Re: [PATCH v2] Place ENTRY_POINT in .text.unlikely section [BZ #28153]

["H.J. Lu via Libc-alpha" <libc-alpha@sourceware.org>]

On Sat, Jul 31, 2021 at 10:06 AM H.J. Lu <hjl.tools@gmail.com> wrote:
>
> On Sat, Jul 31, 2021 at 9:36 AM Florian Weimer <fweimer@redhat.com> wrote:
> >
> > * H. J. Lu via Libc-alpha:
> >
> > > diff --git a/sysdeps/aarch64/start.S b/sysdeps/aarch64/start.S
> > > index 417da8802b..e46e01ed0b 100644
> > > --- a/sysdeps/aarch64/start.S
> > > +++ b/sysdeps/aarch64/start.S
> > > @@ -42,7 +42,7 @@
> > >                                       NULL
> > >   */
> > >
> > > -     .text
> > > +     .section .text.unlikely,"ax",%progbits
> > >  ENTRY(_start)
> > >       /* Create an initial frame with 0 LR and FP */
> > >       cfi_undefined (x30)
> >
> > I don't think it's correct to place code that runs during every process
> > start into .text.unlikely.  Surely we can avoid that page fault.
> >
> > Can we fix the ENTRY_POINT assumption in profiling instead?
>
> We can do
>
> diff --git a/csu/gmon-start.c b/csu/gmon-start.c
> index b3432885b3..83322fd586 100644
> --- a/csu/gmon-start.c
> +++ b/csu/gmon-start.c
> @@ -48,7 +48,7 @@
>  #ifdef ENTRY_POINT_DECL
>  ENTRY_POINT_DECL(extern)
>  #else
> -extern char ENTRY_POINT[];
> +extern char entry_point[] asm (__SYMBOL_PREFIX "main");
>  #endif
>  extern char etext[];
>
> @@ -56,7 +56,7 @@ extern char etext[];
>  # ifdef ENTRY_POINT_DECL
>  #  define TEXT_START ENTRY_POINT
>  # else
> -#  define TEXT_START &ENTRY_POINT
> +#  define TEXT_START &entry_point
>  # endif
>  #endif
>
> But this may only work with BFD linker which places .text.startup
> section before .text section.

Another option is to place _start in .text.startup which leaves

    *(.text.unlikely .text.*_unlikely .text.unlikely.*)
    *(.text.exit .text.exit.*)

sections out of profiling records.

-- 
H.J.