arm-tdep: sort mapping symbols after parsing all minimal symbols

arm-tdep: sort mapping symbols after parsing all minimal symbols

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.