[LLVMbugs] [Bug 23893] New: False positive (divide by zero), involving ALIGNOF()

[bugzilla-daemon at llvm.org]

https://llvm.org/bugs/show_bug.cgi?id=23893

            Bug ID: 23893
           Summary: False positive (divide by zero), involving ALIGNOF()
           Product: clang
           Version: 3.4
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: Static Analyzer
          Assignee: kremenek at apple.com
          Reporter: arnsholt at gmail.com
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

In the MoarVM project, we have some code that looks like this (computing
offsets for struct members):

    size_t cur_size;
    size_t align = ALIGNOF(void *);
    /* Stuff we don't care about here, setting cur_size */
    if (cur_size % align)
        cur_size += align - cur_size % align;

The static analyzer thinks that `align` is initialized to 0 and thus that
there's a potential divide-by-zero in the modulo operation. This is clearly not
possible, since the smallest possible ALIGNOF value is 1.

Our ALIGNOF is implemented like this:

    #define ALIGNOF(t) ((char *)(&((struct { char c; t _h; } *)0)->_h) - (char
*)0)

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