[Bug tree-optimization/52904] -Wstrict-overflow false alarm with bounded loop
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=52904 Andrew Pinski changed: What|Removed |Added Target Milestone|--- |8.0 Status|REOPENED|RESOLVED Resolution|--- |FIXED See Also||https://gcc.gnu.org/bugzill ||a/show_bug.cgi?id=31130 --- Comment #14 from Andrew Pinski --- The xfail was removed with r8-395 which removed -fno-strict-overflow usage.
[Bug tree-optimization/52904] -Wstrict-overflow false alarm with bounded loop
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=52904 Richard Biener changed: What|Removed |Added Keywords||xfail Status|RESOLVED|REOPENED Resolution|WORKSFORME |--- --- Comment #13 from Richard Biener --- Re-opening because I am fixing a bug that makes the diagnostic re-appear (it was "fixed" by a bug that caused us to meet [0, +INF] and [1, +INF(OVF)] as [0, +INF] rather than [0, +INF(OVF)]).
[Bug tree-optimization/52904] -Wstrict-overflow false alarm with bounded loop
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=52904 --- Comment #12 from kugan at gcc dot gnu.org --- Author: kugan Date: Mon Aug 18 06:28:35 2014 New Revision: 214084 URL: https://gcc.gnu.org/viewcvs?rev=214084root=gccview=rev Log: gcc/testsuite 2014-08-18 Kugan Vivekanandarajah kug...@linaro.org PR tree-optimization/52904 * gcc.dg/pr52904.c: New test. Added: trunk/gcc/testsuite/gcc.dg/pr52904.c Modified: trunk/gcc/testsuite/ChangeLog
[Bug tree-optimization/52904] -Wstrict-overflow false alarm with bounded loop
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=52904
kugan at gcc dot gnu.org changed:
What|Removed |Added
Status|NEW |RESOLVED
CC||kugan at gcc dot gnu.org
Resolution|--- |WORKSFORME
--- Comment #9 from kugan at gcc dot gnu.org ---
Incorrect warning is gone from the latest trunk build. VRP is now able to
detect the correct range. This bug can now be closed.
Value ranges after VRP:
channel_8: [1, 1023] EQUIVALENCES: { channel_10 } (1 elements)
channel_10: [1, 1024]
channel_15: [0, 1023]
.MEM_16: VARYING
Removing basic block 5
wait_reading_process_output ()
{
int channel;
int nfds;
bb 2:
bb 3:
# channel_15 = PHI channel_10(3), 0(2)
foo (channel_15);
channel_10 = channel_15 + 1;
if (channel_10 != 1024)
goto bb 3;
else
goto bb 4;
bb 4:
return 0;
}
[Bug tree-optimization/52904] -Wstrict-overflow false alarm with bounded loop
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=52904 Manuel López-Ibáñez manu at gcc dot gnu.org changed: What|Removed |Added CC||manu at gcc dot gnu.org --- Comment #10 from Manuel López-Ibáñez manu at gcc dot gnu.org --- (In reply to kugan from comment #9) Incorrect warning is gone from the latest trunk build. VRP is now able to detect the correct range. This bug can now be closed. Since it was probably fixed by accident, it will get unfixed by accident again. If you don't add the testcase to the regression testsuite, this will happen again: https://gcc.gnu.org/wiki/HowToPrepareATestcase If you add a testcase pointing to this PR, when it fails people can come back here and read the discussion.
[Bug tree-optimization/52904] -Wstrict-overflow false alarm with bounded loop
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=52904 --- Comment #11 from kugan at gcc dot gnu.org --- I agree. I will post a patch to add this test-case and let the maintainers decide if this is necessary.
[Bug tree-optimization/52904] -Wstrict-overflow false alarm with bounded loop
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52904 Laurent Rineau Laurent.Rineau__gcc at normalesup dot org changed: What|Removed |Added CC||Laurent.Rineau__gcc@normale ||sup.org --- Comment #5 from Laurent Rineau Laurent.Rineau__gcc at normalesup dot org --- With the following gcc version: gcc (GCC) 4.8.2 20131017 (Red Hat 4.8.2-1) I have a similar result: $ gcc -c -Wstrict-overflow -O2 v.i v.i: In function ‘wait_reading_process_output’: v.i:14:6: warning: assuming signed overflow does not occur when simplifying conditional to constant [-Wstrict-overflow] if (nfds 0) ^ That diagnostic seems right, according to the documentation of -Wstrict-overflow.
