[clang] [clang][CodeGen] Additional fixes for #114062 (PR #128166)

[Alex Voicu via cfe-commits]

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@@ -2352,6 +2353,22 @@ Value *ScalarExprEmitter::VisitCastExpr(CastExpr *CE) {
     Value *Src = Visit(const_cast<Expr*>(E));
     llvm::Type *SrcTy = Src->getType();
     llvm::Type *DstTy = ConvertType(DestTy);
+
+    // FIXME: this is a gross but seemingly necessary workaround for an issue
+    // manifesting when a target uses a non-default AS for indirect sret args,
+    // but the source HLL is generic, wherein a valid C-cast or reinterpret_cast
+    // on the address of a local struct that gets returned by value yields an
+    // invalid bitcast from the a pointer to the IndirectAS to a pointer to the
+    // DefaultAS. We can only do this subversive thing because sret args are
+    // manufactured and them residing in the IndirectAS is a target specific
+    // detail, and doing an AS cast here still retains the semantics the user
+    // expects. It is desirable to remove this iff a better solution is found.
+    if (SrcTy != DstTy && isa<llvm::Argument>(Src) &&
+        cast<llvm::Argument>(Src)->hasStructRetAttr())
+      return CGF.CGM.getTargetCodeGenInfo().performAddrSpaceCast(
+          CGF, Src, E->getType().getAddressSpace(), DestTy.getAddressSpace(),
+          DstTy);
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AlexVlx wrote:

It carries the LLVM AS, not the AST AS. Since this is a target hook, I assume the intention is to allow a target to override this interface and take into account language intended AS mappings, rather than how the values being cast got codegen-ed? For example, it is plausible, if not currently done (?) that `LangAS::Foo` and `LangAS::Bar` are aliases / map to the same target AS etc., so it'd be possible to simply NOP this and return the source.

https://github.com/llvm/llvm-project/pull/128166