[LLVMbugs] [Bug 14973] New: infinite loop optimized away

bugzilla-daemon at llvm.org bugzilla-daemon at llvm.org
Wed Jan 16 17:17:09 PST 2013


             Bug #: 14973
           Summary: infinite loop optimized away
           Product: clang
           Version: trunk
          Platform: PC
        OS/Version: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: -New Bugs
        AssignedTo: unassignedclangbugs at nondot.org
        ReportedBy: dhazeghi at yahoo.com
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

We have found an inconsistency in the following testcase (derived from
pr33992.c in the gcc testsuite) using clang 3.2 and trunk.  When compiled
without optimization, the loop does not terminate.  However, with -O1 or above,
clang optimizes away the loop.  This behavior appears incorrect, as overflow
behavior is well defined with unsigned int.  Using -fwrapv and -ftrapv does not
change this result.

$ clang -O0 reduced.c
$ ./a.out 
^C [hangs]
$ clang -O1 reduced.c
$ ./a.out 
$ clang -v
clang version 3.3 (trunk 172630)
Target: x86_64-pc-linux-gnu
Thread model: posix
$ cat reduced.c
void foo () 
  unsigned int i = 0;

  for (i = 0; ; i++) { }

int main()

  return 0;

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