      return DAG.getNode(ISD::SRA, DL, VT, N0.getOperand(0), Sum);
+    if (ISD::matchBinaryPredicate(N1, N0.getOperand(1), SumOfShifts)) {
+      SDValue ShiftValue;
+      if (VT.isVector())
+        ShiftValue = DAG.getBuildVector(ShiftVT, DL, ShiftValues);
+      else
+        ShiftValue = ShiftValues[0];
+      return DAG.getNode(ISD::SRA, DL, VT, N0.getOperand(0), ShiftValue);
     }
   }