On Tue, Jul 7, 2020 at 6:03 AM Hans-Peter Nilsson via Gcc-patches <gcc-patches@gcc.gnu.org> wrote: > > We say very little about reads and writes to aggregate / > compound objects, just scalar objects (i.e. assignments don't > cause reads). Let's lets say something safe about aggregate > objects, but only for those that are the same size as a scalar > type. > > There's an equal-sounding section (Volatiles) in extend.texi, > but this seems a more appropriate place, as specifying the > behavior of a standard qualifier.
Hmm, might be true only up to word-mode size, not, say, __int128_t. Also very likely only in case the object has the same alignment as the naturally aligned integer type. Richard. > gcc: > PR middle-end/94600 > * doc/implement-c.texi (Qualifiers implementation): Add blurb > about access to the whole of a volatile aggregate object, only for > same-size as scalar object. > --- > gcc/doc/implement-c.texi | 4 ++++ > 1 file changed, 4 insertions(+) > > diff --git a/gcc/doc/implement-c.texi b/gcc/doc/implement-c.texi > index 692297b69c4..d64922b28ad 100644 > --- a/gcc/doc/implement-c.texi > +++ b/gcc/doc/implement-c.texi > @@ -576,6 +576,10 @@ are of scalar types, the expression is interpreted by > GCC as a read of > the volatile object; in the other cases, the expression is only evaluated > for its side effects. > > +When an object of aggregate type has the same size as a scalar type, GCC > +handles an access to the whole of that volatile aggregate type object > +equal to an access to that volatile same-sized scalar type object. > + > @end itemize > > @node Declarators implementation > -- > 2.11.0 >
