https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85662
Jakub Jelinek <jakub at gcc dot gnu.org> changed:
What |Removed |Added
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CC| |jason at gcc dot gnu.org
--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
In any case, I think the problem is related to the delayed folding, C++
constexpr handling not liking pointer constants and fold_offsetof_1 done as
pointer addition rather than integer addition. With delayed folding and
fold_build_pointer_plus called by fold_offsetof_1 the nested expressions aren't
really folded. We could easily fix it up by doing a cp_fold (together with
recursive cp_fold_r though, because cp_fold isn't fully recursive) on the
result of fold_offsetof in the C++ FE, that way we'd get a constant whenever
possible.
On the other side, we wouldn't then reject __builtin_offsetof used with not
valid constexpr expressions in the second argument (say out of bound array
access etc. subtracted from itself).
Say:
constexpr int a[5];
struct S { int b, c[5]; };
constexpr int d = __builtin_offsetof (S, c[(&a[6] - &a[6]) + 2]);
Another possibility is in fold_offsetof_1, if we detect the TREE_CONSTANT base,
if it is a pointer typed INTEGER_CST, build corresponding sizetype INTEGER_CST
instead and use normal PLUS_EXPR folding (or do we actually want any folding at
all?) instead of fold_build_pointer_plus.
