(Thanks for upstream-tracker BTW. It has been really useful.)
On Fri, Jul 12, 2013 at 06:49:59PM +0400, Andrey Ponomarenko wrote:
I've found an O(N^2) loop in the libdw library (N - number of
symbols in the library). The dwfl_module_getsym function is called N
times, but it calls resolve_symbol function each time and it loops
through the number of symbols again.
So `readelf --debug-dump=info shared_object.so` command is very slow
on large input objects. As an example, it takes several minutes to
analyse libOpenImageIO.so.1.1.11 (27mb), but readelf from binutils
takes just several seconds to do it.
If the goal is just to have a faster eu-readelf -w than binutils
readelf -w then just run it with -N and it will not do any address
to symbol lookups and it should be faster.
But you are right that -N/--numeric-addresses isn't the default.
And by default eu-readelf will do address symbol lookups (because
that makes the output much richer than plain binutils readelf gives)
and those are indeed pretty slow atm.
The gelf_getsymshndx, gelf_getshdr and elf_getscn functions from
libelf are called 2 billion times and it takes about 60% of total
execution time. Another 26% of execution time are taken by the
search_table function that is called by the dwfl_module_addrsym and
loops through the number of symbols too. The dwfl_module_addrsym is
called for each symbol. So it also takes O(N^2).
How can dwfl_module_getsym and dwfl_module_addrsym functions may be
optimised to not to loop through all symbols in the library in order
to find information for just one of them? This can make the library
to be 10 times faster.
Those functions work with the "raw" .dynsyn or .symtab (in the ELF,
separate .debug file or in the .gnu_debugdata). Those tables aren't
sorted by address (just by whether they are local or global, we
prefer globals so we search those first in the hope not to have to
do a full N search) so we have to search them linearly.
They are really designed for doing a quick lookup for a single address.
If you really need all the addresses/symbols then iterating through
them all with dwfl_module_getsymtab() and dwfl_module_getsym (), then
you can create a search table for them all.
So readelf.c could be considered to use dwfl_module_addrsym "wrongly"
since in general it will call it for lots of different addresses. We
should probably build a search table. And maybe even do that not just
in readelf.c but add it to libdwfl for use with dwfl_module_addrsym