On Fri, 4 Apr 2025, Ard Biesheuvel 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 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 <[email protected]>
>
> gcc/ChangeLog:
>
> * config/i386/i386.cc (x86_function_profiler): Take
> ix86_direct_extern_access into account when generating calls
> to __fentry__()
Wrong changelog (reused from the earlier patch?)
The entry should mention PR category/number.
> --- 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);
I guess this is written this way because the caller is supposed to handle the
!flat_plt case, but then this should be an assert:
else if (!TARGET_PECOFF && flag_pic)
{
gcc_assert (flat_plt);
fprintf (file, "1:\tcall\t%s@PLT\n", target);
}
Note that this patch touches only 64-bit codegen, -m32 will yield
call *mcount@GOT as before (the previous patch also touched only 64-bit case).
Alexander
> 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)
> {
> if (ASSEMBLER_DIALECT == ASM_INTEL)
> fprintf (file, "1:\tcall\t[QWORD PTR %s@GOTPCREL[rip]]\n",
>
