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

[Jakub Jelinek <jakub@redhat.com>]

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

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

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