[clang] [CIR] Check union tail padding in CallConvLowering (PR #233897)

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<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Henrich Lauko (xlauko)

<details>
<summary>Changes</summary>

isSupportedType checked a union's members but not its tail padding,
which the union's size includes. Incomplete-record padding asserted in
computeStructSize, and scalable-vector padding was lowered as a byval
of meaningless size. Check the padding like a member, so both are
reported as NYI.

Assisted-by: Claude Code (Claude Opus 5.5)

---
Full diff: https://github.com/llvm/llvm-project/pull/233897.diff


2 Files Affected:

- (modified) clang/lib/CIR/Dialect/Transforms/CallConvLoweringPass.cpp (+4) 
- (modified) clang/test/CIR/Transforms/abi-lowering/x86_64-aggregate-nyi.cir (+9) 


``````````diff
diff --git a/clang/lib/CIR/Dialect/Transforms/CallConvLoweringPass.cpp b/clang/lib/CIR/Dialect/Transforms/CallConvLoweringPass.cpp
index 3b9156ee95bb5b..9615fc16519176 100644
--- a/clang/lib/CIR/Dialect/Transforms/CallConvLoweringPass.cpp
+++ b/clang/lib/CIR/Dialect/Transforms/CallConvLoweringPass.cpp
@@ -241,6 +241,10 @@ static bool isSupportedType(mlir::Type ty, const DataLayout &dl) {
                       [&](mlir::Type m) { return isSupportedType(m, dl); }))
       return false;
     if (recTy.isUnion()) {
+      // The size query below reads the tail padding too.
+      if (mlir::Type padding = cast<cir::UnionType>(ty).getPadding();
+          padding && !isSupportedType(padding, dl))
+        return false;
       llvm::ArrayRef<mlir::Type> members = recTy.getMembers();
       uint64_t recordBits = dl.getTypeSizeInBits(recTy).getFixedValue();
       // Refuse a union that could reach SSEUP at a size no coerce exists for.
diff --git a/clang/test/CIR/Transforms/abi-lowering/x86_64-aggregate-nyi.cir b/clang/test/CIR/Transforms/abi-lowering/x86_64-aggregate-nyi.cir
index c264b26e150416..d04f75cc2781a7 100644
--- a/clang/test/CIR/Transforms/abi-lowering/x86_64-aggregate-nyi.cir
+++ b/clang/test/CIR/Transforms/abi-lowering/x86_64-aggregate-nyi.cir
@@ -39,6 +39,7 @@
 !rec_AtomicWrapper = !cir.struct<{data !rec_S1, pad !cir.array<!s8i x 2>}>
 !rec_HoldsAllPad = !cir.struct<"HoldsAllPad" {data !s32i, empty !rec_E}>
 !rec_Incomplete = !cir.struct<"Incomplete" incomplete>
+!rec_UIncompletePad = !cir.union<"UIncompletePad" {data !s32i}, padding = {!rec_Incomplete}>
 
 module attributes {
   dlti.dl_spec = #dlti.dl_spec<
@@ -175,4 +176,12 @@ module attributes {
   }
 
   // CHECK: not yet implemented for type '!cir.struct<"Incomplete" incomplete>'
+
+  // A union's size includes its tail padding, so the padding is checked like
+  // a member.
+  cir.func @take_incomplete_padding(%arg0: !rec_UIncompletePad) {
+    cir.return
+  }
+
+  // CHECK: not yet implemented for type '!cir.union<"UIncompletePad"
 }

``````````

</details>


https://github.com/llvm/llvm-project/pull/233897