On Tue, Feb 27, 2024 at 10:13:14AM +0100, Jakub Jelinek wrote:
> For __libc_start_main, glibc surely just could use no_callee_saved_registers
> attribute, because that is typically the outermost frame in backtrace,
> there is no need to save those there.
> And for kernel if it really wants it and nothing will use the backtraces,
> perhaps the patch wouldn't need to be reverted completely but just guarded
> the implicit no_callee_saved_registers treatment of noreturn
> functions on -mcmodel=kernel or -fno-asynchronous-unwind-tables.
Guarding on -fno-asynchronous-unwind-tables isn't a good idea,
with just -g we emit in that case unwind info in .debug_frame section
and even that shouldn't break, and we shouldn't generate different code for
-g vs. -g0.
The problem with the changes is that it breaks the unwinding and debugging
experience not just in the functions on which the optimization triggers,
but on all functions in the backtrace as well.
So, IMHO either revert the changes altogether, or guard on -mcmodel=kernel
(but talk to kernel people on linux-toolchains if that is what they actually
want).
Jakub
