Re: RFC: C++ delayed folding merge

[Jason Merrill <jason at redhat dot com>]

On 11/09/2015 02:28 PM, Jason Merrill wrote:
> On 11/09/2015 04:08 AM, Richard Biener wrote:
> > On Mon, 9 Nov 2015, Jason Merrill wrote:
> >
> > > I'm planning to merge the C++ delayed folding branch this week, but I
> > > need to
> > > get approval of the back end changes (the first patch attached).
> > > Most of
> > > these are the introduction of non-folding variants of convert_to_*,
> > > but there
> > > are a few others.
> > >
> > > One question: The branch changes 'convert' to not fold its result,
> > > and it's
> > > not clear to me whether that's part of the expected behavior of a
> > > front end
> > > 'convert' function or not.
> >
> > History.  convert is purely frontend (but shared, unfortunately between
> > all frontends).  I would expect that FEs that do not do delayed folding
> > expect convert to fold.
> >
> > > Also, I'm a bit uncertain about merging this at the end of stage 1,
> > > since it's
> > > a large internal change with relatively small user impact; it just
> > > improves
> > > handling of constant expression corner cases.  I'm inclined to go
> > > ahead with
> > > it at this point, but I'm interested in contrary opinions.
> >
> > I welcome this change as it should allow cleaning up the FE-middle-end
> > interface a bit more.  It should be possible to remove all
> > NON_LVALUE_EXPR adding/removal from the middle-end folders.
> >
> > Looks like the backend patch included frontend parts but as far as I
> > skimmed it only
> >
> > diff --git a/gcc/fold-const.c b/gcc/fold-const.c
> > index 5e32901..d754a90 100644
> > --- a/gcc/fold-const.c
> > +++ b/gcc/fold-const.c
> > @@ -2091,6 +2091,17 @@ fold_convert_const (enum tree_code code, tree
> > type,
> > tree arg1)
> >         else if (TREE_CODE (arg1) == REAL_CST)
> >          return fold_convert_const_fixed_from_real (type, arg1);
> >       }
> > +  else if (TREE_CODE (type) == VECTOR_TYPE)
> > +    {
> > +      if (TREE_CODE (arg1) == VECTOR_CST
> > +         && TYPE_MAIN_VARIANT (type) == TYPE_MAIN_VARIANT (TREE_TYPE
> > (arg1))
> > +         && TYPE_VECTOR_SUBPARTS (type) == VECTOR_CST_NELTS (arg1))
> > +       {
> > +         tree r = copy_node (arg1);
> > +         TREE_TYPE (arg1) = type;
> > +         return r;
> > +       }
> > +    }
> >
> >
> > looks suspicious.  The issue here is that the vector elements will
> > have the wrong type after this simple handling.
>
> I was aiming to just handle simple cv-qualifier changes; that's why the
> TYPE_MAIN_VARIANT comparison is there.
>
> > If you fix that you can as well handle all kind of element type
> > changes via recursing to fold_convert_const (that includes
> > float to int / int to float changes).
>
> But I'll try this.

Like so?

Jason

diff --git a/gcc/c-family/c-lex.c b/gcc/c-family/c-lex.c
index ae16cfc..d8c7faf 100644
diff --git a/gcc/fold-const.c b/gcc/fold-const.c
index f9e5064..927e623 100644
--- a/gcc/fold-const.c
+++ b/gcc/fold-const.c
@@ -2095,6 +2095,25 @@ fold_convert_const (enum tree_code code, tree type, tree arg1)
       else if (TREE_CODE (arg1) == REAL_CST)
 	return fold_convert_const_fixed_from_real (type, arg1);
     }
+  else if (TREE_CODE (type) == VECTOR_TYPE)
+    {
+      if (TREE_CODE (arg1) == VECTOR_CST
+	  && TYPE_VECTOR_SUBPARTS (type) == VECTOR_CST_NELTS (arg1))
+	{
+	  int len = TYPE_VECTOR_SUBPARTS (type);
+	  tree elttype = TREE_TYPE (type);
+	  tree *v = XALLOCAVEC (tree, len);
+	  for (int i = 0; i < len; ++i)
+	    {
+	      tree elt = VECTOR_CST_ELT (arg1, i);
+	      tree cvt = fold_convert_const (code, elttype, elt);
+	      if (cvt == NULL_TREE)
+		return NULL_TREE;
+	      v[i] = cvt;
+	    }
+	  return build_vector (type, v);
+	}
+    }
   return NULL_TREE;
 }