[PR tree-optimization/19786] fix alias grouping lossage

[Alexandre Oliva <aoliva at redhat dot com>]

The problem here was that we added type tags to other tag's may-alias
lists without adding them to the corresponding bitmaps.  Later on,
when group_aliases performed an union of the bitmaps and discarded the
lists, we lost information about the aliases, which enabled LIM to
hoist a pointer access out of a loop because it appeared to be
invariant, since the VDEF supposed to modify it was missing.

Thanks to Jakub for having isolated the source of the problem, and
Diego for discussing tree-ssa alias analysis with me for a few hours
today.

Here's the patch I'm testing.  Ok to install if it bootstraps and
regtests successfully?

Index: gcc/ChangeLog
from  Alexandre Oliva  <aoliva@redhat.com>

	PR tree-optimization/19786
	* tree-ssa-alias.c (compute_flow_insensitive_aliasing): Add one
	tag to another's may-alias bitmap when adding to the other's list.

Index: gcc/tree-ssa-alias.c
===================================================================
RCS file: /cvs/gcc/gcc/gcc/tree-ssa-alias.c,v
retrieving revision 2.69
diff -u -p -r2.69 tree-ssa-alias.c
--- gcc/tree-ssa-alias.c 17 Feb 2005 16:19:42 -0000 2.69
+++ gcc/tree-ssa-alias.c 21 Feb 2005 19:15:40 -0000
@@ -1116,6 +1116,7 @@ compute_flow_insensitive_aliasing (struc
 	      /* Since TAG2 does not have any aliases of its own, add
 		 TAG2 itself to the alias set of TAG1.  */
 	      add_may_alias (tag1, tag2);
+	      SET_BIT (may_aliases1, var_ann (tag2)->uid);
 	    }
 	}
     }
Index: gcc/testsuite/ChangeLog
from  Alexandre Oliva  <aoliva@redhat.com>

	PR tree-optimization/19786
	* g++.dg/tree-ssa/pr19786.C: New.

Index: gcc/testsuite/g++.dg/tree-ssa/pr19786.C
===================================================================
RCS file: gcc/testsuite/g++.dg/tree-ssa/pr19786.C
diff -N gcc/testsuite/g++.dg/tree-ssa/pr19786.C
--- /dev/null	1 Jan 1970 00:00:00 -0000
+++ gcc/testsuite/g++.dg/tree-ssa/pr19786.C 21 Feb 2005 19:15:54 -0000
@@ -0,0 +1,48 @@
+// { dg-do run }
+/* { dg-options "-O2" } */
+
+// We used to get alias grouping wrong on this one, hoisting accesses
+// to the vector's end out of the loop.
+
+#include <vector>
+#include <cassert>
+
+struct A
+{
+  double unused;      // If I remove it => it works.
+  std::vector<int> v;
+
+  A() : v(1) {}
+};
+
+inline // If not inline => it works.
+A g()
+{
+  A r;
+  r.v.resize(2);
+  r.v[0] = 1;
+
+  while (!r.v.empty() && r.v.back() == 0)
+    r.v.pop_back();
+
+  return r;
+}
+
+A f(const A &a)
+{
+  if (a.v.empty())  return a;
+  if (a.v.empty())  return a;
+
+  // A z = g(); return z;  // If I return like this => it works.
+  return g();
+}
+
+int main()
+{
+  A a;
+  A b;
+  A r = f(a);
+  assert(r.v.size() != 0);
+
+  return 0;
+}

-- 
Alexandre Oliva             http://www.ic.unicamp.br/~oliva/
Red Hat Compiler Engineer   aoliva@{redhat.com, gcc.gnu.org}
Free Software Evangelist  oliva@{lsd.ic.unicamp.br, gnu.org}