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

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

On Wed, Jan 12, 2022 at 12:12 PM Florian Weimer <fweimer@redhat.com> wrote:
>
> * H. J. Lu:
>
> > On Wed, Jan 12, 2022 at 11:37 AM Florian Weimer <fweimer@redhat.com> wrote:
> >>
> >> * 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.
> >
> > Please open a glibc bug with a reproducer and CC me.
>
> Done: <https://sourceware.org/bugzilla/show_bug.cgi?id=28771>
>

OPTIMIZE_FOR_GCC_5 enables 2 things: one is let GCC handle
-fPIC and the other is use

register int reg asm ("ebp")

which is unreliable.   You want to let GCC handle -fPIC and
remove CAN_USE_REGISTER_ASM_EBP.  If this is correct,
please combine 2 patches into one and reference BZ #28771
in the commit log.

Thanks.

-- 
H.J.