https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77295
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |NEW
Last reconfirmed| |2016-08-19
Ever confirmed|0 |1
--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
Why does the implementation not simply copy the whole union with an assignment?
It's easier for GCC to optimize that.
Because with the current one you'll have
<bb 2>:
MEM[(struct &)this_7(D)] ={v} {CLOBBER};
_1 = x_9(D)->which;
this_7(D)->which = _1;
_2 = x_9(D)->which;
switch (_2) <default: <L2>, case 1: <L0>, case 2: <L1>>
<L0>:
_3 = x_9(D)->D.2254.v1;
this_7(D)->D.2254.v1 = _3;
goto <bb 5> (<L2>);
<L1>:
_4 = x_9(D)->D.2254.v2;
this_7(D)->D.2254.v2 = _4;
<L2>:
return;
where it's hard to argue the desired transform is profitable in general
(x->which might get known via inlining and if you'd optimize the above
earlier then you'd at least lose TBAA info on the stores / loads).
Also note sth else above - we read from x->which twice (this may point
to x). There's a related PR somewhere about this missed optimization,
with a patch (that has cost even if that special situation doesn't apply,
thus I didn't pursue it).
