Re: [PATCH] i386: Guard noreturn no-callee-saved-registers optimization with -mnoreturn-no-callee-saved-registers [PR38534]

["H.J. Lu" <hjl.tools@gmail.com>]

On Wed, Feb 28, 2024 at 10:20 PM Hongtao Liu <crazylht@gmail.com> wrote:
>
> On Wed, Feb 28, 2024 at 4:54 PM Jakub Jelinek <jakub@redhat.com> wrote:
> >
> > Hi!
> >
> > Adding Hongtao and Honza into the loop as the ones who acked the original
> > patch.
> >
> > The no_callee_saved_registers by default for noreturn functions change can
> > break in-process backtrace(3) or backtraces from debugger or other process
> > (quite often, any time the noreturn function decides to use the bp register
> > and any of the parent frames uses a frame pointer; the unwinder just crashes
> > in the libgcc unwinder case, gdb prints stack corrupted message), so I'd
> > like to save bp register in that case:
> >
> > https://gcc.gnu.org/pipermail/gcc-patches/2024-February/646591.html
> I think this patch makes sense and LGTM, we save and restore frame
> pointer for noreturn.
> >
> > and additionally the no_callee_saved_registers by default for noreturn
> > functions change can make debugging harder, again not localized to the
> > noreturn function, but any of its callers.  So, if say glibc abort function
> > implementation needs a lot of normally callee-saved registers, no matter how
> > users recompile their apps, they will see garbage or optimized out
> > vars/parameters in their code unless they rebuild their glibc with -O0.
> > So, I think we should guard that by a non-default option:
> >
> > https://gcc.gnu.org/pipermail/gcc-patches/2024-February/646649.html
> So it turns off the optimization for noreturn functions by default,
> I'm not sure about this.
> Any comments, H.J?

We need BP for backtrace.  I don't think we need to save other
registers.  True, GDB may not see function parameters.  But
optimization always has this impact.  When I need to debug a
program, I always use -O0 or -Og.

> >
> > Plus we need to somehow make sure to emit DW_CFA_undefined for the modified
> > but not saved normally callee-saved registers, so that we at least don't get
> > garbage in debug info.  H.J. posted some patches for that, so far I wasn't
> > happy about the implementation but the actual change is desirable.
> >
> > Your thoughts on this?
> >
> >         Jakub
> >
>
>
> --
> BR,
> Hongtao



-- 
H.J.