[Bug ada/82813] warning: '.builtin_memcpy' writing between 2 and 6 bytes into a region of size 0 overflows the destination [-Wstringop-overflow=]

[ebotcazou at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82813

--- Comment #2 from Eric Botcazou <ebotcazou at gcc dot gnu.org> ---
It's unreachable code:

  <bb 56> [0.23%] [count: INV]:
  # DEBUG S117b.276 => &D.9593
  .builtin_memcpy (&MEM[(void *)&D.9593], pretmp_305, _250);

The only predecessor is:

  <bb 55> [0.32%] [count: INV]:
  L113b_224 = start_130 + _322;
  # DEBUG L114b => 1
  # DEBUG make_util__ensure_absolute_path__B_4__L_5__TTS117bSP1___L => 1
  # DEBUG make_util__ensure_absolute_path__B_4__L_5__TTS117bSP1___U =>
L113b_224
  # DEBUG D.7306 => 1
  # DEBUG iftmp.271 => 1 + 18446744073709551615
  if (L113b_224 != 0)
    goto <bb 54>; [64.00%]
  else
    goto <bb 56>; [36.00%]

whose only predecessor is:

  <bb 23> [0.63%] [count: INV]:
  _37 = parent$P_BOUNDS_1->UB0;
  _38 = parent$P_BOUNDS_1->LB0;
  _330 = iftmp.245_24 - start_130;
  _322 = _330 + 1;
  _250 = (sizetype) j_231;
  pretmp_305 = &MEM[(void *)sw.246_6][1 ...]{lb: 1 sz: 1};
  if (_37 >= _38)
    goto <bb 24>; [50.00%]
  else
    goto <bb 55>; [50.00%]

So <bb 56> is reachable only if parent$P_BOUNDS_1->UB0 < parent$P_BOUNDS_1->LB0
is true.

But this condition is caught by the block just above:

                  if Parent'Length = 0 then
                     Do_Fail
                       ("relative search path switches ("""
                        & Sw
                        & """) are not allowed");

  <bb 17> [7.26%] [count: INV]:
  _19 = parent$P_BOUNDS_1->LB0;
  _20 = parent$P_BOUNDS_1->UB0;
  if (_19 > _20)
    goto <bb 18>; [33.00%]
  else
    goto <bb 19>; [67.00%]

<bb 19> is a dominator of <bb 23> so <bb 23> is reachable only if
parent$P_BOUNDS_1->LB0 > parent$P_BOUNDS_1->UB0 is false, which is the opposite
of the above condition, so <bb 56> is not reachable.