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* [Bug c/101653] New: Bad code generated when optimizing nested for loops
@ 2021-07-27 22:48 phd at phd dot re
2021-07-27 22:55 ` [Bug tree-optimization/101653] [9/10/11/12 Regression] " pinskia at gcc dot gnu.org
` (3 more replies)
0 siblings, 4 replies; 5+ messages in thread
From: phd at phd dot re @ 2021-07-27 22:48 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101653
Bug ID: 101653
Summary: Bad code generated when optimizing nested for loops
Product: gcc
Version: 11.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: phd at phd dot re
Target Milestone: ---
Not sure if this was already reported.
Bad code is generated when optimizing nested for loops:
int main(void)
{
unsigned a, b, c = 0; // issue only occurs for unsigned
for (a = 0; a < 10; a++) // upper limit for variable a doesn't really
matter
{
for (b = 0; b < 2; b++) // bug only for b < 2, no issue for other
limits
{
c++; // first iteration: a == 0, b == 0, c == 1
if (c < a) // c will *never* be smaller than a from now on
{
return 123; // and yet this is somehow reachable with -O1/2/3
}
}
}
return 0; // with -O0 this returns 0 properly
}
Affected gcc versions: 9.1+ (including 11.1 and trunk)
Not affected versions: 8.5-
Bug occurs for -O1, -O2, -O3 (return 123)
No issue for -O0 (properly returns 0)
Both C and C++ are affected.
Live demo:
https://godbolt.org/z/nanc9osq5
^ permalink raw reply [flat|nested] 5+ messages in thread* [Bug tree-optimization/101653] [9/10/11/12 Regression] Bad code generated when optimizing nested for loops 2021-07-27 22:48 [Bug c/101653] New: Bad code generated when optimizing nested for loops phd at phd dot re @ 2021-07-27 22:55 ` pinskia at gcc dot gnu.org 2021-07-27 23:01 ` jakub at gcc dot gnu.org ` (2 subsequent siblings) 3 siblings, 0 replies; 5+ messages in thread From: pinskia at gcc dot gnu.org @ 2021-07-27 22:55 UTC (permalink / raw) To: gcc-bugs https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101653 Andrew Pinski <pinskia at gcc dot gnu.org> changed: What |Removed |Added ---------------------------------------------------------------------------- Keywords| |wrong-code Summary|Bad code generated when |[9/10/11/12 Regression] Bad |optimizing nested for loops |code generated when | |optimizing nested for loops Component|c |tree-optimization Target Milestone|--- |9.5 ^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug tree-optimization/101653] [9/10/11/12 Regression] Bad code generated when optimizing nested for loops 2021-07-27 22:48 [Bug c/101653] New: Bad code generated when optimizing nested for loops phd at phd dot re 2021-07-27 22:55 ` [Bug tree-optimization/101653] [9/10/11/12 Regression] " pinskia at gcc dot gnu.org @ 2021-07-27 23:01 ` jakub at gcc dot gnu.org 2021-07-27 23:09 ` pinskia at gcc dot gnu.org 2021-07-27 23:13 ` pinskia at gcc dot gnu.org 3 siblings, 0 replies; 5+ messages in thread From: jakub at gcc dot gnu.org @ 2021-07-27 23:01 UTC (permalink / raw) To: gcc-bugs https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101653 Jakub Jelinek <jakub at gcc dot gnu.org> changed: What |Removed |Added ---------------------------------------------------------------------------- CC| |jakub at gcc dot gnu.org Last reconfirmed| |2021-07-27 Status|UNCONFIRMED |NEW Ever confirmed|0 |1 --- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> --- Started with r9-4145-ga81e2c6240655f60a49c16e0d8bbfd2ba40bba51 So possibly related or the same as PR101508 or PR100740 ^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug tree-optimization/101653] [9/10/11/12 Regression] Bad code generated when optimizing nested for loops 2021-07-27 22:48 [Bug c/101653] New: Bad code generated when optimizing nested for loops phd at phd dot re 2021-07-27 22:55 ` [Bug tree-optimization/101653] [9/10/11/12 Regression] " pinskia at gcc dot gnu.org 2021-07-27 23:01 ` jakub at gcc dot gnu.org @ 2021-07-27 23:09 ` pinskia at gcc dot gnu.org 2021-07-27 23:13 ` pinskia at gcc dot gnu.org 3 siblings, 0 replies; 5+ messages in thread From: pinskia at gcc dot gnu.org @ 2021-07-27 23:09 UTC (permalink / raw) To: gcc-bugs https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101653 --- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> --- Confirmed, ivcannon is turning the loops into: <bb 2> [local count: 79093198]: goto <bb 6>; [100.00%] <bb 4> [local count: 357878150]: a_8 = a_11 + 1; c_14 = c_3 + 2; goto <bb 6>; [100.00%] <bb 5> [local count: 79093198]: # _5 = PHI <123(6)> return _5; <bb 6> [local count: 375401868]: # c_3 = PHI <1(2), c_14(4)> # a_11 = PHI <0(2), a_8(4)> # ivtmp_20 = PHI <2(2), ivtmp_1(4)> c_6 = c_3 + 1; ivtmp_1 = ivtmp_20 - 1; if (ivtmp_1 == 0) goto <bb 5>; [5.50%] else goto <bb 4>; [94.50%] Removing all of the if conditions on c < a and turning it into true ... ^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug tree-optimization/101653] [9/10/11/12 Regression] Bad code generated when optimizing nested for loops 2021-07-27 22:48 [Bug c/101653] New: Bad code generated when optimizing nested for loops phd at phd dot re ` (2 preceding siblings ...) 2021-07-27 23:09 ` pinskia at gcc dot gnu.org @ 2021-07-27 23:13 ` pinskia at gcc dot gnu.org 3 siblings, 0 replies; 5+ messages in thread From: pinskia at gcc dot gnu.org @ 2021-07-27 23:13 UTC (permalink / raw) To: gcc-bugs https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101653 Andrew Pinski <pinskia at gcc dot gnu.org> changed: What |Removed |Added ---------------------------------------------------------------------------- Status|NEW |RESOLVED Resolution|--- |DUPLICATE --- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> --- So the loop of PR 100740 is same as this one so marking as a dup. That is both have a double loop and the inner one has loop exit which is bound by comparing of the induction variable that is counting how many times inside the loop it has happened and comparing it against in the inner loop induction variable. *** This bug has been marked as a duplicate of bug 100740 *** ^ permalink raw reply [flat|nested] 5+ messages in thread
end of thread, other threads:[~2021-07-27 23:13 UTC | newest] Thread overview: 5+ messages (download: mbox.gz / follow: Atom feed) -- links below jump to the message on this page -- 2021-07-27 22:48 [Bug c/101653] New: Bad code generated when optimizing nested for loops phd at phd dot re 2021-07-27 22:55 ` [Bug tree-optimization/101653] [9/10/11/12 Regression] " pinskia at gcc dot gnu.org 2021-07-27 23:01 ` jakub at gcc dot gnu.org 2021-07-27 23:09 ` pinskia at gcc dot gnu.org 2021-07-27 23:13 ` pinskia at gcc dot gnu.org
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