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

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

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.

-- 
H.J.