[gdb/symtab] Ignore cold clones

[gdb/symtab] Ignore cold clones

Consider the test-case contained in this patch, compiled for c using gcc-10:
...
$ gcc-10 -x c src/gdb/testsuite/gdb.cp/cold-clone.cc -O2 -g -Wall -Wextra
...

When setting a breakpoint on foo, we get one breakpoint location:
...
$ gdb -q -batch a.out -ex "b foo"
Breakpoint 1 at 0x400560: file cold-clone.cc, line 28.
...

However, when we compile for c++ instead, we get two breakpoint locations:
...
$ gdb -q -batch a.out -ex "b foo" -ex "info break"
Breakpoint 1 at 0x400430: foo. (2 locations)
Num  Type        Disp Enb Address            What
1    breakpoint  keep y   <MULTIPLE>
1.1                   y   0x0000000000400430 in foo() at cold-clone.cc:30
1.2                   y   0x0000000000400560 in foo() at cold-clone.cc:28
...

The additional breakpoint location at 0x400430 corresponds to the cold clone:
...
$ nm a.out | grep foo
0000000000400560 t _ZL3foov
0000000000400430 t _ZL3foov.cold
...
which demangled looks like this:
...
$ nm -C a.out | grep foo
0000000000400560 t foo()
0000000000400430 t foo() [clone .cold]
...

[ Or, in the case of the cc1 mentioned in PR23710:
...
$ nm cc1 | grep do_rpo_vn.*cold
000000000058659d t \
  _ZL9do_rpo_vnP8functionP8edge_defP11bitmap_headbb.cold.138
$ nm -C cc1 | grep do_rpo_vn.*cold
000000000058659d t \
  do_rpo_vn(function*, edge_def*, bitmap_head*, bool, bool) [clone .cold.138]
... ]

The cold clone is a part of the function that is split off from the rest of
the function because it's considered cold (not frequently executed).  So while
the symbol points to code that is part of a function, it doesn't point to a
function entry, so the desirable behaviour for "break foo" is to ignore this
symbol.

When compiling for c, the symbol "foo.cold" is entered as minimal symbol
with the search name "foo.cold", and the lookup using "foo" fails to find that
symbol.

But when compiling for c++, the symbol "foo.cold" is entered as minimal symbol
with both the mangled and demangled name, and for the demangled name
"foo() [clone .cold]" we get the search name "foo" (because
cp_search_name_hash stops hashing at '('), and the lookup using "foo" succeeds.

Fix this by recognizing the cold clone suffix and returning false for such a
minimal symbol in msymbol_is_function.

Tested on x86_64-linux.

gdb/ChangeLog:

2021-06-01  Tom de Vries  <tdevries@suse.de>

	PR symtab/26096
	* minsyms.c (msymbol_is_cold_clone): New function.
	(msymbol_is_function): Use msymbol_is_cold_clone.

gdb/testsuite/ChangeLog:

2021-06-01  Tom de Vries  <tdevries@suse.de>

	PR symtab/26096
	* gdb.cp/cold-clone.cc: New test.
	* gdb.cp/cold-clone.exp: New file.