[LLVMdev] How to access llvm Types from the codegen?

[Rafael Espindola]

In order to the code generators to lower functions arguments that have
the "byval" attribute, they would have to access the original argument
Type. For example, on linux x86_64 {i64, i64} should be passed on
registers and {i64, i64, i64} goes on the stack.

The problem is that when looking at (for example) FORMAL_ARGUMENTS,
the only thing that is present is the type of the pointer itself (i64
for example).

I can see three options to solve the problem:

1) Make FORMAL_ARGUMENTS have one SrcValueSDNode per argument in
addition to the flags and make CALL use SrcValueSDNode for the
arguments. The calling convention functions (CC_X86_64_C) can then be
modified to take a Type as an additional argument.

2) Implement part of the ABI on the DAG construction. In this approach
a struct that should be passed on registers would result on a series
of arguments at the DAG level, similar to what is done by llvm-gcc
today, but one lever lower (llvm -> DAG instead of C -> llvm).
Arguments that should go on the stack result in an argument tagged as
a implicit pointer that the code generators can compute as an offset
from the frame pointer. In order to find the offset only the size of
the struct has to be added to the DAG somehow.

3) Similar to 2, but, for arguments that should go on registers, let
llvm-gcc do the splitting. In this approach the "byval" argument would
be a much simpler extension. It would mean: this pointer is to be
computed as an offset from the frame pointer.

In order to implement options 2 or 3 we have to add the structure size
to the DAG somehow. Suggestions?

I prefer option 3, but I would like to know the opinion from other developers.

Thanks,
-- 
Rafael Avila de Espindola

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