On Tue, 8 Apr 2025 at 15:33, H.J. Lu <hjl.to...@gmail.com> wrote:
>
> On Tue, Apr 8, 2025 at 3:46 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 also ensures that -mnop-mcount works as expected when
> > emitting the PLT based profiling calls.
> >
> > Note that only 64-bit codegen is affected by this change or by the
> > commit referenced above; -m32 will yield 'call *mcount@GOT()' as before.
> >
> > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=119386
> >
> > Signed-off-by: Ard Biesheuvel <a...@kernel.org>
> >
> > gcc/ChangeLog:
> >
> > PR target/119386
> > * config/i386/i386.cc (x86_print_call_or_nop): Add @PLT suffix
> > where appropriate.
> > (x86_function_profiler): Fall through to x86_print_call_or_nop()
> > for PIC codegen when flag_plt is set.
> >
> > gcc/testsuite/ChangeLog:
> >
> > PR target/119386
> > * gcc.target/i386/pr119386-1.c: New test.
> > * gcc.target/i386/pr119386-2.c: New test.
> > ---
> > gcc/config/i386/i386.cc | 8 +++++++-
> > gcc/testsuite/gcc.target/i386/pr119386-1.c | 11 +++++++++++
> > gcc/testsuite/gcc.target/i386/pr119386-2.c | 11 +++++++++++
> > 3 files changed, 29 insertions(+), 1 deletion(-)
> >
> > diff --git a/gcc/config/i386/i386.cc b/gcc/config/i386/i386.cc
> > index be5e27fc391..0b238c3dddc 100644
> > --- a/gcc/config/i386/i386.cc
> > +++ b/gcc/config/i386/i386.cc
> > @@ -23154,6 +23154,12 @@ 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)
> > + {
> > + gcc_assert (flag_plt);
> > +
> > + fprintf (file, "1:\tcall\t%s@PLT\n", target);
> > + }
> > else
> > fprintf (file, "1:\tcall\t%s\n", target);
> > }
> > @@ -23317,7 +23323,7 @@ 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)
> > {
> > if (ASSEMBLER_DIALECT == ASM_INTEL)
> > fprintf (file, "1:\tcall\t[QWORD PTR
> > %s@GOTPCREL[rip]]\n",
> > diff --git a/gcc/testsuite/gcc.target/i386/pr119386-1.c
> > b/gcc/testsuite/gcc.target/i386/pr119386-1.c
> > new file mode 100644
> > index 00000000000..7930fc6f28d
> > --- /dev/null
> > +++ b/gcc/testsuite/gcc.target/i386/pr119386-1.c
> > @@ -0,0 +1,11 @@
> > +/* PR target/119386 */
> > +/* { dg-do compile { target *-*-linux* } } */
> > +/* { dg-require-effective-target lp64 } */
>
> Can this be dropped?
>
I copied that from another test, assuming it would limit the testing
to x86_64. Is there a better way to achieve that?
> > +/* { dg-options "-O2 -fpic -pg" } */
> > +/* { dg-final { scan-assembler "mcount@PLT" } } */
> > +
> > +int
> > +main ()
> > +{
> > + return 0;
> > +}
> > diff --git a/gcc/testsuite/gcc.target/i386/pr119386-2.c
> > b/gcc/testsuite/gcc.target/i386/pr119386-2.c
> > new file mode 100644
> > index 00000000000..6334b9b9072
> > --- /dev/null
> > +++ b/gcc/testsuite/gcc.target/i386/pr119386-2.c
> > @@ -0,0 +1,11 @@
> > +/* PR target/119386 */
> > +/* { dg-do compile { target *-*-linux* } } */
> > +/* { dg-require-effective-target lp64 } */
>
> Can this be dropped?
>
> > +/* { dg-options "-O2 -fpic -fno-plt -pg" } */
> > +/* { dg-final { scan-assembler "mcount@GOTPCREL" } } */
>
> Different scans for ia32 and ! ia32?
>
I did not consider IA32 at all - can we just omit it?
