On Tue, 2014-11-11 at 11:43 +0100, Richard Biener wrote:
> On Tue, Nov 11, 2014 at 8:26 AM, Jakub Jelinek <ja...@redhat.com> 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.
> > 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 see two ways out here. One is to add overloads to all the functions
> taking the special types like
>
> tree
> gimple_assign_rhs1 (gimple *);
>
> or simply add
>
> gassign *operator ()(gimple *g) { return as_a <gassign *> (g); }
>
> into a gimple-compat.h header which you include in places that
> are not converted "nicely".
Thanks for the suggestions.
Am I missing something, or is the gimple-compat.h idea above not valid C
++?
Note that "gimple" is still a typedef to
gimple_statement_base *
(as noted before, the gimple -> gimple * change would break everyone
else's patches, so we talked about that as a followup patch for early
stage3).
Given that, if I try to create an "operator ()" outside of a class, I
get this error:
‘gassign* operator()(gimple)’ must be a nonstatic member function
which is emitted from cp/decl.c's grok_op_properties:
/* An operator function must either be a non-static member function
or have at least one parameter of a class, a reference to a class,
an enumeration, or a reference to an enumeration. 13.4.0.6 */
I tried making it a member function of gimple_statement_base, but that
doesn't work either: we want a conversion
from a gimple_statement_base * to a gassign *, not
from a gimple_statement_base to a gassign *.
Is there some syntactic trick here that I'm missing? Sorry if I'm being
dumb (I can imagine there's a way of doing it by making "gimple" become
some kind of wrapped ptr class, but that way lies madness, surely).
> Both avoid manually making the compiler happy (which the
> explicit as_a<> stuff is! It doesn't add any "checking" - it's
> just placing the as_a<> at the callers and thus make the
> runtine ICE fire there).
>
> As much as I don't like "global" conversion operators I don't
> like adding overloads to all of the accessor functions even more.
(nods)
Some other options:
Option 3: only convert the "easy" accessors: the ones I already did in
the /89 patch kit, as reviewed by Jeff, and rebased by me recently,
which is this 92-patch kit:
"[gimple-classes, committed 00/92] Initial slew of commits":
https://gcc.gnu.org/ml/gcc-patches/2014-10/msg02791.html
Doing so converts about half of the gimple_foo_ accessors to taking a
gfoo *, giving a mixture of GIMPLE_CHECK vs subclass use. I believe
the quality of those patches was higher than the later ones on the
branch: I was doing the places that didn't require the invasive/verbose
changes seen in the later patches. Shelve the remaining ~80
increasingly ugly patches, starting a new branch to contain just the
good ones.
Option 4: don't convert any accessors, but instead focus on fields of
structs (e.g. "call_stmt" within a cgraph_edge), and on params of other
functions (e.g. phi-manipulation code). That way we'd avoid the
inconsistency of some accessors using GIMPLE_CHECK and some using
subclasses - all would continue to consistently use GIMPLE_CHECK, but
there would be some extra type-checking and self-documentation of the
expected statement kinds in the code.
FWIW, option 3 is my preferred approach (I've already done the bulk of
the work and it's already been reviewed; it would need an update merger
from trunk, and there's the big gimple to gimple * fixup you wanted).
> Whether you enable them generally or just for selected files
> via a gimple-compat.h will be up to you (but I'd rather get
> rid of them at some point).
>
> Note this allows seamless transform of "random" functions
> taking a gimple now but really only expecting a single kind.
>
> Note that we don't absolutely have to rush this all in for GCC 5.
> Being the very first for GCC 6 stage1 is another possibility.
> We just should get it right.
Thanks
Dave
