Re: [ANNOUNCE] pahole and other DWARF2 utilities

Arnaldo Carvalho de Melo
2006-10-31T20:45:46Z

On Tue, Oct 31, 2006 at 02:22:37PM -0300, Arnaldo Carvalho de Melo wrote:
> On Mon, Oct 30, 2006 at 08:33:34PM -0800, Andrew Morton wrote:
> > On Mon, 30 Oct 2006 18:33:19 -0300
> > Arnaldo Carvalho de Melo <acme@mandriva.com> wrote:
> >
> > > I've been working on some DWARF2 utilities and thought that it
> > > is about time I announce it to the community, so that what is already
> > > available can be used by people interested in reducing structure sizes
> > > and otherwise taking advantage of the information available in the elf
> > > sections of files compiled with 'gcc -g' or in the case of the kernel
> > > with CONFIG_DEBUG_INFO enabled, so here it goes the description of said
> > > tools:
> > >
> > > pahole: Poke-a-Hole is a tool to find out holes in structures, holes
> > > being defined as the space between members of functions due to alignemnt
> > > rules that could be used for new struct entries or to reorganize
> > > existing structures to reduce its size, without more ado lets see what
> > > that means:
> > >
> > > ...
> > >
> > > Further ideas on how to use the DWARF2 information include tools
> > > that will show where inlines are being used, how much code is added by
> > > inline functions,
> >
> > It would be quite useful to be able to identify inlined functions which are
> > good candidates for uninlining.

For now people can take a look at:

http://oops.merseine.nu:81/acme/net.ipv4.tcp.o.pahole

Where all the types in headers included from net/ipv4/tcp.c that have
holes can be seen, for instance:

/* /pub/scm/linux/kernel/git/acme/net-2.6/include/linux/dqblk_xfs.h:143
* */
struct fs_quota_stat {
__s8 qs_version; /* 0 1 */

/* XXX 1 bytes hole, try to pack */

__u16 qs_flags; /* 2 2 */
__s8 qs_pad; /* 4 1 */

/* XXX 3 bytes hole, try to pack */

fs_qfilestat_t qs_uquota; /* 8 20 */
fs_qfilestat_t qs_gquota; /* 28 20 */
__u32 qs_incoredqs; /* 48 4 */
__s32 qs_btimelimit; /* 52 4 */
__s32 qs_itimelimit; /* 56 4 */
__s32 qs_rtbtimelimit; /* 60 4 */
__u16 qs_bwarnlimit; /* 64 2 */
__u16 qs_iwarnlimit; /* 66 2 */
}; /* size: 68, sum members: 64, holes: 2, sum holes: 4 */

See? two holes, that can be combined and reduce the size of this
struct by 4 bytes, just moving qs_pad to be defined just before
qs_flags, many more holes are there to harvest :-)

Of course, mistakes from the past for structs that are exported
to userspace have to be kept that way, and in other cases where grouping
members for cacheline locality optimizations, etc.

- Arnaldo