[DAG] During DAG legalization keep opaque constants even after expanding.

The included test case would return the incorrect results, because the expansion
of an shift with a constant shift amount of 0 would generate undefined behavior.

This is because ExpandShiftByConstant assumes that all shifts by constants with
a value of 0 have already been optimized away. This doesn't happen for opaque
constants and usually this isn't a problem, because opaque constants won't take
this code path - they are not supposed to. In the case that the opaque constant
has to be expanded by the legalizer, the legalizer would drop the opaque flag.
In this case we hit the limitations of ExpandShiftByConstant and create incorrect
code.

This commit fixes the legalizer by not dropping the opaque flag when expanding
opaque constants and adding an assertion to ExpandShiftByConstant to catch this
not supported case in the future.

This fixes <rdar://problem/16718472>

llvm-svn: 207304

1 files changed, 8 insertions, 3 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/LegalizeIntegerTypes.cpp b/llvm/lib/CodeGen/SelectionDAG/LegalizeIntegerTypes.cpp
index f5e583b3283..e622ca2b609 100644
--- a/llvm/lib/CodeGen/SelectionDAG/LegalizeIntegerTypes.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/LegalizeIntegerTypes.cpp
@@ -1272,6 +1272,7 @@ std::pair <SDValue, SDValue> DAGTypeLegalizer::ExpandAtomic(SDNode *Node) {
 /// and the shift amount is a constant 'Amt'.  Expand the operation.
 void DAGTypeLegalizer::ExpandShiftByConstant(SDNode *N, unsigned Amt,
                                              SDValue &Lo, SDValue &Hi) {
+  assert(Amt && "Expected zero shifts to be already optimized away.");
   SDLoc DL(N);
   // Expand the incoming operand to be shifted, so that we have its parts
   SDValue InL, InH;
@@ -1714,9 +1715,13 @@ void DAGTypeLegalizer::ExpandIntRes_Constant(SDNode *N,
                                              SDValue &Lo, SDValue &Hi) {
   EVT NVT = TLI.getTypeToTransformTo(*DAG.getContext(), N->getValueType(0));
   unsigned NBitWidth = NVT.getSizeInBits();
-  const APInt &Cst = cast<ConstantSDNode>(N)->getAPIntValue();
-  Lo = DAG.getConstant(Cst.trunc(NBitWidth), NVT);
-  Hi = DAG.getConstant(Cst.lshr(NBitWidth).trunc(NBitWidth), NVT);
+  auto Constant = cast<ConstantSDNode>(N);
+  const APInt &Cst = Constant->getAPIntValue();
+  bool IsTarget = Constant->isTargetOpcode();
+  bool IsOpaque = Constant->isOpaque();
+  Lo = DAG.getConstant(Cst.trunc(NBitWidth), NVT, IsTarget, IsOpaque);
+  Hi = DAG.getConstant(Cst.lshr(NBitWidth).trunc(NBitWidth), NVT, IsTarget,
+                       IsOpaque);
 }
 
 void DAGTypeLegalizer::ExpandIntRes_CTLZ(SDNode *N,