[Bug target/128387] New: i386: conflicting cf_check and nocf_check attributes on the same function accepted without warning and drop ENDBR

[220245569 at seu dot edu.cn via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=128387

            Bug ID: 128387
           Summary: i386: conflicting cf_check and nocf_check attributes
                    on the same function accepted without warning and drop
                    ENDBR
           Product: gcc
           Version: 16.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: 220245569 at seu dot edu.cn
                CC: 220255623 at seu dot edu.cn, jianhao.xu at seu dot edu.cn
  Target Milestone: ---
            Target: x86_64-*-* i?86-*-*

Reproducer
----------
$ cat trigger.c
__attribute__((cf_check))
int __attribute__((nocf_check))
conflicting_attrs (int x)
{
  __asm__ volatile ("" : "+r" (x));
  return x + 1;
}

__attribute__((cf_check))
int control_cf_check (int x)
{
  __asm__ volatile ("" : "+r" (x));
  return x + 2;
}

Compile flag: x86_64-linux-gnu-gcc -O2 -fcf-protection=branch -mmanual-endbr
-Wall -Wextra -S trigger.c

conflicting_attrs:
.LFB0:
        .cfi_startproc
        leal    1(%rdi), %eax   # no warning emitted; endbr64 dropped
        ret
        .cfi_endproc

control_cf_check:
.LFB1:
        .cfi_startproc
        endbr64
        leal    2(%rdi), %eax
        ret
        .cfi_endproc

Root cause
----------
cf_check and nocf_check have opposite meanings, but cf_check is registered on
DECL_ATTRIBUTES (decl_required = true in ix86_gnu_attributes,
gcc/config/i386/i386-options.cc:4213) while nocf_check is registered on
TYPE_ATTRIBUTES (type_required = true in c_common_attribute_table,
gcc/c-family/c-attribs.cc:634).  Neither attribute handler checks the other
list, so applying both to the same function emits no -Wattributes diagnostic
and rest_of_insert_endbr_and_patchable_area (i386-features.cc:3014) drops
endbr64.

Suggested Fix
-------------
Diagnose the conflict between cf_check on DECL_ATTRIBUTES (decl) and
nocf_check on TYPE_ATTRIBUTES (TREE_TYPE (decl)) with a -Wattributes warning.