[CIR][Transforms] Fix CallConv Function Lowering

[bruteforceboy]

Consider the following code snippet tmp.c:

typedef struct {
  int a, b;
} S;

void foo(S s) {}

Running bin/clang tmp.c -fclangir -Xclang -emit-llvm -Xclang -fclangir-call-conv-lowering -S -o -, we get:

loc(fused["tmp.c":5:1, "tmp.c":5:16]): error: 'llvm.bitcast' op result #0 must be LLVM-compatible non-aggregate type, but got '!llvm.struct<"struct.S", (i32, i32)>'

We can also produce a similar error from this:

typedef struct {
  int a, b;
} S;

S init() { S s; return s; }

gives:

loc(fused["tmp.c":5:17, "tmp.c":5:24]): error: 'llvm.bitcast' op operand #0 must be LLVM-compatible non-aggregate type, but got '!llvm.struct<"struct.S", (i32, i32)>'

I've traced the errors back to lib/CIR/Dialect/Transforms/TargetLowering/LowerFunction.cpp in LowerFunction::buildAggregateStore, castReturnValue, and buildAggregateBitcast.

withElementType(SrcTy) is currently commented out/ignored in LowerFunction.cpp, but it is important.

This PR adds/fixes this and updates one test.

[sitio-couto]

Hey @bruteforceboy!

Could you double-check if you specified the target triple when testing?
This PR updates the test for the x86-64 ABI, but the original codegen example doesn't match what I'm seeing in Compiler Explorer—there's no LLVM IR bitcast here.

I added cir.bitcast in TargetLowering because CIR pointers aren't opaque like LLVM's; meaning the source value's type must match the target pointee's type. It's there to circumvent CIR's type-checking. Ideally, this bitcast should be dropped altogether when lowering to LLVM (something I forgot to do).

[bruteforceboy]

Hey @bruteforceboy!

Could you double-check if you specified the target triple when testing? This PR updates the test for the x86-64 ABI, but the original codegen example doesn't match what I'm seeing in Compiler Explorer—there's no LLVM IR bitcast here.

I added cir.bitcast in TargetLowering because CIR pointers aren't opaque like LLVM's; meaning the source value's type must match the target pointee's type. It's there to circumvent CIR's type-checking. Ideally, this bitcast should be dropped altogether when lowering to LLVM (something I forgot to do).

You're right! I passed -fclangir to clang to produce the llvm above. If removing the bitcast fixes this, I can close the PR.

[bcardosolopes]

Closing on behalf of @bruteforceboy, feel free to reopen if more work is going to be done here

[bruteforceboy]

@sitio-couto Just to clarify. I just realized my clang version did not support opaque pointers, and that is why the LLVM IR above was produced.