[llvm-bugs] [Bug 33341] New: Possible false positive with -Wcast-align for aligned Class

via llvm-bugs llvm-bugs at lists.llvm.org
Wed Jun 7 02:21:11 PDT 2017


            Bug ID: 33341
           Summary: Possible false positive with -Wcast-align for aligned
           Product: clang
           Version: 4.0
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: normal
          Priority: P
         Component: Static Analyzer
          Assignee: kremenek at apple.com
          Reporter: zhengtao.lee at autodesk.com
                CC: llvm-bugs at lists.llvm.org

Created attachment 18586
  --> https://bugs.llvm.org/attachment.cgi?id=18586&action=edit
test code

we have a class which contains 3 doubles and they should be aligned to 8.
however consider the code below

class AcGeVector3d
    double x,y,z;
inline double*
asDblArray(const AcGeVector3d& vec)
    static_assert(alignof(decltype(&vec)) == 4,"hmm?");
    static_assert(alignof(AcGeVector3d*) == 4,"hmm?");
    static_assert(alignof(double*) == 4, "hmm?");
    static_assert(alignof(double[3]) == 8, "hmm?");
    static_assert(alignof(AcGeVector3d) == 8, "hmm?");
    static_assert(alignof(decltype(vec)) == 8, "hmm?");
    static_assert(alignof(decltype(*&vec)) == 8, "hmm?");

    static_assert(alignof(AcGeVector3d) == alignof(double[3]), "AcGeVector3d)
== alignof(double[3])");
    static_assert(alignof(AcGeVector3d*) == alignof(double*), "AcGeVector3d*)
== alignof(double*)");
    static_assert(sizeof(AcGeVector3d) == sizeof(double[3]), "AcGeVector3d) ==
//#pragma clang diagnostic push
//#pragma clang diagnostic ignored "-Wcast-align"
    return (double*)&vec;
//#pragma clang diagnostic pop
bool test(const AcGeVector3d& n)
    double* p = asDblArray(n);
    return p != nullptr;

run "clang++.exe" test.cpp -c -Wcast-align -emit-llvm -std=c++14 -target

the output is:
test.cpp:22:12: warning: cast from 'const AcGeVector3d *' to 'double *'
increases required alignment from 4 to 8
    return (double*)&vec;
1 warning generated.

note that none of the static asserts are triggered.

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