[Bug driver/97469] New: __attribute__ ((__target__ ("..."))) resets -mcmodel= values, breaks grub compilation

[slyfox at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97469

            Bug ID: 97469
           Summary: __attribute__ ((__target__ ("..."))) resets -mcmodel=
                    values, breaks grub compilation
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: driver
          Assignee: unassigned at gcc dot gnu.org
          Reporter: slyfox at gcc dot gnu.org
  Target Milestone: ---

Bug initially observed as a build failure on grub-2.04. There 32-bit PLT
relocations get generated against external unresolved symbols when
zstd_decompress.c is compiled in -mcmode=large mode.

The trigger seems to be '__attribute__((__target__("bmi2")))' that throws away
'-mcmode=large' commandline flag.

Here is the minimal example extracted (gcc-10 generates correct 'movabs/call
%reg' code, gcc-11 generates incorrect 'call <symbol>' code):

$ bash -x ./mk.bash 
+ args=(-O1 -fno-asynchronous-unwind-tables -fno-ident -mcmodel=large -fno-PIE)
+ cat a.c
// grub builds zstd_decompress.c (zstd.module) in -mcmodel=large
// (and -freestanding) mode. grub assumes there will be no dynamic
// PLT relocations. grub_memmove() is symbol external (dynamic)
// to zstd.module.

extern void grub_memmove(void);

__attribute__((__target__("bmi2"))) void a_bmi(void) {
  // expect:
  //   movabsq $grub_memmove, %rbx
  //   call    *%rbx
  // actual (bug):
  //   call    grub_memmove
  for (;;)
    grub_memmove();
}

void a_nobmi(void) {
  // expect/actual:
  //   movabsq $grub_memmove, %rbx
  //   call    *%rbx
  for (;;)
    grub_memmove();
}
+ gcc-10.2.0 -O1 -fno-asynchronous-unwind-tables -fno-ident -mcmodel=large
-fno-PIE -S a.c
+ cat a.s
        .file   "a.c"
        .text
        .globl  a_bmi
        .type   a_bmi, @function
a_bmi:
        pushq   %rbx
        movabsq $grub_memmove, %rbx
.L2:
        call    *%rbx
        jmp     .L2
        .size   a_bmi, .-a_bmi
        .globl  a_nobmi
        .type   a_nobmi, @function
a_nobmi:
        pushq   %rbx
        movabsq $grub_memmove, %rbx
.L5:
        call    *%rbx
        jmp     .L5
        .size   a_nobmi, .-a_nobmi
        .section        .note.GNU-stack,"",@progbits
+ gcc-11.0.0 -O1 -fno-asynchronous-unwind-tables -fno-ident -mcmodel=large
-fno-PIE -S a.c
+ cat a.s
        .file   "a.c"
        .text
        .globl  a_bmi
        .type   a_bmi, @function
a_bmi:
        subq    $8, %rsp
.L2:
        call    grub_memmove
        jmp     .L2
        .size   a_bmi, .-a_bmi
        .globl  a_nobmi
        .type   a_nobmi, @function
a_nobmi:
        pushq   %rbx
        movabsq $grub_memmove, %rbx
.L5:
        call    *%rbx
        jmp     .L5
        .size   a_nobmi, .-a_nobmi
        .section        .note.GNU-stack,"",@progbits