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Somebody on IRC reported a while ago that loading a big ARM program in
GDB was very slow. Their profiling pointed out that a big amount of
time was spent in
VEC_safe_insert (arm_mapping_symbol_s, *map_p, idx, &new_map_sym);
I was able to verify this as well.
ARM mapping symbols are special ELF symbols named $a, $d and $t
indicating that symbols starting at this address up to the next mapping
symbol (in terms of address) are of type "ARM code", "data" and "Thumb
code", respectively. GDB records these symbols in vectors (one for each
section) in arm-tdep.c. These vectors are sorted by symbol address, to
allow for quick lookup. The current approach is to insert new symbols
at the right position to keep the vectors sorted at all time. This is
done based on the assumption that mapping symbols come already almost
sorted from the binary, as explains this comment in
arm_record_special_symbol:
/* Assume that most mapping symbols appear in order of increasing
value. If they were randomly distributed, it would be faster to
always push here and then sort at first use. */
Well, it turns out this is not the case. The original reporter
mentioned that mapping symbols in their binaries are not nearly sorted,
and this is not my experience either (at least in the binary used in the
benchmark below). So if the values don't come nearly sorted, doing
insertions to keep the vectors sorted ends up being of the order of
number_of_mapping_symbols ^ 2.
This patch changes it just like the comment above says, to just append
to the vector in arm_record_special_symbol and sort the vector on first
use.
Benchmark
=========
I have done some benchmarks using an --enable-targets=all GDB, compiled
with -O2, running on x86-64 and parsing file
dce18d22e5c2ecb6a3a57372f4e6ef614130bc.debug from this package:
https://launchpad.net/ubuntu/+source/firefox/66.0.3+build1-0ubuntu1/+build/16608691/+files/firefox-dbg_66.0.3+build1-0ubuntu1_armhf.deb
This file is the separate debug info for libxul.so (part of firefox) for
ARM.
I have added some traces to measure the execution time of just
elf_symtab_read and ran GDB like this:
./gdb --data-directory=data-directory -nx -batch .../path/to/usr/lib/debug/.build-id/65/dce18d22e5c2ecb6a3a57372f4e6ef614130bc.debug
Since the new code sorts the vectors on first use, it would be difficult
to benchmark it as-is and be fair, since the "before" version does more
work in elf_symtab_read. So I have actually benchmarked a version of
the patch that did sort all the vectors at the end of elf_symtab_read,
so the sorting would be considered in the measured execution time.
Here's the measured execution time of elf_symtab_read, averaged on 3
runs:
insert sorted (before): 28.678s
sort after (after): 1.760s
And here's the total execution time of the command above (just one run).
The time is now mostly spent in reading DWARF.
insert sorted: 71.12s user 2.71s system 99% cpu 1:14.03 total
sort after: 46.42s user 2.60s system 99% cpu 49.147 total
I tried for fun on my Raspberry Pi 3, the run time of
elf_symtab_read goes from ~259s to ~9s, reading the same file.
gdb/ChangeLog:
* arm-tdep.c (struct arm_per_objfile) <section_maps_sorted>: New
field.
(arm_find_mapping_symbol): Sort mapping symbol vectors on first
use.
(arm_record_special_symbol): Don't insert new symbol in sorted
position, push it at the end.
README for GNU development tools This directory contains various GNU compilers, assemblers, linkers, debuggers, etc., plus their support routines, definitions, and documentation. If you are receiving this as part of a GDB release, see the file gdb/README. If with a binutils release, see binutils/README; if with a libg++ release, see libg++/README, etc. That'll give you info about this package -- supported targets, how to use it, how to report bugs, etc. It is now possible to automatically configure and build a variety of tools with one command. To build all of the tools contained herein, run the ``configure'' script here, e.g.: ./configure make To install them (by default in /usr/local/bin, /usr/local/lib, etc), then do: make install (If the configure script can't determine your type of computer, give it the name as an argument, for instance ``./configure sun4''. You can use the script ``config.sub'' to test whether a name is recognized; if it is, config.sub translates it to a triplet specifying CPU, vendor, and OS.) If you have more than one compiler on your system, it is often best to explicitly set CC in the environment before running configure, and to also set CC when running make. For example (assuming sh/bash/ksh): CC=gcc ./configure make A similar example using csh: setenv CC gcc ./configure make Much of the code and documentation enclosed is copyright by the Free Software Foundation, Inc. See the file COPYING or COPYING.LIB in the various directories, for a description of the GNU General Public License terms under which you can copy the files. REPORTING BUGS: Again, see gdb/README, binutils/README, etc., for info on where and how to report problems.