Re: [gimple-classes, committed 4/6] tree-ssa-tail-merge.c: Use gassign

[David Malcolm <dmalcolm at redhat dot com>]

On Tue, 2014-11-11 at 08:26 +0100, Jakub Jelinek wrote:
> On Mon, Nov 10, 2014 at 05:27:50PM -0500, David Malcolm wrote:
> > On Sat, 2014-11-08 at 14:56 +0100, Jakub Jelinek wrote:
> > > On Sat, Nov 08, 2014 at 01:07:28PM +0100, Richard Biener wrote:
> > > > To be constructive here - the above case is from within a
> > > > GIMPLE_ASSIGN case label
> > > > and thus I'd have expected
> > > > 
> > > >     case GIMPLE_ASSIGN:
> > > >       {
> > > >         gassign *a1 = as_a <gassign *> (s1);
> > > >         gassign *a2 = as_a <gassign *> (s2);
> > > >       lhs1 = gimple_assign_lhs (a1);
> > > >       lhs2 = gimple_assign_lhs (a2);
> > > >       if (TREE_CODE (lhs1) != SSA_NAME
> > > >           && TREE_CODE (lhs2) != SSA_NAME)
> > > >         return (operand_equal_p (lhs1, lhs2, 0)
> > > >                 && gimple_operand_equal_value_p (gimple_assign_rhs1 (a1),
> > > >                                                  gimple_assign_rhs1 (a2)));
> > > >       else if (TREE_CODE (lhs1) == SSA_NAME
> > > >                && TREE_CODE (lhs2) == SSA_NAME)
> > > >         return vn_valueize (lhs1) == vn_valueize (lhs2);
> > > >       return false;
> > > >       }
> > > > 
> > > > instead.  That's the kind of changes I have expected and have approved of.
> > > 
> > > But even that looks like just adding extra work for all developers, with no
> > > gain.  You only have to add extra code and extra temporaries, in switches
> > > typically also have to add {} because of the temporaries and thus extra
> > > indentation level, and it doesn't simplify anything in the code.
> > 
> > The branch attempts to use the C++ typesystem to capture information
> > about the kinds of gimple statement we expect, both:
> >   (A) so that the compiler can detect type errors, and
> >   (B) as a comprehension aid to the human reader of the code
> > 
> > The ideal here is when function params and struct field can be
> > strengthened from "gimple" to a subclass ptr.  This captures the
> > knowledge that every use of a function or within a struct has a given
> > gimple code.
> 
> I just don't like all the as_a/is_a stuff enforced everywhere,
> it means more typing, more temporaries, more indentation.
> So, as I view it, instead of the checks being done cheaply (yes, I think
> the gimple checking as we have right now is very cheap) under the
> hood by the accessors (gimple_assign_{lhs,rhs1} etc.), those changes
> put the burden on the developers, who has to check that manually through
> the as_a/is_a stuff everywhere, more typing and uglier syntax.
> I just don't see that as a step forward, instead a huge step backwards.
> But perhaps I'm alone with this.

I agree with you w.r.t. my later patches.   I like my initial set of 89
patches, but I overdid things with the attempt to convert all of the
gimple_assign_ accessors.

> Can you e.g. compare the size of - lines in your patchset combined, and
> size of + lines in your patchset?  As in, if your changes lead to less
> typing or more.

I don't know if this data matches the proposed interpretation
(autoindentation in emacs is wonderful), but here goes:

Diff of my current working copy vs last merge point, ignoring ChangeLog
additions, obtaining lines starting "-", piping into wc:

$ git diff fddbd0194b01f44c5b5f16379fd5405dcf6d71c0 $(git diff
fddbd0194b01f44c5b5f16379fd5405dcf6d71c0 | diffstat -lp1 | grep -v
ChangeLog) | grep -e "^-" | wc
   6169   31032  272916

Likewise for lines starting with "+":

$ git diff fddbd0194b01f44c5b5f16379fd5405dcf6d71c0 $(git diff
fddbd0194b01f44c5b5f16379fd5405dcf6d71c0 | diffstat -lp1 | grep -v
ChangeLog) | grep -e "^+" | wc
   7295   36566  309380

so the + lines have 13% more bytes than the - lines


Dave