Allow to truncate left shift with non-constant shift amount

That is pretty common for clang to produce code like
(shl %x, (and %amt, 31)). In this situation we can still perform
trunc (shl) into shl (trunc) conversion given the known value
range of shift amount.

Differential Revision: https://reviews.llvm.org/D34723

llvm-svn: 306499

1 files changed, 12 insertions, 10 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 225c4826f16..90642d72286 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -8210,18 +8210,20 @@ SDValue DAGCombiner::visitTRUNCATE(SDNode *N) {
   if (N0.getOpcode() == ISD::SHL && N0.hasOneUse() &&
       (!LegalOperations || TLI.isOperationLegalOrCustom(ISD::SHL, VT)) &&
       TLI.isTypeDesirableForOp(ISD::SHL, VT)) {
-    if (const ConstantSDNode *CAmt = isConstOrConstSplat(N0.getOperand(1))) {
-      uint64_t Amt = CAmt->getZExtValue();
-      unsigned Size = VT.getScalarSizeInBits();
-
-      if (Amt < Size) {
-        SDLoc SL(N);
-        EVT AmtVT = TLI.getShiftAmountTy(VT, DAG.getDataLayout());
+    SDValue Amt = N0.getOperand(1);
+    KnownBits Known;
+    DAG.computeKnownBits(Amt, Known);
+    unsigned Size = VT.getScalarSizeInBits();
+    if (Known.getBitWidth() - Known.countMinLeadingZeros() <= Log2_32(Size)) {
+      SDLoc SL(N);
+      EVT AmtVT = TLI.getShiftAmountTy(VT, DAG.getDataLayout());
 
-        SDValue Trunc = DAG.getNode(ISD::TRUNCATE, SL, VT, N0.getOperand(0));
-        return DAG.getNode(ISD::SHL, SL, VT, Trunc,
-                           DAG.getConstant(Amt, SL, AmtVT));
+      SDValue Trunc = DAG.getNode(ISD::TRUNCATE, SL, VT, N0.getOperand(0));
+      if (AmtVT != Amt.getValueType()) {
+        Amt = DAG.getZExtOrTrunc(Amt, SL, AmtVT);
+        AddToWorklist(Amt.getNode());
       }
+      return DAG.getNode(ISD::SHL, SL, VT, Trunc, Amt);
     }
   }