llvm dev - May 2009 - [LLVMdev] Mapping field names to GEP indices in clang-compiled C

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)

On Mon, May 11, 2009 at 10:56 AM, Jeffrey Yasskin <jyasskin at google.com>
wrote:
> 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?
First-off, this sort of question is more appropriate for cfe-dev;
please direct any follow-up questions there.

clang's current behavior here is a bug, but it's a low priority to fix
because the generated IR isn't incorrect, just somewhat difficult to
read.

The only completely reliable solution I can think of is generating
something like "int Foo_offsets[] = { offsetof(struct Foo, x),
offsetof(struct Foo, y)};", then use some bitcasting to do the
arithmetic.

-Eli