[Bug 1251169] [NEW] during compilation with -O2 a function call is moved to the wrong place
Public bug reported: OS: Ubuntu 12.04.3 LTS clang version: 3.0-6ubuntu3 In the attached code the function foo2 has a side effect on its second argument. So, despite the fact that value of v1 has been previously set to 0, foo1 sould return 0. But if this code was compiled with -O2 (or higher optimization level) foo1 returns 1. Decompilation shows that after optimization the call of foo2 is placed after the check (v1!=0). ** Affects: clang (Ubuntu) Importance: Undecided Status: New ** Tags: optimizer ** Attachment added: Example code https://bugs.launchpad.net/bugs/1251169/+attachment/3908688/+files/clang_bug.c -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1251169 Title: during compilation with -O2 a function call is moved to the wrong place To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/clang/+bug/1251169/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
[Bug 1250883] [NEW] Optimizer's mistake
Public bug reported:
OS: Ubuntu 12.04.3 LTS
gcc version: 4.6.3-1ubuntu5
It's a slightly modificated test case from gcc-4.6.3 testsuite
(920612-1.c):
int main(){
int a = 0;
if ((int)((~0u) (1 + a))+1 0)
abort();
exit(0);
}
It operates fine after compilation with -O0 or -O1 option (exit is
executed), but changes behavior if compiled with -O2 or -O3 (abort is
executed). Decompilation shows that optimizer reduces the code down to
abort(); . Besides, if variable a is absent, bug is absent even with
O3 optimization. So in simple case the optimizer takes into account the
overflow, but misses it after a trivial change of the code.
** Affects: gcc-4.6 (Ubuntu)
Importance: Undecided
Status: New
** Tags: optimizer
** Description changed:
OS: Ubuntu 12.04.3 LTS
gcc version: 4.6.3-1ubuntu5
It's a slightly modificated test case from gcc-4.6.3 testsuite
(920612-1.c):
int main(){
- int a = 0;
- if ((int)((~0u) (1 + a))+1 0)
- abort();
- exit(0);
+ int a = 0;
+ if ((int)((~0u) (1 + a))+1 0)
+ abort();
+ exit(0);
}
It operates fine after compilation with -O0 or -O1 option (exit is
- executed), but changes behaiviour if compiled with -O2 or -O3 (abort is
+ executed), but changes behavior if compiled with -O2 or -O3 (abort is
executed). Decompilation shows that optimizer reduces the code down to
abort(); . Besides, if variable a is absent, bug is absent even with
O3 optimization. So in simple case the optimizer takes into account the
overflow, but misses it after a trivial change of the code.
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https://bugs.launchpad.net/bugs/1250883
Title:
Optimizer's mistake
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[Bug 1250904] [NEW] variable is optimized out by mistake
Public bug reported:
OS: Ubuntu 12.04.3 LTS
gcc version: 4.6.3-1ubuntu5
It's a modificated test case from gcc-4.6.3 testsuite (reassoc-11.c):
int main(int a, int b, int c, int d){
int e = (a ^ b) ^ (c ^ d);
int f = (c ^ a) ^ (b ^ d);
return ( * ((int *)(((long unsigned ) c) + (((long unsigned ) e) - ((long
unsigned ) c)) *
(f = (~(f ^ (~((b 2 | f 2) - (( b | f) 2) ^ f;
}
Those terrifying expression in braces should be evaluated to e ( (f
= (~(f ^ (~((b 2 | f 2) - (( b | f) 2)) is a predicate
and always equal to 1) and so test should return 0. But if the
optimization is applied (O1 and greater) test returns some other value.
Decompilation shows that e and f are optimized out. f value is
evaluated in place, but evaluation of e is also optimized out. Then
expression in braces is evaluated to some garbage value (interesting
that compiler handles somehow dereferencing of e even after it was
optimized out).
Example is quite exotic, but, in my opinion, reveals incorrect
optimizer's behavior.
** Affects: gcc-4.6 (Ubuntu)
Importance: Undecided
Status: New
** Tags: optimizer
** Description changed:
OS: Ubuntu 12.04.3 LTS
gcc version: 4.6.3-1ubuntu5
It's a modificated test case from gcc-4.6.3 testsuite (reassoc-11.c):
int main(int a, int b, int c, int d){
- int e = (a ^ b) ^ (c ^ d);
- int f = (c ^ a) ^ (b ^ d);
- return ( * ((int *)(((long unsigned ) c) + (((long unsigned ) e) -
((long unsigned ) c)) *
-
(f = (~(f ^ (~((b 2 | f 2)
- (( b | f) 2) ^ f;
+ int e = (a ^ b) ^ (c ^ d);
+ int f = (c ^ a) ^ (b ^ d);
+ return ( * ((int *)(((long unsigned ) c) + (((long unsigned ) e) -
((long unsigned ) c)) *
+ (f = (~(f ^ (~((b 2 | f 2) - (( b | f) 2) ^ f;
}
Those terrifying expression in braces should be evaluated to e ( (f
= (~(f ^ (~((b 2 | f 2) - (( b | f) 2)) is a predicate
and always equal to 1) and so test should return 0. But if the
optimization is applied (O1 and greater) test returns some other value.
Decompilation shows that e and f are optimized out. f value is
evaluated in place, but evaluation of e is also optimized out. Then
expression in braces is evaluated to some garbage value (interesting
that compiler handles somehow dereferencing of e even after it was
optimized out).
Example is quite exotic, but, in my opinion, reveals incorrect
optimizer's behavior.
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https://bugs.launchpad.net/bugs/1250904
Title:
variable is optimized out by mistake
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[Bug 1250941] [NEW] optimization of a recursive function cause segmentation fault during execution (in very rare cases)
Public bug reported:
OS: Ubuntu 12.04.3 LTS
gcc version: 4.6.3-1ubuntu5
It's a modificated test case from gcc-4.6.3 testsuite (2412-2.c):
int f(int a){
int x = (( a)[(( a % 10) * ( a % 10)) % 10) (((10 - ( a % 10)) * (10
- ( a % 10))) % 10))) * 314160879)]);
if (x==0)
return 1;
return f(x-1);
}
int main(int argc,char **argv)
{
if (f (2) != 1)
abort ();
exit (0);
}
( a % 10) * ( a % 10)) % 10) (((10 - ( a % 10)) * (10 - ( a %
10))) % 10))) is a predicate and always equal to 0. So x is
actually equal to a.
As you can see this test should exit successfully, but if it was
compiled with optimization (O2 and higher, O1 works fine) segmentation
fault happens during execution.
** Affects: gcc-4.6 (Ubuntu)
Importance: Undecided
Status: New
** Tags: optimizer
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Title:
optimization of a recursive function cause segmentation fault during
execution (in very rare cases)
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