Question about setjmp_aux.c on MIPS

[Florian Weimer]

* Huang Pei:

> Hi, 
> On Thu, Nov 26, 2020 at 01:37:53PM +0100, Florian Weimer wrote:
>> * Huang Pei:
>> 
>> > For ld.so, it is not intended to use floating point instruction, but the
>> > __sigsetjmp from _dl_catch_exception touch FP regs repeatedly as loading
>> > DSOs, which cause unneeded FP context saving and restoring between tasks
>> > that does not use FP at all, I wonder if this can be avoided.
>> 
>> I think I have posted patches to move _dl_catch_exception into ld.so
>> (“elf: Rework exception handling in the dynamic loader [BZ #25486]”).
>> Once they are merged, you could special-case setjmp for use within ld.so
>> not to save & restore floating point state when compiled for
>> IS_IN (rtld).
>> 
> I got it, there is one more __sigsetjmp call from __libc_start_main in
> libc.so, not from _dl_catch_exception, like this one:
> ......
>
> (gdb) c
> Continuing.
>
> Breakpoint 1, __sigsetjmp_aux (env=0xffffbff2a0, savemask=0,
> sp=1099507430048, fp=0, gp=1099376871424) at
> ../sysdeps/mips/mips64/setjmp_aux.c:39
> 39        asm volatile ("s.d $f24, %0" : : "m"
> (env[0].__jmpbuf[0].__fpregs[0]));
> (gdb) bt
> #0  __sigsetjmp_aux (env=0xffffbff2a0, savemask=0, sp=1099507430048,
> fp=0, gp=1099376871424) at ../sysdeps/mips/mips64/setjmp_aux.c:39
> #1  0x000000fff7ded7c0 in __libc_start_main (warning: GDB can't find the
> start of the function at 0xfff7f9704f.
> main=0xfff7f966e0, argc=<optimized out>, argv=0xffffbff3e0,
> init=<optimized out>, fini=<optimized out>, rtld_fini=<optimized out>,
>     stack_end=<optimized out>) at libc-start.c:283
>     #2  0x000000fff7f97050 in ?? ()
> (gdb) 
>
> ......
>
> Can __libc_start_main call this special-case setjmp?

It should be possible if __libc_start_main is also built in such a way
that it does not floating-point registers.  __libc_start_main does not
return into user code, so it does not matter if it actually preserves
callee-saved registers.

Thanks,
Florian
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