[LLVMdev] struct as a function argument

[Chris Lattner]

On Oct 2, 2007, at 1:03 AM, Domagoj Babic wrote:
> Hi all,
>
> I have the same problem. My guess is that when a structure
> is passed as a parameter, you cast it into an array for optimization
> reasons (less parameters, less stack space).

This is not an optimization.  This behavior is to be ABI complaint  
when emitting code for your OS.

That said, this is not a very good way to do this.  Rafael is working  
on adding the 'byref' attribute which will support this much more  
gracefully in the future.

> This is, certainly, a reasonable optimization, but makes
> inter-procedural static analysis more complex. Is there a way to
> disable it (my guess is that this should be doable by passing
> some parameter to llvm-gcc)?

The easiest way to disable this is to hack it out of llvm-gcc.   
Change this (gcc/llvm-abi.h):

#ifndef LLVM_SHOULD_PASS_AGGREGATE_IN_INTEGER_REGS
#define LLVM_SHOULD_PASS_AGGREGATE_IN_INTEGER_REGS(X) \
    !isSingleElementStructOrArray(type)
#endif

to:

#undef LLVM_SHOULD_PASS_AGGREGATE_IN_INTEGER_REGS
#define LLVM_SHOULD_PASS_AGGREGATE_IN_INTEGER_REGS(X) 0

Note that you won't be able to mix and match .o files with GCC where  
structs are passed by value.

-Chris