Re: [patch] Add --compress-debug-sections option to gas

["H.J. Lu" <hjl dot tools at gmail dot com>]

On Mon, Jun 28, 2010 at 2:39 PM, Cary Coutant <ccoutant@google.com> wrote:
> A couple of years ago, Craig Silverstein submitted a patch to add
> support for compressed debug sections generated by the linker:
>
> ?http://sourceware.org/ml/binutils/2008-06/msg00203.html
>
> The attached patch extends this support to the assembler, adding a new
> --compress-debug-sections option to gas, and adding the necessary
> support (and fixing a couple of bugs in the original patch) for
> reading compressed sections in relocatables to bfd, objdump, and
> readelf.
>
> Tested on x86_64 by compiling with and without
> -Wa,--compress-debug-sections, and comparing readelf -w output and
> objdump -W output. No regressions in the test suite.
>
> A patch for gold to read files with compressed debug sections is in progress.
>
> I have to admit that I'm not happy with a couple of things:
>
> (1) In both bfd/dwarf2.c and binutils/objdump.c, in order to
> uncompress the section before relocating it, I call
> bfd_get_section_contents() to get the unrelocated section contents,
> then decompress the contents and put a pointer to the decompressed
> contents in sec->contents, setting the SEC_IN_MEMORY flag so that
> bfd_simple_get_relocated_section_contents() would not go read the
> section again. I couldn't find a simpler way to apply relocations
> after decompression.
>
> (2) In gas/write.c, I used a similar trick to signal write_contents()
> that the frags had already been collected and compressed. An
> alternative I considered was to simply replace the frag list with a
> single frag containing the entire compressed section contents, but
> that seemed a bit more complicated.
>
> Advice on these issues is welcome.
>
> -cary
>
> p.s. I'm not sure why my regenerated libbfd.h is different from the
> original -- nothing changed except for parameters to
> bfd_generic_get_relocated_section_contents() that used to be unnamed.
> Did "make headers" change behavior since the last time that file was
> regenerated? Should I just leave this out of the patch?
>
>
> bfd/ChangeLog:
>
> ? ? ? ?* bfd-in2.h: Regenerate.
> ? ? ? ?* compress.c (bfd_compress_section_contents): New function.
> ? ? ? ?* dwarf2.c (read_and_uncompress_section): New function.
> ? ? ? ?(read_section): Call it.
> ? ? ? ?(find_line): Likewise.
> ? ? ? ?* libbfd.h: Regenerate.
>
> binutils/ChangeLog:
>
> ? ? ? ?* objdump.c (load_specific_debug_section): Decompress section contents
> ? ? ? ?before applying relocations.
> ? ? ? ?* readelf.c (load_specific_debug_section): Update section size after
> ? ? ? ?decompression.
>
> gas/ChangeLog:
>
> ? ? ? ?* as.c (parse_args): Add --compress-debug-sections and
> ? ? ? ?--nocompress-debug-sections.
> ? ? ? ?* as.h (flag_compress_debug): New variable.
> ? ? ? ?* write.c (compress_debug): New function.
> ? ? ? ?(write_contents): Check for uncompressed section.
> ? ? ? ?(write_object_file): Compress debug sections if requested.
>

Hi Cary,

I think it should be mentioned on gas/NEWS. Also you should add
the new options to "as --help".

Thanks.



-- 
H.J.