When we try to thread across a back edge in the CFG we have a check to
ensure that we don't use temporary equivalences which are invalidated by
traversal of the back edge to simplify the final conditional.
About a year ago I added code to pick up secondary threading
opportunities after an initial jump threading was successful. However,
I failed to account for equivalences which were invalidated by the back
edge traversal in those new cases.
This patch extracts the check into its own function, then calls it from
the 3 locations where it's necessary.
Bootstrapped and regression tested on x86_64-unknown-linux-gnu.
Installed onto the trunk.
Oh how I want to rewrite all this code....
Jeff
ps. First time commiting using git svn, hopefully I did all the steps
correctly.
* tree-ssa-threadedge.c (cond_arg_set_in_bb): New function extracted
from thread_across_edge.
(thread_across_edge): Use it in all cases where we might thread
across a back edge.
* gcc.c-torture/execute/pr54985.c: New test.
diff --git a/gcc/testsuite/gcc.c-torture/execute/pr54985.c
b/gcc/testsuite/gcc.c-torture/execute/pr54985.c
new file mode 100644
index 0000000..678c9f4
--- /dev/null
+++ b/gcc/testsuite/gcc.c-torture/execute/pr54985.c
@@ -0,0 +1,36 @@
+
+typedef struct st {
+ int a;
+} ST;
+
+int __attribute__((noinline,noclone))
+foo(ST *s, int c)
+{
+ int first = 1;
+ int count = c;
+ ST *item = s;
+ int a = s->a;
+ int x;
+
+ while (count--)
+ {
+ x = item->a;
+ if (first)
+ first = 0;
+ else if (x >= a)
+ return 1;
+ a = x;
+ item++;
+ }
+ return 0;
+}
+
+extern void abort (void);
+
+int main ()
+{
+ ST _1[2] = {{2}, {1}};
+ if (foo(_1, 2) != 0)
+ abort ();
+ return 0;
+}
diff --git a/gcc/tree-ssa-threadedge.c b/gcc/tree-ssa-threadedge.c
index 105e3ab..491aa9f 100644
--- a/gcc/tree-ssa-threadedge.c
+++ b/gcc/tree-ssa-threadedge.c
@@ -572,6 +572,44 @@ simplify_control_stmt_condition (edge e,
return cached_lhs;
}
+/* Return TRUE if the statement at the end of e->dest depends on
+ the output of any statement in BB. Otherwise return FALSE.
+
+ This is used when we are threading a backedge and need to ensure
+ that temporary equivalences from BB do not affect the condition
+ in e->dest. */
+
+static bool
+cond_arg_set_in_bb (edge e, basic_block bb, int n)
+{
+ ssa_op_iter iter;
+ use_operand_p use_p;
+ gimple last = gsi_stmt (gsi_last_bb (e->dest));
+
+ /* E->dest does not have to end with a control transferring
+ instruction. This can occurr when we try to extend a jump
+ threading opportunity deeper into the CFG. In that case
+ it is safe for this check to return false. */
+ if (!last)
+ return false;
+
+ if (gimple_code (last) != GIMPLE_COND
+ && gimple_code (last) != GIMPLE_GOTO
+ && gimple_code (last) != GIMPLE_SWITCH)
+ return false;
+
+ FOR_EACH_SSA_USE_OPERAND (use_p, last, iter, SSA_OP_USE | SSA_OP_VUSE)
+ {
+ tree use = USE_FROM_PTR (use_p);
+
+ if (TREE_CODE (use) == SSA_NAME
+ && gimple_code (SSA_NAME_DEF_STMT (use)) != GIMPLE_PHI
+ && gimple_bb (SSA_NAME_DEF_STMT (use)) == bb)
+ return true;
+ }
+ return false;
+}
+
/* TAKEN_EDGE represents the an edge taken as a result of jump threading.
See if we can thread around TAKEN_EDGE->dest as well. If so, return
the edge out of TAKEN_EDGE->dest that we can statically compute will be
@@ -705,19 +743,8 @@ thread_across_edge (gimple dummy_cond,
safe to thread this edge. */
if (e->flags & EDGE_DFS_BACK)
{
- ssa_op_iter iter;
- use_operand_p use_p;
- gimple last = gsi_stmt (gsi_last_bb (e->dest));
-
- FOR_EACH_SSA_USE_OPERAND (use_p, last, iter, SSA_OP_USE | SSA_OP_VUSE)
- {
- tree use = USE_FROM_PTR (use_p);
-
- if (TREE_CODE (use) == SSA_NAME
- && gimple_code (SSA_NAME_DEF_STMT (use)) != GIMPLE_PHI
- && gimple_bb (SSA_NAME_DEF_STMT (use)) == e->dest)
- goto fail;
- }
+ if (cond_arg_set_in_bb (e, e->dest, 1))
+ goto fail;
}
stmt_count = 0;
@@ -758,7 +785,9 @@ thread_across_edge (gimple dummy_cond,
address. If DEST is not null, then see if we can thread
through it as well, this helps capture secondary effects
of threading without having to re-run DOM or VRP. */
- if (dest)
+ if (dest
+ && ((e->flags & EDGE_DFS_BACK) == 0
+ || ! cond_arg_set_in_bb (taken_edge, e->dest, 2)))
{
/* We don't want to thread back to a block we have already
visited. This may be overly conservative. */
@@ -816,11 +845,16 @@ thread_across_edge (gimple dummy_cond,
e3 = taken_edge;
do
{
- e2 = thread_around_empty_block (e3,
- dummy_cond,
- handle_dominating_asserts,
- simplify,
- visited);
+ if ((e->flags & EDGE_DFS_BACK) == 0
+ || ! cond_arg_set_in_bb (e3, e->dest, 3))
+ e2 = thread_around_empty_block (e3,
+ dummy_cond,
+ handle_dominating_asserts,
+ simplify,
+ visited);
+ else
+ e2 = NULL;
+
if (e2)
{
e3 = e2;
