Optimize .gdb_index symbol name searching

As mentioned in the previous patch, .gdb_index name lookup got
significantly slower with the previous patch.

This patch addresses that, and in the process makes .gdb_index name
searching faster than what we had before the previous patch, even.
Using the same test:

 $ cat script.cmd
 set pagination off
 set $count = 0
 while $count < 400
   complete b string_prin
   printf "count = %d\n", $count
   set $count = $count + 1
 end

 $ time gdb --batch -q ./gdb-with-index -ex "source script.cmd"

I got, before the previous patch (-O2, x86-64):

 real    0m1.773s
 user    0m1.737s
 sys     0m0.040s

and after this patch:

 real    0m1.361s
 user    0m1.315s
 sys     0m0.040s

The basic idea here is simple: instead of always iterating over all
the symbol names in the index, we build an accelerator/sorted name
table and binary search names in it.

Later in the series, we'll want to support wild matching for C++ too,
so this mechanism already considers that.  For example, say that
you're looking up functions/methods named "func", no matter the
containing namespace/class.  If we sorted the table by qualified name,
then we obviously wouldn't be able to find those symbols with a binary
search:

  func
  ns1::a::b::func
  ns1::b::func
  ns2::func

(function symbol names in .gdb_index have no parameter info, like psymbols)

To address that out, we put an entry for each name component in the
sorted table.  something like this:

  Table Entry       Actual symbol
  ---------------------------------
  func              func

  func              ns1::a::b::func
  b::func           ns1::a::b::func
  a::b::func        ns1::a::b::func
  ns1::a::b::func   ns1::a::b::func

  func              ns1::b::func
  b::func           ns1::b::func
  ns1::b::func      ns1::b::func

  func              ns2::func
  ns2::func         ns2::func

Which sorted results in this:

  Table Entry       Actual symbol
  ---------------------------------
  a::b::func        ns1::a::b::func
  b::func           ns1::a::b::func
  b::func           ns1::b::func
  func              func
  func              ns1::a::b::func
  func              ns1::b::func
  func              ns2::func
  ns1::a::b::func   ns1::a::b::func
  ns1::b::func      ns1::b::func
  ns2::func         ns2::func

And we can binary search this.

Note that a binary search approach works for both completion and
regular lookup, while a name hashing approach only works for normal
symbol looking, since obviously "fun" and "func" have different
hashes.

At first I was a bit wary of these tables potentially growing GDB's
memory significantly.  But I did an experiment that convinced it's not
a worry at all.  I hacked gdb to count the total number of entries in
all the tables, attached that gdb to my system/Fedora's Firefox
(Fedora's debug packages uses .gdb_index), did "set max-completions
unlimited", and then hit "b [TAB]" to cause everything to expand.

That resulted in 1351355 name_components.  Each entry takes 8 bytes,
so that's 10810840 bytes (ignoring std::vector overhead), or ~10.3 MB.
That's IMO too small to worry about, given GDB was using over 7400MB
total at that point.  I.e., we're talking about 0.1% increase.

dw2_expand_symtabs_matching unit tests covering this will be added in
a follow up patch.

If the size of this table turns out to be a concern, I have an idea to
reduce the size of the table further at the expense of a bit more code
-- the vast majority of the name offsets are either 0 or fit in
8-bits:

 total name_component = 1351355, of which,
 name_component::name_offset instances need  0 bits = 679531
 name_component::name_offset instances need  8 bits = 669526
 name_component::name_offset instances need 16 bits = 2298
 name_component::name_offset instances need 32 bits = 0
 name_component::idx instances need 0 bits  = 51
 name_component::idx instances need 8 bits  = 8361
 name_component::idx instances need 16 bits = 280329
 name_component::idx instances need 32 bits = 1062614

so we could have separate tables for 0 name_offset, 8-bit name_offset
and 32-bit name_offset.  That'd give us roughly:

 679531 * 0 + 669526 * 1 + 2298 * 4 + 1062614 * 4 = 4929174, or ~4.7MB

with only 8-bit and 32-bit tables, that'd be:

 1349057 * 1 + 2298 * 4 + 4 * 1351355 = 6763669 bytes, or ~6.5MB.

I don't think we need to bother though.

I also timed:

 $ time gdb --batch -q -p `pidof firefox`
 $ time gdb --batch -q -p `pidof firefox` -ex "b main"
 $ time gdb --batch -q -p `pidof firefox` -ex "set max-completion unlimited" -ex "complete b "

and compared before previous patch vs this patch, and I didn't see a
significant difference, seemingly because time to read debug info
dominates.  The "complete b " variant of the test takes ~2min
currently...  (I have a follow up series that speeds that up
somewhat.)

gdb/ChangeLog:
2017-11-08  Pedro Alves  <palves@redhat.com>

	* dwarf2read.c (byte_swap, MAYBE_SWAP): Move higher up in file.
	(struct name_component): New.
	(mapped_index::name_components): New field.
	(mapped_index::symbol_name_at): New method.
	(dwarf2_read_index): Call mapped_index ctor.
	(dw2_map_matching_symbols): Add comment about name_components
	table.
	(dw2_expand_symtabs_matching): Factor part to...
	(dw2_expand_symtabs_matching_symbol): ... this new function.
	Build name components table, and lookup symbols in it before
	calling the name matcher.
	(dw2_expand_marked_cus): New, factored out from
	dw2_expand_symtabs_matching.
	(dwarf2_per_objfile_free): Call the mapped_index's dtor.

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