Re: The 'cold' function attribute and GDB

[Kevin Buettner <kevinb@redhat.com>]

On Thu, 02 May 2019 18:23:20 +0300
Eli Zaretskii <eliz@gnu.org> wrote:

> > Date: Thu, 2 May 2019 00:38:49 -0700
> > From: Kevin Buettner <kevinb@redhat.com>
> > Cc: gdb-patches@sourceware.org, Simon Marchi <simark@simark.ca>
> > 
> > Can you show me what is printed for the following commands?
> > 
> > disassemble print_vectorlike
> > x/3i print_vectorlike
> > b print_vectorlike
> > 
> > For the disassemble command, I expect both ranges to be displayed. 
> > The x and b commands should print insns / set a breakpoint on the entry
> > pc (which should NOT be the cold address) of the function.  If one or more
> > of these things aren't happening, then something is going wrong
> > somewhere.  
> 
> Here:
> 
>   (gdb) disassemble /m print_vectorlike
>   Dump of assembler code for function print_vectorlike:
>   Address range 0x11d6521 to 0x11d81d6:
>   1367	{
>      0x011d6521 <+0>:	push   %ebp
>      0x011d6522 <+1>:	mov    %esp,%ebp
>      0x011d6524 <+3>:	push   %edi
>      0x011d6525 <+4>:	push   %esi
>      0x011d6526 <+5>:	push   %ebx
>      0x011d6527 <+6>:	sub    $0x7c,%esp
>   [...]
>   Address range 0x12e1ce5 to 0x12e1f3b:
>   1824	      emacs_abort ();
>      0x012e1f36 <+593>:	call   0x12e7b40 <emacs_abort>
> 
>   End of assembler dump.
> 
>   (gdb) x/3i print_vectorlike
>      0x11d6521 <print_vectorlike>:        push   %ebp
>      0x11d6522 <print_vectorlike+1>:      mov    %esp,%ebp
>      0x11d6524 <print_vectorlike+3>:      push   %edi
>   (gdb) break print_vectorlike
>   Breakpoint 1 at 0x11d6521: file print.c, line 1367.
> 
> Seems to be according to your expectations.

Yes, this all looks good to me.

> Btw, if I use a source line number whose code is in the 'cold' area,
> then I see this:
> 
>   (gdb) break print.c:1824
>   Breakpoint 1 at 0x12e1f36: file print.c, line 1824.
>   (gdb) info line print.c:1824
>   Line 1824 of "print.c"
>      starts at address 0x12e1f36 <print_vectorlike.cold.65+593>
>      and ends at 0x12e1f3b <print_vectorlike.cold.65+598>.

I'm not especially surprised that GDB chose a nearby minimal symbol to
print here.  I am surprised at the rather large offsets from that
symbol, however.  In the example that I studied last year, I think
that the first offset was 0.

I'll try building emacs...

Kevin