[Bug 1250883] [NEW] Optimizer's mistake

[Shkarnikov Sergey]

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

Status in “gcc-4.6” package in Ubuntu:
  New

Bug description:
  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.

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