On Wed, Apr 9, 2025 at 1:53 AM Ard Biesheuvel <[email protected]> wrote:
>
> From: Ard Biesheuvel <[email protected]>
>
> 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 <[email protected]>
>
> 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 | 12 ++++++++++++
> 3 files changed, 30 insertions(+), 1 deletion(-)
> create mode 100644 gcc/testsuite/gcc.target/i386/pr119386-1.c
> create mode 100644 gcc/testsuite/gcc.target/i386/pr119386-2.c
>
> 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..174d00f1e27
> --- /dev/null
> +++ b/gcc/testsuite/gcc.target/i386/pr119386-1.c
> @@ -0,0 +1,11 @@
> +/* PR target/119386 */
> +/* { dg-do compile { target *-*-linux* } } */
> +/* { dg-options "-O2 -fpic -pg" } */
> +/* { dg-final { scan-assembler "call\[ \t\]mcount@PLT" { target { ! ia32 } }
> } } */
> +/* { dg-final { scan-assembler "call\[ \t\]\\*mcount@GOT\\(" { target ia32 }
> } } */
This is wrong for ia32, which should also generate "call 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..808dd9298ce
> --- /dev/null
> +++ b/gcc/testsuite/gcc.target/i386/pr119386-2.c
> @@ -0,0 +1,12 @@
> +/* PR target/119386 */
> +/* { dg-do compile { target *-*-linux* } } */
> +/* { dg-options "-O2 -fpic -fno-plt -pg" } */
> +/* { dg-final { scan-assembler "call\[ \t\]\\*mcount@GOTPCREL\\(" { target {
> ! ia32 } } } } */
> +/* { dg-final { scan-assembler "call\[ \t\]\\*mcount@GOT\\(" { target ia32 }
> } } */
> +
> +
> +int
> +main ()
> +{
> + return 0;
> +}
> --
> 2.49.0.504.g3bcea36a83-goog
>
--
H.J.
