[LLVMbugs] [Bug 22077] New: MemorySanitizer false positive with -O2 or higher

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=22077

            Bug ID: 22077
           Summary: MemorySanitizer false positive with -O2 or higher
           Product: clang
           Version: 3.5
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: -New Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: oliverst at online.de
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

Created attachment 13616
  --> http://llvm.org/bugs/attachment.cgi?id=13616&action=edit
preprocessed C source code

This code

#include <string>

void func(const char* s)
{
    if(s== NULL)
    {
    }
}

int main()
{
    func((std::string("const ") + "manager").c_str());
    return 0;
}

generates the following warning when compiled with "-fsanitize=memory
-fsanitize-memory-track-origins -O2". It also happens with -O3, but not with
-O1 or without optimizations.

==13491== WARNING: MemorySanitizer: use-of-uninitialized-value
    #0 0x7f74e825e04b in main (/home/notroot/trunk/a.out+0x8f04b)
    #1 0x7f74e6dbbec4 in __libc_start_main
/build/buildd/eglibc-2.19/csu/libc-start.c:287
    #2 0x7f74e825da5c in _start (/home/notroot/trunk/a.out+0x8ea5c)

  Uninitialized value was created by an allocation of '' in the stack frame of
function 'main'
    #0 0x7f74e825db60 in main (/home/notroot/trunk/a.out+0x8eb60)

SUMMARY: MemorySanitizer: use-of-uninitialized-value ??:0 main

I am using the official clang 3.5 binaries from llvm.org on ubuntu 14.04.

clang version 3.5.0 (tags/RELEASE_350/final)
Target: x86_64-unknown-linux-gnu
Thread model: posix
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.7
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.7.3
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.8
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.8.2
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.9
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.9.1
Selected GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.8
Candidate multilib: .;@m64
Selected multilib: .;@m64

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