[cfe-dev] Passing StructType arguments directly

[David Meyer]

Hello,

I am working with Ivan Krasin on the PNaCl target in Clang. Our ABI
requires that aggregates are passed as first class types in bitcode.

For example:
void foo(struct bar x);

Should compile to:
declare void foo(%struct.bar %x)

To implement this, I added a custom ABIInfo for PNaCl which uses
ABIArgInfo::Direct for aggregate arguments. However, the result of
this is:

define void @foo(i32 %x.coerce0, i32 %x.coerce1)

This reason appears to be code in lib/CodeGen/CGCall.cpp which always
coerces structure types:

      // If the coerce-to type is a first class aggregate, we flatten it and
      // pass the elements. Either way is semantically identical, but fast-isel
      // and the optimizer generally likes scalar values better than FCAs.
      if (llvm::StructType *STy =
            dyn_cast<llvm::StructType>(ArgI.getCoerceToType())) {
        Ptr = Builder.CreateBitCast(Ptr, llvm::PointerType::getUnqual(STy));

        for (unsigned i = 0, e = STy->getNumElements(); i != e; ++i) {
          assert(AI != Fn->arg_end() && "Argument mismatch!");
          AI->setName(Arg->getName() + ".coerce" + Twine(i));
          llvm::Value *EltPtr = Builder.CreateConstGEP2_32(Ptr, 0, i);
          Builder.CreateStore(AI++, EltPtr);
        }
      }

What's the right way to bypass this step (only for target PNaCl) so
that I can actually pass aggregates directly?

Thanks,
- pdox