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
- representation of struct field offsets Sandra Loosemore
-
