On Sun, Apr 6, 2025 at 8:54 AM H.J. Lu <hjl.to...@gmail.com> wrote:
>
> On Fri, Apr 4, 2025 at 12:01 AM Ard Biesheuvel <ardb+...@google.com> wrote:
> >
> > From: Ard Biesheuvel <a...@kernel.org>
> >
> > Commit bde21de1205 ("i386: Honour -mdirect-extern-access when calling
> > __fentry__") updated the logic that emits mcount() / __fentry__() calls
> > into function prologues when profiling is enabled, to avoid GOT-based
> > indirect calls when a direct call would suffice.
> >
> > There are two problems with that change:
> > - it relies on -mdirect-extern-access rather than -fno-plt to decide
> > whether or not a direct [PLT based] call is appropriate;
> > - for the PLT case, it falls through to x86_print_call_or_nop(), which
> > does not emit the @PLT suffix, resulting in the wrong relocation to be
> > used (R_X86_64_PC32 instead of R_X86_64_PLT32)
> >
> > Fix this by testing flag_plt instead of ix86_direct_extern_access, and
> > updating x86_print_call_or_nop() to take flag_pic and flag_plt into
> > account. This ensures that -mnop-mcount works as expected when emitting
> > the PLT based profiling calls.
> >
> > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=119386
> >
> > Signed-off-by: Ard Biesheuvel <a...@kernel.org>
> >
> > gcc/ChangeLog:
> >
> > * config/i386/i386.cc (x86_function_profiler): Take
> > ix86_direct_extern_access into account when generating calls
> > to __fentry__()
> > ---
> > gcc/config/i386/i386.cc | 4 +++-
> > 1 file changed, 3 insertions(+), 1 deletion(-)
> >
> > diff --git a/gcc/config/i386/i386.cc b/gcc/config/i386/i386.cc
> > index be5e27fc391..6be6ddcbb3c 100644
> > --- a/gcc/config/i386/i386.cc
> > +++ b/gcc/config/i386/i386.cc
> > @@ -23154,6 +23154,8 @@ x86_print_call_or_nop (FILE *file, const char
> > *target)
> > if (flag_nop_mcount || !strcmp (target, "nop"))
> > /* 5 byte nop: nopl 0(%[re]ax,%[re]ax,1) */
> > fprintf (file, "1:" ASM_BYTE "0x0f, 0x1f, 0x44, 0x00, 0x00\n");
> > + else if (!TARGET_PECOFF && flag_pic && flag_plt)
> > + fprintf (file, "1:\tcall\t%s@PLT\n", target);
> > else
> > fprintf (file, "1:\tcall\t%s\n", target);
> > }
> > @@ -23317,7 +23319,7 @@ x86_function_profiler (FILE *file, int labelno
> > ATTRIBUTE_UNUSED)
> > break;
> > case CM_SMALL_PIC:
> > case CM_MEDIUM_PIC:
> > - if (!ix86_direct_extern_access)
> > + if (!flag_plt)
>
> Should ix86_force_load_from_GOT_p be extended to handle this?
Never mind. !flag_plt is appropriate here. ix86_direct_extern_access doesn't
apply to function reference.
>
> > {
> > if (ASSEMBLER_DIALECT == ASM_INTEL)
> > fprintf (file, "1:\tcall\t[QWORD PTR
> > %s@GOTPCREL[rip]]\n",
> > --
> > 2.49.0.504.g3bcea36a83-goog
> >
>
Tests are missing.
--
H.J.
