[LLVMdev] lowering varargs, rewriting types

[Jay Foad]

Hi,

I'm looking at writing a custom backend targetting a proprietary virtual
machine. I'm basing it on the existing C and MSIL backends.

The VM has a defined ABI for varargs functions: the caller allocates some
memory to hold the arguments and passes a pointer to this memory as an extra
parameter in the call. I'd like to write a varargs lowering pass that
represents this ABI explicitly in the LLVM code, replacing varargs
intrinsics with explicit operations on pointers. Does a pass like this
already exist?

I think part of the work of this pass would be rewriting function types
throughout the entire module. For example, if a global variable has a
pointer-to-varargs-function type (or even a structure type with a
pointer-to-varargs-function buried deep within it) then that type will have
to be rewritten so that the function type has an extra pointer parameter
instead of the "..." at the end of the parameter list. As I understand it
you can't change the type of a global variable in place, so I'd have to add
a new variable with the tweaked type and delete the original one. It seems
like quite a lot of work to have to rewrite large chunks of the module like
this when all I want to do is make a small change to function types. Is
there any support for doing this kind of tweaking of types across a whole
module?

An alternative to tweaking types across the whole module would be to only
change the types of varargs function definitions, and to insert bitcasts at
every use of a varargs function and every call with a varargs function type.
But that doesn't sound very clean!

I'd appreciate any thoughts on this.

Thanks,
Jay.
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