[Bug tree-optimization/119797] Incorrect Warning about Array Subscript out of Range
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=119797
Richard Biener changed:
What|Removed |Added
Status|UNCONFIRMED |NEW
Ever confirmed|0 |1
Last reconfirmed||2025-04-15
--- Comment #4 from Richard Biener ---
(In reply to Christoph Rüthing from comment #3)
> @Andrew, thank you for checking. You mean in case r >= 16 we hit the
> dedicated path where "if (r >= 16)" is true and in this branch we know that
> we will access out of bounds and thus issue the warning?
Yes.
> When further thinking about this it makes sense like this, still it is a
> little bit unintuitive for a user to see this. Especially, since the warning
> goes away when changing the ordering. With
>
> const uint32_t ce = BUG[r];
>
> const uint32_t s = Test<0U>(r);
> const uint32_t e = Test<1U>(r);
>
> const uint32_t cs = BUG[r];
>
> the warning goes away.
Here 'cs' is CSEd to 'ce' and thus the access is before the if()s which
means that jump threading does not isolate the case of BUG[r] when r
is known >= 16.
Jump threading basically results in array-bound warnings that are given
because you don't constrain the index to the size of the array. Similar
as to if we were to diagnose
uint32_t Bug(unt8_r r)
{
return BUG[r];
}
with a "warning: BUG[r] might access BUG out-of-bounds" which wouldn't
be very useful. In your case you do indirectly constrain the index,
and in one path you constrain it to be always out-of-bounds which is
why we warn.
We just fail to give you this extra information ...
So confirmed, but I'd say it's a duplicate of other instances of this kind.
[Bug tree-optimization/119797] Incorrect Warning about Array Subscript out of Range
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=119797 --- Comment #3 from Christoph Rüthing --- @Andrew, thank you for checking. You mean in case r >= 16 we hit the dedicated path where "if (r >= 16)" is true and in this branch we know that we will access out of bounds and thus issue the warning? When further thinking about this it makes sense like this, still it is a little bit unintuitive for a user to see this. Especially, since the warning goes away when changing the ordering. With const uint32_t ce = BUG[r]; const uint32_t s = Test<0U>(r); const uint32_t e = Test<1U>(r); const uint32_t cs = BUG[r]; the warning goes away.
[Bug tree-optimization/119797] Incorrect Warning about Array Subscript out of Range
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=119797 --- Comment #2 from Andrew Pinski --- There is a patch which should improve/fix this situtation proposed for GCC 16 (it was actually submitted a few months ago but reviews on it has been slow). Basically because of optimization (jump threading) we know in some cases r will be out of bounds. In Bug, either add: if (r >= std::size(BUG)) std::unreachable (); // or //__builtin_unreachable(); or something similar at the begining of the function to signal that r being greater than the size of the BUG array is invalid.
[Bug tree-optimization/119797] Incorrect Warning about Array Subscript out of Range
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=119797 Andrew Pinski changed: What|Removed |Added Component|c++ |tree-optimization Keywords||diagnostic --- Comment #1 from Andrew Pinski --- Note the warning is not exactly wrong just missing extra information on why it is being emitted in this case.
