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gcov: option to instrument whole basic block graph, no use of spanning tree optimization
- From: Holger Blasum <hbl at sysgo dot com>
- To: gcc-patches at gcc dot gnu dot org
- Date: Sun, 7 Nov 2010 22:54:23 +0100
- Subject: gcov: option to instrument whole basic block graph, no use of spanning tree optimization
Hello,
The attached patch introduces an option to instrument the whole basic
block graph and not just its spanning tree.
Rationale: gcov is not robust in a threaded environment, the patch
makes it more robust.
In particular, in threaded environments on can observe two effects:
(1) underreporting when coverage counters are overwritten by another
instance of a thread after a thread switch.
(2) generation of arbitrary (even negative) values by subtraction when
reconstructing the whole tree from the used by the spanning tree
optimization mechanism. (Recall that subtraction is messy numerically.)
With the current gcov framework (I think) one cannot do anything
really generic about (1). However, (2) can be fixed by optionally
disabling the spanning tree optimization.
Question: Why is fixing only (2) useful?
Answer: One often wants to have the
property P: if a counter is non-zero then its
path has had some (potentially underreported but non-zero) coverage
So fixing (2) is valuable because when (2) is fixed then P holds
whereas when (2) is not fixed (I think) due to potential cancellation
effects P does not necessarily hold.
Moreover I think fixing (2) also gives
property Q: if a path has had some (potentially underreported but
non-zero) coverage, then its counter is non-zero (converse of P)
because in the event of coverage loss by concurrency at least the
overwriting counter is never 0.
The following program is used for demonstration.
$cat uncontrolled.c
#include <pthread.h>
#include <stdio.h>
#include <sys/io.h>
void *worker (void *args) {
int i;
int j = 0;
for (i = 0; i< 100000*1000; i++) {
j++;
if (j % 100000 == 0) {
printf("j now is %d.\n", j);
}
}
}
int main ()
{
iopl(3);
pthread_t t1, t2;
pthread_create (&t1, NULL, worker, NULL);
pthread_create (&t2, NULL, worker, NULL);
pthread_join (t1, NULL);
pthread_join (t2, NULL);
return 0;
}
... when run via ...
$ gcc -fprofile-arcs -ftest-coverage -O0 -o uncontrolled uncontrolled.c -lpthread
$ ./uncontrolled
$ gcov uncontrolled
... we observe effects (1) and (2)
-: 0:Source:uncontrolled.c
-: 0:Graph:uncontrolled.gcno
-: 0:Data:uncontrolled.gcda
-: 0:Runs:1
-: 0:Programs:1
-: 1:#include <pthread.h>
-: 2:#include <stdio.h>
-: 3:#include <sys/io.h>
-: 4:
-1067519: 5:void *worker (void *args) {
-: 6: int i;
-1067519: 7: int j = 0;
182760737: 8: for (i = 0; i< 100000*1000; i++) {
182760735: 9: j++;
182760735: 10: if (j % 100000 == 0) {
2000: 11: printf("j now is %d.\n", j);
-: 12: }
-: 13: }
-: 14:
2: 15:}
-: 16:
1: 17:int main ()
-: 18:{
1: 19: iopl(3);
-: 20: pthread_t t1, t2;
1: 21: pthread_create (&t1, NULL, worker, NULL);
1: 22: pthread_create (&t2, NULL, worker, NULL);
1: 23: pthread_join (t1, NULL);
1: 24: pthread_join (t2, NULL);
1: 25: return 0;
-: 26:}
When run via the patched version (with -fprofile-whole-graph) this becomes:
$ gcc -fprofile-whole-graph -fprofile-arcs -ftest-coverage -O0 -o uncontrolled uncontrolled.c -lpthread
$ ./uncontrolled
$ gcov uncontrolled
... we only observe effect (1)
$ cat uncontrolled.c.gcov
-: 0:Source:uncontrolled.c
-: 0:Graph:uncontrolled.gcno
-: 0:Data:uncontrolled.gcda
-: 0:Runs:1
-: 0:Programs:1
-: 1:#include <pthread.h>
-: 2:#include <stdio.h>
-: 3:#include <sys/io.h>
-: 4:
2: 5:void *worker (void *args) {
-: 6: int i;
2: 7: int j = 0;
187663616: 8: for (i = 0; i< 100000*1000; i++) {
187879212: 9: j++;
187879212: 10: if (j % 100000 == 0) {
2000: 11: printf("j now is %d.\n", j);
-: 12: }
-: 13: }
-: 14:
2: 15:}
-: 16:
1: 17:int main ()
-: 18:{
1: 19: iopl(3);
-: 20: pthread_t t1, t2;
1: 21: pthread_create (&t1, NULL, worker, NULL);
1: 22: pthread_create (&t2, NULL, worker, NULL);
1: 23: pthread_join (t1, NULL);
1: 24: pthread_join (t2, NULL);
1: 25: return 0;
-: 26:}
For further discussion see also section 4 of
http://sysrun.haifa.il.ibm.com/hrl/greps2007/papers/gcov-on-an-embedded-system.pdf . Actually I'm submitting here because someone had noticed the paper and then asked the question why this isn't fixed yet :-)
Regards,
--
Holger Blasum (SYSGO AG)
Index: gcc/opts.c
===================================================================
--- gcc/opts.c (revision 166172)
+++ gcc/opts.c (working copy)
@@ -1973,6 +1973,10 @@
profile_data_prefix = xstrdup (arg);
break;
+ case OPT_fprofile_whole_graph:
+ flag_profile_whole_graph = 1;
+ break;
+
case OPT_fprofile_use_:
profile_data_prefix = xstrdup (arg);
flag_profile_use = true;
Index: gcc/profile.c
===================================================================
--- gcc/profile.c (revision 166172)
+++ gcc/profile.c (working copy)
@@ -1019,9 +1019,10 @@
/* Create spanning tree from basic block graph, mark each edge that is
on the spanning tree. We insert as many abnormal and critical edges
as possible to minimize number of edge splits necessary. */
+
+ if (!flag_profile_whole_graph)
+ find_spanning_tree (el);
- find_spanning_tree (el);
-
/* Fake edges that are not on the tree will not be instrumented, so
mark them ignored. */
for (num_instrumented = i = 0; i < num_edges; i++)
Index: gcc/common.opt
===================================================================
--- gcc/common.opt (revision 166172)
+++ gcc/common.opt (working copy)
@@ -1279,6 +1279,10 @@
Common Joined RejectNegative
Enable common options for generating profile info for profile feedback directed optimizations, and set -fprofile-dir=
+fprofile-whole-graph
+Common Var(flag_profile_whole_graph, 0) RejectNegative
+Use whole basic block graph for profiling, do not use spanning tree optimization (more memory usage, but better conservative approximation for multithreaded execution).
+
fprofile-use
Common Var(flag_profile_use)
Enable common options for performing profile feedback directed optimizations