[Bug middle-end/121593] Miscompiles small function to produce endless(?) loop

[pinskia at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121593

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |INVALID
             Status|UNCONFIRMED                 |RESOLVED

--- Comment #16 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
This is NOT a GCC bug.

`char*` and `qchar8_t *` don't alias according to the C++ aliasing rules.

So:
            auto* &i = reinterpret_cast<const qchar8_t *&>(it);


and then incrrementing i will not work correctly.

This is why -fno-strict-aliasing works.

So the correct fix is:
            constexpr char32_t invalidValue = 0xFFFFFFFF;
            static_assert(invalidValue > QChar::LastValidCodePoint);
            const qchar8_t *i = (const qchar8_t *)it;
            const qchar8_t *e = (const qchar8_t *)end;
            const char32_t result = QUtf8Functions::nextCharacterFromUtf8(i, e,
invalidValue);
            it = (const char *)i;
            end = (const char *)e;
            return result == invalidValue ? NextResult{U'\0', true}
                                          : NextResult{result, false};