[Bug tree-optimization/52904] -Wstrict-overflow false alarm with bounded loop
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52904 --- Comment #6 from eggert at gnu dot org --- That diagnostic seems right, according to the documentation of -Wstrict-overflow. The diagnostic is right only in the sense that it is correctly reporting that GCC does not deduce that signed overflow cannot possibly occur in this function. It is not right in the common sense that a programmer would ordinarily want, i.e., that the program may have a bug because signed overflow might lead to undefined behavior. (Surely the diagnostic is supposed to be for the benefit of programmers trying to find potential bugs in their programs, not for the benefit of GCC maintainers trying to explain how GCC works internally. :-)
[Bug tree-optimization/52904] -Wstrict-overflow false alarm with bounded loop
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52904 --- Comment #7 from Laurent Rineau Laurent.Rineau__gcc at normalesup dot org --- In the test case, nfds cannot overflow, because of two reasons: - nfds is only incremented from 0, and -fstrict-overflow allows gcc to suppose it will not overflow, - the number of iterations of the loop cannot allow nfds to overflow, even without -fstrict-overflow. Gcc warns that the test (nfds 0) is useless, because of -fstrict-overflow. The developer has two solutions: - remove that test, - or compile with -fno-strict-overflow.
[Bug tree-optimization/52904] -Wstrict-overflow false alarm with bounded loop
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52904 --- Comment #8 from eggert at gnu dot org --- On 12/12/2013 10:19 AM, Laurent.Rineau__gcc at normalesup dot org wrote: The developer has two solutions: - remove that test, - or compile with -fno-strict-overflow. Sure, and because of this problem, GNU Emacs has chosen #2, that is, Emacs doesn't use -Wstrict-overflow any more. (Removing the test would unnecessarily complicate Emacs, since the test is needed on some platforms that are conditionally compiled away in this stripped-down example.) A goal of -Wstrict-overflow, at least at the 1 level, is to not generate false alarms, so that it's generally useful in real programs. This bug report gives one example where the goal is not being met. Emacs currently has a half dozen or so such examples of this (please see below for what the current Emacs bzr trunk would output, if we enabled this warning) and I thought that the GCC developers would find it useful to see one of them. If you're not interested, that's OK too; Emacs will continue to not use -Wstrict-overflow. In file included from dispnew.c:26:0: dispnew.c: In function ‘update_window’: lisp.h:749:30: warning: assuming signed overflow does not occur when simplifying conditional to constant [-Wstrict-overflow] return num lower ? lower : num = upper ? num : upper; dispnew.c: In function ‘update_frame_1’: dispnew.c:4490:19: warning: assuming signed overflow does not occur when simplifying conditional to constant [-Wstrict-overflow] pause_p = 0 i i FRAME_LINES (f) - 1; fileio.c: In function ‘Finsert_file_contents’: fileio.c:3630:11: warning: assuming signed overflow does not occur when simplifying conditional to constant [-Wstrict-overflow] if (nread 0) ^ fileio.c:3632:16: warning: assuming signed overflow does not occur when simplifying conditional to constant [-Wstrict-overflow] else if (nread 0) ^ eval.c: In function ‘backtrace_eval_unrewind’: eval.c:3496:3: warning: assuming signed overflow does not occur when simplifying conditional to constant [-Wstrict-overflow] for (; distance 0; distance--) ^ intervals.c: In function ‘offset_intervals’: intervals.c:1364:6: warning: assuming signed overflow does not occur when simplifying conditional to constant [-Wstrict-overflow] if (length == TOTAL_LENGTH (tree)) ^ intervals.c:1379:9: warning: assuming signed overflow does not occur when simplifying conditional to constant [-Wstrict-overflow] while (left_to_delete 0) ^
[Bug tree-optimization/52904] -Wstrict-overflow false alarm with bounded loop
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52904
Richard Guenther rguenth at gcc dot gnu.org changed:
What|Removed |Added
Keywords||missed-optimization
Status|UNCONFIRMED |NEW
Last reconfirmed||2012-04-10
CC||rguenth at gcc dot gnu.org
Ever Confirmed|0 |1
--- Comment #4 from Richard Guenther rguenth at gcc dot gnu.org 2012-04-10
12:16:54 UTC ---
It's the
if (nfds 0)
conditional that we warn on. We cannot currently derive an upper bound
for nfds based on the number of iterations of the loop as it is not
an induction variable we can analyze.
int
wait_reading_process_output (void)
{
int nfds = 0;
int channel;
for (channel = 0; channel 1024; ++channel)
{
if (foo (channel))
nfds++;
}
if (nfds 0)
return 1;
This is in some way a missed optimization in value-range analysis as well.
[Bug tree-optimization/52904] -Wstrict-overflow false alarm with bounded loop
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52904 Andrew Pinski pinskia at gcc dot gnu.org changed: What|Removed |Added Keywords||diagnostic Component|c |tree-optimization --- Comment #3 from Andrew Pinski pinskia at gcc dot gnu.org 2012-04-08 10:05:26 UTC --- (In reply to comment #2) Created attachment 27115 [details] simplified version of Emacs code, illustrating the bug Re-uploading the test case, marked as text/plain this time, so it's easier to download. You did not have to re-upload the testcase, you can change the MIME type of the atttachment after the fact.
