[Bug tree-optimization/99918] New: suboptimal code for bool bitfield tests

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99918

            Bug ID: 99918
           Summary: suboptimal code for bool bitfield tests
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

GCC does a better job folding operations involving plain Booleans than it does
with bool bit-fields.  The example below shows that in f() the return statement
is folded to zero while in g() it's not.  This is behind a class of
-Wmaybe-uninitialized warnings.

$ cat z.c && gcc -O2 -S -Wall -fdump-tree-optimized=/dev/stdout z.c
struct A { _Bool i, j; };

_Bool f (struct A a)
{
  if (a.i)
    a.j = 0;
  else
    a.j = a.i;

  return a.j;    // folded to 0
}

struct B { _Bool i: 1, j: 1; };

_Bool g (struct B b)
{
  if (b.i)
    b.j = 0;
  else
    b.j = b.i;

  return b.j;    // not folded
}


;; Function f (f, funcdef_no=0, decl_uid=1946, cgraph_uid=1, symbol_order=0)

_Bool f (struct A a)
{
  <bb 2> [local count: 1073741824]:
  return 0;

}



;; Function g (g, funcdef_no=1, decl_uid=1953, cgraph_uid=2, symbol_order=1)

Removing basic block 5
_Bool g (struct B b)
{
  _Bool b$j;
  unsigned char _1;
  unsigned char _2;
  _Bool _3;

  <bb 2> [local count: 1073741824]:
  _1 = VIEW_CONVERT_EXPR<unsigned char>(b);
  _2 = _1 & 1;
  if (_2 != 0)
    goto <bb 4>; [50.00%]
  else
    goto <bb 3>; [50.00%]

  <bb 3> [local count: 536870913]:
  _3 = b.i;

  <bb 4> [local count: 1073741824]:
  # b$j_5 = PHI <0(2), _3(3)>
  return b$j_5;

}