https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99258
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |jsm28 at gcc dot gnu.org,
| |rguenth at gcc dot gnu.org
Keywords| |wrong-code
Last reconfirmed| |2021-02-25
Ever confirmed|0 |1
Component|tree-optimization |middle-end
Status|UNCONFIRMED |NEW
--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
Confirmed. Happens already during gimplification which does
else if (COMPLETE_TYPE_P (TREE_TYPE (*expr_p))
&& TYPE_MODE (TREE_TYPE (*expr_p)) != BLKmode)
{
/* Historically, the compiler has treated a bare reference
to a non-BLKmode volatile lvalue as forcing a load. */
...
}
else
/* We can't do anything useful with a volatile reference to
an incomplete type, so just throw it away. Likewise for
a BLKmode type, since any implicit inner load should
already have been turned into an explicit one by the
gimplification process. */
*expr_p = NULL;
throwing away the load. GIMPLE indeed requires a LHS for a load and
we'd need to introduce a temporary aggregate here. Which should be
possible for non-VLA, non TREE_ADDRESSABLE types. Thus the condition
could be amended to not look at the types mode but verify its size is
constant and the type is not addressable (though the code explicitely
says a TREE_ADDRESSABLE type is OK). Note relaxing the mode check
to constant size can cause arbitrarily big temporaries being generated
on the stack so I'm not sure that's good. Alternatively open-coding
elementwise reads would be a possibility (for a load into a temporary
we might emit a memcpy call in the end).
Alternatively we could invent some internal function taking an aggregate
argument and emit
.USE (*x);
and leave the actual generation of the load to RTL expansion.
