Re: [PATCH][gdb/symtab] Ignore cold clones

[Simon Marchi via Gdb-patches <gdb-patches@sourceware.org>]

On 2021-05-31 10:15 a.m., Tom de Vries wrote:
> Hi,
> 
> 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.
> 
> Any comments?

Is this related to

https://sourceware.org/pipermail/gdb-patches/2021-May/179371.html

?

Simon