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From: "gabravier at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/94911] New: Failure to optimize comparisons
 of VLA sizes
Date: Fri, 01 May 2020 21:04:06 +0000
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: tree-optimization
X-Bugzilla-Version: 10.0
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D94911

            Bug ID: 94911
           Summary: Failure to optimize comparisons of VLA sizes
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gabravier at gmail dot com
  Target Milestone: ---

inline void assume(_Bool b)
{
    if (!b)
        __builtin_unreachable();
}

_Bool f(int n)
{
    assume(n >=3D 1);
    typedef int A[n];
    ++n;
    A a;
    int b[n];
    n -=3D 2;
    typedef int C[n];
    C c;

    return (sizeof(a) < sizeof(b)) && (sizeof(a) > sizeof(c));
}

This C code (this will have different results in C++ with GCC) should always
`return true`. LLVM makes this transformation, but GCC does not.

Also, extra question, is the fact that this always returns `false` when
compiled as C++ normal ? Clang has it return `true` if compiled as C++.=