------- Additional Comments From aoliva at gcc dot gnu dot org 2005-02-21
19:30 -------
Subject: [PR tree-optimization/19786] fix alias grouping lossage
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 <[EMAIL PROTECTED]>
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 <[EMAIL PROTECTED]>
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 [EMAIL PROTECTED], gcc.gnu.org}
Free Software Evangelist [EMAIL PROTECTED], gnu.org}
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19786
