[llvm] DAG: Fix asserting in error case for powi softening (PR #147237)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-selectiondag

Author: Matt Arsenault (arsenm)

<details>
<summary>Changes</summary>

We need to return the expected legalized type, not the original
in this case. Also swap to returning poison, and avoid contraction
in error message.

---
Full diff: https://github.com/llvm/llvm-project/pull/147237.diff


1 Files Affected:

- (modified) llvm/lib/CodeGen/SelectionDAG/LegalizeFloatTypes.cpp (+5-4) 


``````````diff
diff --git a/llvm/lib/CodeGen/SelectionDAG/LegalizeFloatTypes.cpp b/llvm/lib/CodeGen/SelectionDAG/LegalizeFloatTypes.cpp
index 7dfb43164be37..64d9c1586796f 100644
--- a/llvm/lib/CodeGen/SelectionDAG/LegalizeFloatTypes.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/LegalizeFloatTypes.cpp
@@ -724,11 +724,13 @@ SDValue DAGTypeLegalizer::SoftenFloatRes_ExpOp(SDNode *N) {
   RTLIB::Libcall LC = IsPowI ? RTLIB::getPOWI(N->getValueType(0))
                              : RTLIB::getLDEXP(N->getValueType(0));
   assert(LC != RTLIB::UNKNOWN_LIBCALL && "Unexpected fpowi.");
+  EVT NVT = TLI.getTypeToTransformTo(*DAG.getContext(), N->getValueType(0));
+
   if (!TLI.getLibcallName(LC)) {
     // Some targets don't have a powi libcall; use pow instead.
     // FIXME: Implement this if some target needs it.
-    DAG.getContext()->emitError("Don't know how to soften fpowi to fpow");
-    return DAG.getUNDEF(N->getValueType(0));
+    DAG.getContext()->emitError("do not know how to soften fpowi to fpow");
+    return DAG.getPOISON(NVT);
   }
 
   if (DAG.getLibInfo().getIntSize() !=
@@ -736,10 +738,9 @@ SDValue DAGTypeLegalizer::SoftenFloatRes_ExpOp(SDNode *N) {
     // If the exponent does not match with sizeof(int) a libcall to RTLIB::POWI
     // would use the wrong type for the argument.
     DAG.getContext()->emitError("POWI exponent does not match sizeof(int)");
-    return DAG.getUNDEF(N->getValueType(0));
+    return DAG.getPOISON(NVT);
   }
 
-  EVT NVT = TLI.getTypeToTransformTo(*DAG.getContext(), N->getValueType(0));
   SDValue Ops[2] = { GetSoftenedFloat(N->getOperand(0 + Offset)),
                      N->getOperand(1 + Offset) };
   SDValue Chain = IsStrict ? N->getOperand(0) : SDValue();

``````````

</details>


https://github.com/llvm/llvm-project/pull/147237