[LLVMbugs] [Bug 12239] New: False positive with count-down loop

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=12239

             Bug #: 12239
           Summary: False positive with count-down loop
           Product: clang
           Version: 3.0
          Platform: Macintosh
        OS/Version: MacOS X
            Status: NEW
          Severity: normal
          Priority: P
         Component: Static Analyzer
        AssignedTo: kremenek at apple.com
        ReportedBy: dave.swofford at gmail.com
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified


I think I have found a false-positive warning with the static analyzer (I say
"think" because every other time I've been convinced that a warning was a false
positive, I eventually realized that I knew something the compiler couldn't
know, and the warning goes away with the appropriate "assert").  With a loop 
that is counting down toward zero, the analyzer may fail to realize that all
elements of an array have been initialized before they are subsequently
referenced.

E.g., with the following example (main.c)...

#include <assert.h>
int m = 3;
int main(void)
{
    int i, x[4];
    assert(m > 1 && m <= 3);
    x[m] = 1.0;
    for (i = m; i > 0; i--)
        x[i - 1] = 2*x[i];
    for (i = m; i > 0; i--)
        x[i] *= x[i - 1];
    return 0;
}

"<path-to>/clang --analyze main.c" reports:

main.c:15:8: warning: The left expression of the compound assignment is an
uninitialized value. The computed value will also be garbage
                x[i] *= x[i - 1];
                ~~~~ ^

But unless I'm missing something, every element of x referenced in the second
loop is initialized in the first loop.

This happens with both the version of clang installed via the Apple developer
tools as well as a freshly installed copy of clang 3.0:

$  /Developer/usr/bin/clang --version
Apple clang version 1.7 (tags/Apple/clang-77) (based on LLVM 2.9svn)
Target: x86_64-apple-darwin10
Thread model: posix

$ ~/tools/clang_llvm/clang+llvm-3.0-x86_64-apple-darwin11/bin/clang --version
clang version 3.0 (tags/RELEASE_30/final)
Target: x86_64-apple-darwin10.8.0
Thread model: posix

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