[Bug tree-optimization/102631] -Wmaybe-uninitialized cannot see through a series of PHIs

[aldyh at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102631

--- Comment #3 from Aldy Hernandez <aldyh at gcc dot gnu.org> ---
(In reply to Aldy Hernandez from comment #2)
> Created attachment 51562 [details]
> similar problem on aarch64 bootstrap

$ ./cc1plus calls-aarch64.ii -O2 -quiet -Wall
In function ‘void mark_stack_region_used(poly_uint64, poly_uint64)’,
    inlined from ‘rtx_def* emit_library_call_value_1(int, rtx, rtx,
libcall_type, machine_mode, int, rtx_mode_t*)’ at
/home/aldyh/src/gcc/gcc/calls.c:4536:29:
/home/aldyh/src/gcc/gcc/calls.c:206:26: warning: ‘const_upper’ may be used
uninitialized in this function [-Wmaybe-uninitialized]
  206 |       stack_usage_map[i] = 1;
      |       ~~~~~~~~~~~~~~~~~~~^~~
/home/aldyh/src/gcc/gcc/calls.c: In function ‘rtx_def*
emit_library_call_value_1(int, rtx, rtx, libcall_type, machine_mode, int,
rtx_mode_t*)’:
/home/aldyh/src/gcc/gcc/calls.c:202:30: note: ‘const_upper’ was declared here
  202 |   unsigned HOST_WIDE_INT const_lower, const_upper;
      |                              ^~~~~~~~~~~

As I've described here:

   https://gcc.gnu.org/pipermail/gcc-patches/2021-October/581045.html

If you take the calls.ii file from the aarch64 bootstrap and break on
the warning, you can see that the uninitalized use is for
const_upper_3934 here:

 <bb 102> [local count: 315357954]:
  # const_upper_3934 = PHI <const_upper_3937(D)(101), _6707(293)>
  if (_881 != 0)
    goto <bb 103>; [50.00%]
  else
    goto <bb 106>; [50.00%]

  <bb 103> [local count: 157678977]:
  if (const_upper_3934 > _6699)
    goto <bb 105>; [89.00%]
  else
    goto <bb 294>; [11.00%]

  <bb 294> [local count: 17344687]:

  <bb 104> [local count: 157678977]:
  goto <bb 107>; [100.00%]

  <bb 105> [local count: 140334290]:
  stack_usage_map.481_3930 = stack_usage_map;
  _6441 = const_upper_3934 - _6699;
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
PROBLEMATIC READ HERE
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  _4819 = stack_usage_map.481_3930 + _6699;
  __builtin_memset (_4819, 1, _6441);
  goto <bb 104>; [11.00%]

const_upper_3934 could be undefined if it comes from BB101
(const_upper_3937(D)), but it only gets read for _881 != 0, so it
shouldn't warn.

This looks very similar.

The source is here, which is obviously properly guarded:

static void
mark_stack_region_used (poly_uint64 lower_bound, poly_uint64 upper_bound)
{
  unsigned HOST_WIDE_INT const_lower, const_upper;
  const_lower = constant_lower_bound (lower_bound);
  if (upper_bound.is_constant (&const_upper))
    for (unsigned HOST_WIDE_INT i = const_lower; i < const_upper; ++i)
      stack_usage_map[i] = 1;
  else
    stack_usage_watermark = MIN (stack_usage_watermark, const_lower);
}