[Bug rtl-optimization/80770] New: suboptimal code negating a 1-bit _Bool field

["msebor at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80770

            Bug ID: 80770
           Summary: suboptimal code negating a 1-bit _Bool field
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

For all targets I tried (powerpc64, sparcv9, and x86_64), GCC emits worse code
for the negation of a 1-bit _Bool field than it does for the equivalent _Bool
(non-bit) field.  The following test case shows the difference.  The code for
fb1 should look the same as the code for fb2.  A _Bool bit-field is typically
used as a (space) optimization so emitting bigger and slower code defeats its
sole purpose.

Clang emits the same code for both function for the same targets.

$ cat a.c && gcc -O2 -S -Wall -Wextra -Wconversion -o/dev/stdout a.c
struct S {
  _Bool b1: 1;
  _Bool b;
};

void fb1 (struct S *s)
{
  s->b1 = !s->b1;
}

void fb (struct S *s)
{
  s->b = !s->b;
}

        .file   "a.c"
        .text
        .p2align 4,,15
        .globl  fb1
        .type   fb1, @function
fb1:
.LFB0:
        .cfi_startproc
        movzbl  (%rdi), %eax
        movl    %eax, %edx
        andl    $-2, %eax
        andl    $1, %edx
        xorl    $1, %edx
        orl     %edx, %eax
        movb    %al, (%rdi)
        ret
        .cfi_endproc
.LFE0:
        .size   fb1, .-fb1
        .p2align 4,,15
        .globl  fb
        .type   fb, @function
fb:
.LFB1:
        .cfi_startproc
        xorb    $1, 1(%rdi)
        ret
        .cfi_endproc
.LFE1:
        .size   fb, .-fb
        .ident  "GCC: (GNU) 7.0.1 20170418 (experimental)"
        .section        .note.GNU-stack,"",@progbits
tmp$