https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109720
--- Comment #5 from Richard Biener <rguenth at gcc dot gnu.org> ---
We complain about the m_num_bits read. While PRE isolates it to BB3 even
before PRE that happens unconditionally:
<bb 2> [local count: 118111600]:
nbits ={v} {CLOBBER};
_25 = MEM[(const struct DynamicBitSet &)&set]._size;
nbits._size = _25;
if (_25 <= 64)
goto <bb 3>; [67.00%]
else
goto <bb 4>; [33.00%]
<bb 3> [local count: 79134772]:
_162 = MEM[(const struct DynamicBitSet &)&set].D.151332._fixedSizeBitSet;
nbits.D.151332._fixedSizeBitSet = _162;
goto <bb 13>; [100.00%]
...
<bb 12> [local count: 56691384]:
_46 = (long unsigned int) _40;
_47 = iftmp.2_49 + _46;
MEM[(struct vector *)&nbits + 8B].D.150855._M_impl.D.150172._M_finish = _47;
_29 = MEM[(const struct dynamic_bitset &)&set + 8].m_num_bits;
MEM[(struct dynamic_bitset *)&nbits + 8B].m_num_bits = _29;
_60 = boost::dynamic_bitset<>::m_do_find_from
(&nbits.D.151332._dynamicBitSet, 0);
if (_60 != 18446744073709551615)
goto <bb 13>; [66.92%]
else
goto <bb 33>; [33.08%]
<bb 13> [local count: 103222856]:
# _63 = PHI <_60(12), _25(3)>
_68 = MEM[(const struct dynamic_bitset *)&nbits + 8B].m_num_bits;
PRE just sees the partial redundancy on the edge from BB12.
The read is from dynamic_bitset<Block, Allocator>::to_ulong().
It _looks_ like that maybe the range-for handling code exposes this in a way
that doesn't properly short-cut this read (or that GCC short-circuits a
test there). Difficult to identify in the preprocessed sources.
