RFC: ssa subvariables for complex types

[Aldy Hernandez <aldyh at redhat dot com>]

Hi folks.

While investigating a regression from the V_MUST_DEF removal on mem-ssa,
I've noticed that we were missing out on optimization of certain
stores to complex types (on mainline).

For example, here:

  _Complex int t = 0;
 __real__ t = 2;
 __imag__ t = 2;

we end up with:

  #   t_2 = V_MUST_DEF <t_1>;
  t = __complex__ (0, 0);
  #   t_3 = V_MAY_DEF <t_2>;
  REALPART_EXPR <t> = 2;
  #   t_4 = V_MAY_DEF <t_3>;
  IMAGPART_EXPR <t> = 2;

When we really should be decomposing the field stores into SFTs, like this:

  #   SFT.0_3 = V_MUST_DEF <SFT.0_1>;
  #   SFT.1_4 = V_MUST_DEF <SFT.1_2>;
  t = __complex__ (0, 0);
  #   SFT.1_5 = V_MUST_DEF <SFT.1_4>;
  REALPART_EXPR <t> = 2;
  #   SFT.0_6 = V_MUST_DEF <SFT.0_3>;
  IMAGPART_EXPR <t> = 2;

The problem with not decomposing, is that since we can't account for the
fields themselves, we have to end up using V_MAY_DEFs (instead of V_MUST_DEFs)
for the entire complex type, and later on DCE cannot remove the original
clearring of "t" because we have a V_MUST_DEF followed by a V_MAY_DEF.

I see the original rationale for inhibiting creation of subvariables
on aggregates here:

http://gcc.gnu.org/ml/fortran/2006-01/msg00195.html

But I don't think, memory wise, it should apply to complex types.
This patch will cause the clearring of "t" to be redundant on mainline.
On mem-ssa it doesn't matter, cause we get the case wrong anyhow, but it's
best to describe what's going on-- while I'm at it :).

How does this look?

	* tree-ssa-alias.c (create_overlap_variables_for): Do not inhibit
	creation of subvariables for complex types.

Index: tree-ssa-alias.c
===================================================================
--- tree-ssa-alias.c	(revision 112618)
+++ tree-ssa-alias.c	(working copy)
@@ -2878,7 +2878,8 @@ create_overlap_variables_for (tree var)
 
   up = up_lookup (uid);
   if (!up
-      || up->write_only)
+      || (up->write_only
+	  && TREE_CODE (TREE_TYPE (var)) != COMPLEX_TYPE))
     return;
 
   push_fields_onto_fieldstack (TREE_TYPE (var), &fieldstack, 0, NULL);