Re: [Patch Ping] [RFC] Alias export patch

[Andrey Belevantsev <abel at ispras dot ru>]

Hi Andrew and Diego,

Sorry for the delay in my answer.  Following your suggestions, I've 
tried the attached patch.  It gives the following statistics (gathered 
by write_stat calls in the patch):

C = 433         add_coalesce is called for pointers with different 
points-to sets
O = 523         add_coalesce is called when one pointer has a points-to 
set and the other has not	
a = 10563       new disambiguations from extended alias sets when 
alias_sets_conflict_p is called
x = 1954        new disambiguations from saved points-to sets when 
*_dependence is called

Before the patch, it was like follows:

a = 10453
x = 2035

i.e. ~1000 chances for not coalescing meaningful pointers during a 
bootstrap adds 110 new disambiguations with extended alias sets, but 
removes 81 old ones with points-to sets.  On SPECint2000 with -O3 
-funroll-loops, we have:

before          after
                C = 26
                O = 52
a = 11691       a = 11691
x = 1028        x = 1035

i.e. only 78 interesting calls of add_coalesce that give us 7 new 
disambiguations.  Given that numbers, I haven't tried to measure 
performance impact because it doesn't seem like a lot of new 
opportunities were created.

Does the patch look like what you've said?  If so, maybe the problem is 
in the way we save information from the tree ssa side.  Here's the 
general approach.  For each pointer with non-null SSA_NAME_PTR_INFO 
found in tree operands, we do the following:

 mapped_ptr = var_to_partition_to_var (map, ptr);
 if (mapped_ptr)
    tse_update_var_ann_with_pts_info (mapped_ptr, pi);

where pi is the points-to info for original pointer.  In the latter 
function we extract previously saved points-to set of mapped_ptr, if it 
has one, or create a brand new one if it hasn't.  Then we add all vars 
from ptr's points-to set to the saved set of mapped_ptr.

If you have any ideas on what could be going wrong on the tree ssa side, 
then I'm happy to try them.  If it looks ok, then I might start looking 
on how the saved information survives through RTL optimizers.

Andrey
--- tree-ssa-live.c	(revision 19384)
+++ tree-ssa-live.c	(local)
@@ -1244,12 +1244,38 @@ add_coalesce (coalesce_list_p cl, int p1
 	      int value)
 {
   partition_pair_p node;
+  tree t1, t2;
+  struct ptr_info_def *pi, *qi;
 
   gcc_assert (cl->add_mode);
 
   if (p1 == p2)
     return;
 
+  t1 = partition_to_var (cl->map, p1);
+  t2 = partition_to_var (cl->map, p2);  
+  pi = SSA_NAME_PTR_INFO (t1);
+  qi = SSA_NAME_PTR_INFO (t2);
+
+  if (pi && qi)
+    {
+      /* Do not coalesce when one pointer has a points-to set and 
+         the other hasn't.  */
+      if ((pi->pt_vars && !qi->pt_vars) || (!pi->pt_vars && qi->pt_vars))
+        {
+          write_stat ("O");
+          return; 
+        }
+
+      /* Do not coalesce when pointers have different points-to sets.  */
+      if (pi->pt_vars && qi->pt_vars 
+          && !bitmap_equal_p (pi->pt_vars, qi->pt_vars)) 
+        {
+          write_stat ("C");
+          return; 
+        }
+    }
+
   node = find_partition_pair (cl, p1, p2, true);
 
   node->cost += value;