PR14562 - Truncation of left shift became undef
DAGCombiner::ReduceLoadWidth was converting (trunc i32 (shl i64 v, 32))
into (shl i32 v, 32) into undef. To prevent this, check the shift count
against the final result size.
Patch by: Kevin Schoedel
Reviewed by: Nadav Rotem
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174972 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 5b83149..67fa39d 100644
--- a/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -5163,8 +5163,15 @@
EVT ShImmTy = getShiftAmountTy(Result.getValueType());
if (!isUIntN(ShImmTy.getSizeInBits(), ShLeftAmt))
ShImmTy = VT;
- Result = DAG.getNode(ISD::SHL, N0.getDebugLoc(), VT,
- Result, DAG.getConstant(ShLeftAmt, ShImmTy));
+ // If the shift amount is as large as the result size (but, presumably,
+ // no larger than the source) then the useful bits of the result are
+ // zero; we can't simply return the shortened shift, because the result
+ // of that operation is undefined.
+ if (ShLeftAmt >= VT.getSizeInBits())
+ Result = DAG.getConstant(0, VT);
+ else
+ Result = DAG.getNode(ISD::SHL, N0.getDebugLoc(), VT,
+ Result, DAG.getConstant(ShLeftAmt, ShImmTy));
}
// Return the new loaded value.