Re: [PATCH] Shrink size of dwarf2_per_cu_data

[Christian Biesinger via Gdb-patches <gdb-patches@sourceware.org>]

On Wed, Apr 21, 2021 at 6:04 PM Tom Tromey <tom@tromey.com> wrote:
>
> >>>>> "Gary" == Gary Benson <gbenson@redhat.com> writes:
>
> Gary> Tom Tromey wrote:
> >> I noticed some holes in struct dwarf2_per_cu_data.  This patch
> >> rearranges the type slightly, and shrinks the size of some fields.
> >> This reduces it from 136 bytes to 112 bytes (on x86-64).
>
> Gary> Nice.  Though I'd assumed the compiler would do this.
>
> C and C++ compilers can't generally do this, though other languages like
> Rust and Ada do allow it.
>
> >> I also reduced the size of the DWARF "version" fields in a couple of
> >> spots.  It seemed needless to use a short to hold a value that
> >> ranges from 2 to 5, and this also helped the goal of shrinking
> >> dwarf2_per_cu_data.
>
> Gary> There's other things in there if you were going that way, e.g.:
>
> Gary>   unsigned char addr_size;
> Gary>   unsigned char signed_addr_p;
> Gary>   sect_offset abbrev_sect_off;
>
> Gary>   /* Size of file offsets; either 4 or 8.  */
> Gary>   unsigned int offset_size;
>
> Gary>   /* Size of the length field; either 4 or 12.  */
> Gary>   unsigned int initial_length_size;
>
> Gary> I presume you'd need to lose 8 whole bytes to shrink the struct
> Gary> further, so marking these as <8bit values probably wouldn't help
> Gary> any, but maybe it's worth it?
>
> I looked at this, and it doesn't remove enough space to help.
> Might still be worth doing for clarity, I dunno.
>
> Gary> How many CUs does something big
> Gary> have?
>
> gdb has 1197 as of today, but IMO it is moderately-sized at best.
> I don't have anything really big that's convenient to check.

I tried to approximate the number of CUs that Chrome has (Linux debug build):

$ objdump -g chrome | grep 'Compilation Unit' | wc -l
9233

Plus the parts that are in shared libraries:

$ find -name \*.so | xargs -n 1 objdump -g | grep 'Compilation Unit' | wc -l
57942

I'm not sure if all the .so files get loaded at the same time, so it
may be a bit fewer in practice. Of course this does not include any
CUs that may come from debug symbols for system libraries. Is there a
gdb command that could tell me the exact number?

Christian