Re: vector comparisons in C++

[Marc Glisse <marc dot glisse at inria dot fr>]

On Thu, 13 Sep 2012, Jason Merrill wrote:

> > > I don't know either.
> > >
> > > > +      if (TREE_TYPE (type0) != TREE_TYPE (type1))
> > >
> > > I think this should use same_type_ignoring_top_level_qualifiers_p.
> >
> > Hmm, I assume you mean
> >
> > same_type_ignoring_top_level_qualifiers_p (type0, type1)
> >
> > which would replace both this test and
> >
> > TYPE_VECTOR_SUBPARTS (type0) != TYPE_VECTOR_SUBPARTS (type1)
> >
> > below?
>
> I was thinking just for the first test, but I suppose that would work too. 
> My concern is that vectors of typedefs of the same type need to be 
> compatible.

Oh, so you meant something like:

same_type_ignoring_top_level_qualifiers_p (TREE_TYPE (type0), TREE_TYPE (type1))

? I thought the element types of vectors were already kind of 
canonicalized. build_vector_type uses TYPE_MAIN_VARIANT (although it does 
handle the case where this is not its own TYPE_CANONICAL). And when I 
tried making a vector of T where T was a typedef for const int, I got a 
const vector of int instead, which I thought was pretty cool. But I agree 
that using same_type_ignoring_top_level_qualifiers_p is safer and can't 
hurt, plus it will keep the code close enough to the one in the C 
front-end.

While checking my facts for the previous paragraph, I got an ICE :-(

typedef int vec __attribute__((vector_size(16)));
vec const f(vec x,vec y){return x<y;}

cc.c:2:11: internal compiler error: in prepare_cmp_insn, at optabs.c:4176

The same program compiles with gcc (prepare_cmp_insn isn't called), but 
ICEs with g++. Looking at the 003t.original tree dump, the C one looks 
like:

  return VIEW_CONVERT_EXPR<const vec>(x < y);

while the C++ one looks like:

return <retval> = x < y ? { -1, -1, -1, -1 } : { 0, 0, 0, 0 };

or in raw form:

  gimple_cond <lt_expr, x, y, <D.2200>, <D.2201>>
  gimple_label <<D.2200>>
  gimple_assign <vector_cst, iftmp.0, { -1, -1, -1, -1 }, NULL, NULL>
  gimple_goto <<D.2202>>
  gimple_label <<D.2201>>
  gimple_assign <vector_cst, iftmp.0, { 0, 0, 0, 0 }, NULL, NULL>
  gimple_label <<D.2202>>
  gimple_assign <var_decl, D.2198, iftmp.0, NULL, NULL>
  gimple_return <D.2198>

That doesn't look very vector-like... I'll investigate before resending 
the patch.

> As Mike says, we want code that works in C to work in C++ too as much as 
> possible.  Furthermore, this builtin support would be useful for implementing 
> a C++ class for vector arithmetic, just as it is with std::complex.  I'm not 
> aware of any other portable way to implement such a class.

Ok, thanks to both of you.

--
Marc Glisse