[LLVMdev] Mapping field names to GEP indices in clang-compiled C

[Jeffrey Yasskin]

I'm using clang to compile functions and types written in C into LLVM
IR so that I can inline calls and avoid hand-writing the StructType
definitions. The types clang generates are packed structs instead of
ordinary structs. So for

struct Foo {
  char x;
  int* y;
};

clang produces the type <{i8, i8, i8, i8, i32*}> instead of {i8,
i32*}. To extract the 'y' field from the unpacked struct, I'd use "GEP
..., 1", and it'd be right on all architectures. To extract 'y' from
the packed struct that clang produces, I have to use "GEP ..., 4" on
x86-32, "GEP ..., 8" on x86-64, and who knows what on other
architectures.

Is there a way to get clang to emit a C++-readable mapping from field
names to GEP offsets? Or some other way to avoid special-casing these
offsets for each architecture? If not, what would be the easiest way
to add such an ability?

Thanks,
Jeffrey (and the Unladen Swallow team)