[DAGCombine] Unchecked calls to DAGCombiner::*ExtPromoteOperand

[jacobly]

Summary

Other calls to DAGCombiner::*PromoteOperand check the result, but here it could cause an assertion in getNode. Falling back to any extend in this case instead of failing outright seems correct to me.

[spatel]

Did you find a place where this fails? If so, it would be better to add that test case.

[jacobly]

I would have, but it was triggered by an out of tree backend. In order to trigger it, a backend would need to overload TargetLowering::IsDesirableToPromoteOp to return true for a type for which ISD::SIGN_EXTEND_INREG is marked illegal. In tree, only X86 overloads and sometimes returns true for MVT::i16 yet it marks setOperationAction(ISD::SIGN_EXTEND_INREG, MVT::i16 , Legal);.

[spatel]

Ok - thanks for the explanation. LGTM.

[jacobly]

I don't have commit access, could someone commit this for me please.

lib/CodeGen/SelectionDAG/DAGCombiner.cpp

return DAG.getExtLoad(ExtType, DL, PVT,
                           LD->getChain(), LD->getBasePtr(),
                           MemVT, LD->getMemOperand());
   }
 
   unsigned Opc = Op.getOpcode();
   switch (Opc) {
   default: break;
   case ISD::AssertSext:
-    return DAG.getNode(ISD::AssertSext, DL, PVT,
-                       SExtPromoteOperand(Op.getOperand(0), PVT),
-                       Op.getOperand(1));
+    if (SDValue Op0 = SExtPromoteOperand(Op.getOperand(0), PVT))
+      return DAG.getNode(ISD::AssertSext, DL, PVT, Op0, Op.getOperand(1));
+    break;
   case ISD::AssertZext:
-    return DAG.getNode(ISD::AssertZext, DL, PVT,
-                       ZExtPromoteOperand(Op.getOperand(0), PVT),
-                       Op.getOperand(1));
+    if (SDValue Op0 = ZExtPromoteOperand(Op.getOperand(0), PVT))
+      return DAG.getNode(ISD::AssertZext, DL, PVT, Op0, Op.getOperand(1));
+    break;
   case ISD::Constant: {
     unsigned ExtOpc =
       Op.getValueType().isByteSized() ? ISD::SIGN_EXTEND : ISD::ZERO_EXTEND;
     return DAG.getNode(ExtOpc, DL, PVT, Op);
   }
   }
 
   if (!TLI.isOperationLegal(ISD::ANY_EXTEND, PVT))