On Wed, Feb 23, 2022 at 05:33:57PM +0000, Qing Zhao wrote:
> From my understanding, __builtin_clear_padding (&object), does not _use_ any
> variable,
> therefore, no uninitialized usage warning should be emitted for it.
__builtin_clear_padding (&object)
sometimes expands to roughly:
*(int *)((char *)&object + 32) = 0;
etc., in that case it shouldn't be suppressed in any way, it doesn't read
anything, only stores.
Or at other times it is:
*(int *)((char *)&object + 32) &= 0xfec7dab1;
etc., in that case it reads bytes from the object which can be
uninitialized, we mask some bits off and store.
It is similar to what object.bitfld = 3; expands to,
but usually only after the uninit pass. Though, we have the
optimize_bit_field_compare optimization, that is done very early
and I wonder what uninit does about that. Perhaps it ignores
BIT_FIELD_REFs, I'd need to check that.
Anyway, if we want to disable uninit warnings for __builtin_clear_padding,
we should do that with suppress_warning on the read stmts that load
a byte (or more adjacent ones) before they are masked off and stored again,
so that we don't warn about that.
Jakub
