Hi,
the update of RTL optimizers to use SCEV's loop bounds make them to be
inexpectedly active.
One of reasons is invalid. For loop
int *a;
int t()
{
int i;
for (i=0;i<1000000;i++)
if (a[i])
return 1;
return 0;
}
We get realistic number of iteration estimate to be 9999999. This is quite
wrong. We could however still predict loop:
int t2()
{
int i;
for (i=0;i<3000000;i++)
if (a[i])
abort ();
return 0;
}
This patch implements that by making estimate_numbers_of_iterations_loop to
save the realistic estimate only when all other exits out of the loop are
unlikely
(i.e. EH or predicted by NORETURN or similarly strong heuristic).
Bootstrapped/regtested x86_64-linux, comitted.
Honza
* tree-ssa-loop-niter.c (estimate_numbers_of_iterations_loop): Do not
predict loops with multiple exits realistically.
* cfgloopanal.c (single_likely_exit): New function.
* gcc.dg/unroll_5.c: New testcase.
Index: tree-ssa-loop-niter.c
===================================================================
--- tree-ssa-loop-niter.c (revision 192432)
+++ tree-ssa-loop-niter.c (working copy)
@@ -2965,6 +2965,7 @@ estimate_numbers_of_iterations_loop (str
struct tree_niter_desc niter_desc;
edge ex;
double_int bound;
+ edge likely_exit;
/* Give up if we already have tried to compute an estimation. */
if (loop->estimate_state != EST_NOT_COMPUTED)
@@ -2975,6 +2976,7 @@ estimate_numbers_of_iterations_loop (str
loop->any_estimate = false;
exits = get_loop_exit_edges (loop);
+ likely_exit = single_likely_exit (loop);
FOR_EACH_VEC_ELT (edge, exits, i, ex)
{
if (!number_of_iterations_exit (loop, ex, &niter_desc, false))
@@ -2988,7 +2990,7 @@ estimate_numbers_of_iterations_loop (str
niter);
record_estimate (loop, niter, niter_desc.max,
last_stmt (ex->src),
- true, true, true);
+ true, ex == likely_exit, true);
}
VEC_free (edge, heap, exits);
Index: cfgloopanal.c
===================================================================
--- cfgloopanal.c (revision 192432)
+++ cfgloopanal.c (working copy)
@@ -446,3 +446,40 @@ mark_loop_exit_edges (void)
}
}
+/* Return exit edge if loop has only one exit that is likely
+ to be executed on runtime (i.e. it is not EH or leading
+ to noreturn call. */
+
+edge
+single_likely_exit (struct loop *loop)
+{
+ edge found = single_exit (loop);
+ VEC (edge, heap) *exits;
+ unsigned i;
+ edge ex;
+
+ if (found)
+ return found;
+ exits = get_loop_exit_edges (loop);
+ FOR_EACH_VEC_ELT (edge, exits, i, ex)
+ {
+ if (ex->flags & (EDGE_EH | EDGE_ABNORMAL_CALL))
+ continue;
+ /* The constant of 5 is set in a way so noreturn calls are
+ ruled out by this test. The static branch prediction algorithm
+ will not assign such a low probability to conditionals for usual
+ reasons. */
+ if (profile_status != PROFILE_ABSENT
+ && ex->probability < 5 && !ex->count)
+ continue;
+ if (!found)
+ found = ex;
+ else
+ {
+ VEC_free (edge, heap, exits);
+ return NULL;
+ }
+ }
+ VEC_free (edge, heap, exits);
+ return found;
+}
Index: testsuite/gcc.dg/unroll_5.c
===================================================================
--- testsuite/gcc.dg/unroll_5.c (revision 0)
+++ testsuite/gcc.dg/unroll_5.c (revision 0)
@@ -0,0 +1,25 @@
+/* { dg-do compile } */
+/* { dg-options "-O3 -fdump-rtl-loop2_unroll -funroll-loops" } */
+void abort (void);
+int *a;
+int t()
+{
+ int i;
+ for (i=0;i<1000000;i++)
+ if (a[i])
+ return 1;
+ return 0;
+}
+int t2()
+{
+ int i;
+ for (i=0;i<3000000;i++)
+ if (a[i])
+ abort ();
+ return 0;
+}
+/* { dg-final { scan-rtl-dump-times "upper bound: 999999" 1 "loop2_unroll" } }
*/
+/* { dg-final { scan-rtl-dump-not "realistic bound: 999999" "loop2_unroll" } }
*/
+/* { dg-final { scan-rtl-dump-times "upper bound: 2999999" 1 "loop2_unroll" }
} */
+/* { dg-final { scan-rtl-dump-times "realistic bound: 2999999" 1
"loop2_unroll" } } */
+/* { dg-final { cleanup-rtl-dump "loop2_unroll" } } */
