representation of struct field offsets

[Sandra Loosemore <sandra at codesourcery dot com>]

I've been having a heck of a time figuring out how to translate the offsets for 
struct fields from the DWARF encoding back to GCC's internal encoding for the 
LTO project.  I've got a handle on the DWARF encoding and how to do the 
necessary big/little endian conversions, but for the GCC side, there doesn't 
seem to be any documentation about the relevant macros in the manual, and the 
comments in tree.h don't seem to reflect what is actually going on in the 
representation.

For example, DECL_FIELD_OFFSET is supposed to be "the field position, counting 
in bytes, of the byte containing the bit closest to the beginning of the 
structure", while DECL_FIELD_BIT_OFFSET is supposed to be "the offset, in bits, 
of the first bit of the field from DECL_FIELD_OFFSET".  So I'm quite puzzled 
why, for fields that are not bit fields and that are aligned on byte boundaries, 
the C front end is generating a DECL_FIELD_OFFSET that points to some byte that 
doesn't contain any part of the field, and a non-zero DECL_FIELD_BIT_OFFSET 
instead.  If I make the LTO front end do what the comments in tree.h describe, 
then dwarf2out.c produces incorrect offsets that don't match those from the 
original C file.

I see in stor-layout.c that there are routines to "perform computations that 
convert between the offset/bitpos forms and byte and bit offsets", but what 
exactly are these forms and which values are the ones that I should actually be 
storing inside the FIELD_DECL object?  Is it possible to compute the 
DECL_OFFSET_ALIGN value somehow, given that it's not encoded in the DWARF 
representation?  Trying to reverse-engineer dwarf2out.c isn't turning out to be 
very productive....  :-P

-Sandra