> On 6/19/19 1:53 PM, Jan Hubicka wrote:
> > > > > - ctype = CLASSTYPE_AS_BASE (ctype);
> > > > > + {
> > > > > + if (!tree_int_cst_equal (TYPE_SIZE (ctype),
> > > > > + TYPE_SIZE (CLASSTYPE_AS_BASE (ctype))))
> > > > > + ctype = CLASSTYPE_AS_BASE (ctype);
> > > > > + }
> > > > > tree clobber = build_clobber (ctype);
> > >
> > > I have noticed we build a distinct as-base type in rather more cases than
> > > strictly necessary. For instance when there's a member of reference type
> > > or
> > > we have a non-trivial dtor. (CLASSTYPE_NON_LAYOUT_POD_P gets set by a
> > > bunch
> > > of things that don't affect ABI layout)
> >
> > Avoiding the extra copies at first place would be great. In my
> > understanding the types differ by virtual bases and also by their size
> > since the fake types are not padded to multiply of their alignment.
> > I guess this can be tested ahead of producing the copy and saving some
> > memory...
> >
> > I am not sure if my C++ FE abilities are on par to implement this tough.
>
> I don't think it's simple to fix there, just unfortunate. your
> understanding is correct, and I think your workaround will work. However,
> remember it's possible for T == CLASSTYPE_AS_BASE (T), so might be worth
> checking that before doing the size comparison?
>
> It'd be great to comment on why you're not just using classtype_as_base
> there. I suppose I'm serializing this stuff too, with the same
> inefficiencies ...
Hi,
here is updated patch.
Bootstrapped/regtested x86_64-linux, OK?
It would be still nice to avoid copies at least in the commmon cases -
it is easy to generate many types via templates and having basically
every type twice is not very nice (we also copy all the fields, so
overall memory use can be large).
* decl.c (build_clobber_this): Do not use CLASSTYPE_AS_BASE
when possible.
Index: decl.c
===================================================================
--- decl.c (revision 272506)
+++ decl.c (working copy)
@@ -15229,7 +15229,20 @@ build_clobber_this ()
tree ctype = current_class_type;
if (!vbases)
- ctype = CLASSTYPE_AS_BASE (ctype);
+ {
+ /* When clobbering base type, we need to be careful to not clobber
+ extra padding at the end of structure or virtual bases, which are
+ not considered part of the base by the C++ ABI.
+
+ However try to avoid using CLASSTYPE_AS_BASE when possible because
+ typically this is the only use in the final intermediate language
+ where this type is needed. Doing so avoids need to stream many
+ duplciate type copies to LTO. */
+ if (ctype != CLASSTYPE_AS_BASE (ctype)
+ && !tree_int_cst_equal (TYPE_SIZE (ctype),
+ TYPE_SIZE (CLASSTYPE_AS_BASE (ctype))))
+ ctype = CLASSTYPE_AS_BASE (ctype);
+ }
tree clobber = build_clobber (ctype);
