Re: [PATCH 0/6] Conversion of gimple types to C++ inheritance (v3)

[David Malcolm <dmalcolm@redhat.com>]

On Tue, 2013-11-05 at 12:47 +0100, Richard Biener wrote:
> On Mon, Nov 4, 2013 at 10:43 PM, David Malcolm <dmalcolm@redhat.com> wrote:
> > On Mon, 2013-11-04 at 08:19 -0500, Andrew MacLeod wrote:
> >> On 11/01/2013 06:58 PM, David Malcolm wrote:
> >> > On Fri, 2013-11-01 at 22:57 +0100, Jakub Jelinek wrote:
> >> >> On Fri, Nov 01, 2013 at 05:47:14PM -0400, Andrew MacLeod wrote:
> >> >>> On 11/01/2013 05:41 PM, Jakub Jelinek wrote:
> >> >>>> On Fri, Nov 01, 2013 at 05:36:34PM -0400, Andrew MacLeod wrote:
> >> >>>>>    static inline void
> >> >>>>> ! gimple_call_set_lhs (gimple gs, tree lhs)
> >> >>>>>    {
> >> >>>>> -   GIMPLE_CHECK (gs, GIMPLE_CALL);
> >> >> The checking you are removing here.
> >> >>
> >> >>> What checking?  There ought to be no checking at all in this
> >> >>> example...  gimple_build_call_vec returns a gimple_call, and
> >> >>> gimple_call_set_lhs()  doesn't have to check anything because it
> >> >>> only accepts gimple_call's.. so there is no checking other than the
> >> >>> usual "does my parameter match" that the compiler has to do...
> >> >> and want to replace it by checking of the types at compile time.
> >> >> The problem is that it uglifies the source too much, and, when you
> >> >> actually don't have a gimple_call but supposedly a base class of it,
> >> >> I expect you'd do as_a which is not only further uglification, but has
> >> >> runtime cost also for --enable-checking=release.
> >> > I can have a look next week at every call to gimple_call_set_lhs in the
> >> > tree, and see to what extent we know at compile-time that the initial
> >> > arg is indeed a call (of the ones I quickly grepped just now, most are
> >> > from gimple_build_call and friends, but one was from a gimple_copy).
> >> >
> >> > FWIW I did some performance testing of the is_a/as_a code in the earlier
> >> > version of the patch, and it didn't have a noticable runtime cost
> >> > compared to the GIMPLE_CHECK in the existing code:
> >> > Size of compiler executable:
> >> > http://gcc.gnu.org/ml/gcc-patches/2013-08/msg01920.html
> >> > Compile times:
> >> > http://gcc.gnu.org/ml/gcc-patches/2013-09/msg00171.html
> >> I actually really dislike as_a<> and is_a<>, and  think code needs to be
> >> restructured rather than use them, other than possibly at the very
> >> bottom level when we're allocating memory or something like that, or
> >> some kind of emergency :-)...   If we require frequent uses of those,
> >> I'd be against it, I find them quite ugly.
> >>
> >> Like I said in the other reply, no rush, I don't think any of this
> >> follow up is appropriate this late in stage 1.  It would be more of an
> >> "interest" examination right now.. at least in my opinion...  I suspect
> >> thinks like gimple_assign are more complex cases, but without looking
> >> its hard to tell for sure.
> >
> > I tried converting gimple_call_set_lhs to accept a gimple_call, rather
> > than a gimple, and excitingly, it was easiest to also convert
> > cgraph_edge's call_stmt to also be a gimple_call, rather than just a
> > gimple.
> 
> Does that work (using gimple_call * objects) for our garbage collector?
> That is, does it know it is looking at a 'gimple'?  It doesn't matter for this
> case as all stmts are reachable from struct function as sequence of 'gimple',
> but in general?
Yes (as of r204146, I believe).

For example, the patch converts cgraph_edge's call_stmt to be a
"gimple_call", rather than just a "gimple", but gengtype handles this by
emitting a call to the base class visitor for call_stmt i.e.
gimple_statement_base:

void
gt_ggc_mx_cgraph_edge (void *x_p)
{
  struct cgraph_edge * x = (struct cgraph_edge *)x_p;
  [...snip chain_next/prev handling...]
  [...snip other fields...]
      gt_ggc_m_21gimple_statement_base ((*x).call_stmt);
  [..etc..]
}