[PATCH 0/2] Remove some i386 system call optimizations

[Florian Weimer]

* H. J. Lu:

> On Tue, Jan 11, 2022 at 2:52 PM Florian Weimer <fweimer@redhat.com> wrote:
>>
>> * H. J. Lu:
>>
>> >> > There are
>> >> >
>> >> > /* Since GCC 5 and above can properly spill %ebx with PIC when needed,
>> >> >    we can inline syscalls with 6 arguments if GCC 5 or above is used
>> >> >    to compile glibc.  Disable GCC 5 optimization when compiling for
>> >> >    profiling or when -fno-omit-frame-pointer is used since asm ("ebp")
>> >> >    can't be used to put the 6th argument in %ebp for syscall.  */
>> >> > #if !defined PROF && CAN_USE_REGISTER_ASM_EBP
>> >> > # define OPTIMIZE_FOR_GCC_5
>> >> > #endif
>> >> >
>> >> > If we want to support profiling or -fno-omit-frame-pointer,
>> >> > we need to keep these codes.
>> >>
>> >> This is strictly for %ebp, I think, as indicated by the comment.  %ebx
>> >> does not need to be special-cased for profiling.
>> >>
>> >> I assume that building glibc (with profiling) is proof enough that this
>> >> works?
>> >
>> > You need to test glibc build with -fno-omit-frame-pointer.
>>
>> I tried it, it works (with both patches applied).
>> -fno-omit-frame-pointer is about %ebp, not %ebx.
>>
>> Thanks,
>> Florian
>>
>
> Do they have to be in glibc 2.35?  If not, I prefer to do it after
> 2.35 is branched.

We can't build glibc 2.35 with GCC 12 in Fedora without these changes.

The %ebp avoidance I can activate with a configure argument, but the
macros for optimized %ebx usage are incompatible with .altmacro, as used
by the current Systemtap probes.

Thanks,
Florian