[LLVMdev] Target independency using "opaque"? How to do it else?

[Johannes Schaub - litb]

Hello all, 

I'm writing a backend for our scriptlanguage compiler and I'm currently 
writing an IR module for the runtime library that contains some support 
routines called by generated code. 

The IR module contains calls to "malloc", which depend on the size of 
"size_t". Since I don't know the target when writing the IR module for the 
runtime library, I thought about using an "opaque" type instance in place of 
"size_t". When loading the IR module, I would refine the "opaque" to either 
i64 or i32, depending on which target I'm using. 

For example I currently have

    ; these opaque types are replaced at load time by codegen::RuntimeLib
    %sizet_ty = type opaque
    %intptrt_ty = type opaque

    ; ... then in a function I do:
    %sizeof_value_ty = ptrtoint %value_ty* getelementptr (%value_ty* null, 
i32 1) to i32
   %numBytesToAlloc = mul i32 %num, %sizeof_value_ty
   %numBytesSizeT = bitcast i32 %numBytesToAlloc to %sizet_ty
   %memory = call i8* @malloc(%sizet_ty %numBytesSizeT)

However, it always fails to compile this li-file to bitcode at the bitcast 
instruction, and says 

   error: invalid cast opcode for cast from 'i32' to 'opaque'

It appears one cannot use "opaque" in this way, as some kind of 
"placeholder"? What can I do else to achieve my goal? Thanks for any 
support!