https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109791
--- Comment #9 from Richard Biener <rguenth at gcc dot gnu.org> ---
So the first pass that makes things difficult is reassoc which transforms
_51 = (unsigned long) &MEM <char[3]> [(void *)&str + 2B];
_4 = (unsigned long) __i_44;
_65 = _51 - _4;
_48 = _65 + 18446744073709551615;
_46 = _48 > 13;
if (_46 != 0)
to
_51 = (unsigned long) &MEM <char[3]> [(void *)&str + 2B];
_4 = (unsigned long) __i_44;
_12 = -_4;
_119 = _51 + 18446744073709551615;
_48 = _119 - _4;
_46 = _48 > 13;
if (_46 != 0)
(also leaving garbage around). That's probably because of the PHI biasing
and __i_44 being defined by a PHI. niter analysis produces
# of iterations (((unsigned long) &MEM <char[3]> [(void *)&str + 2B] -
(unsigned long) __i_44) + 18446744073709551615) / 2, bounded by
9223372036854775807
but doesn't expand __i_44 (not sure if we'd fold the thing then). The
gimplifier folds stmts w/o following SSA edges when we eventually
re-gimplify those expressions for insertion. If we expand offsetting of
invariant bases we get instead
# of iterations ((unsigned long) &MEM <char[3]> [(void *)&str + 2B] - (unsigned
long) (&str + (_69 + 1))) / 2, bounded by 9223372036854775807
but that's still not simplified. I think ptr_difference_const should
handle this - of course the difference here isn't const...
/* Try folding difference of addresses. */
(simplify
(minus (convert ADDR_EXPR@0) (convert @1))
(if (tree_nop_conversion_p (type, TREE_TYPE (@0)))
(with { poly_int64 diff; }
(if (ptr_difference_const (@0, @1, &diff))
{ build_int_cst_type (type, diff); }))))
(simplify
(minus (convert @0) (convert ADDR_EXPR@1))
(if (tree_nop_conversion_p (type, TREE_TYPE (@0)))
(with { poly_int64 diff; }
(if (ptr_difference_const (@0, @1, &diff))
{ build_int_cst_type (type, diff); }))))
it works fine when adding
(simplify
(minus (convert ADDR_EXPR@0) (convert (pointer_plus @1 @2)))
(if (tree_nop_conversion_p (type, TREE_TYPE (@0)))
(with { poly_int64 diff; }
(if (ptr_difference_const (@0, @1, &diff))
(minus { build_int_cst_type (type, diff); } (convert @2))))))
then we get
# of iterations (1 - (unsigned long) _69) / 2, bounded by
9223372036854775807
and the diagnostic is gone.
