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Subject: [Bug c++/100309] New: [11 regression] false positive
 -Wstringop-overflow/stringop-overread/array-bounds on reinterpret_cast'd
 integers
Date: Wed, 28 Apr 2021 08:27:18 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D100309

            Bug ID: 100309
           Summary: [11 regression] false positive
                    -Wstringop-overflow/stringop-overread/array-bounds on
                    reinterpret_cast'd integers
           Product: gcc
           Version: 11.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: sciresm.gccbugzilla at gmail dot com
  Target Milestone: ---

Created attachment 50697
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=3D50697&action=3Dedit
Minimal test case code.

Bug occurs in GCC 11.1.0, but none of the 10.x releases.

It appears that GCC is now inferring a size of 0 when doing
reinterpret_cast<void*>(ConstantInteger); when doing std::memcpy/std::memset
to/from the result pointers, bogus warnings are emitted about reading/writi=
ng
to regions of zero size.

My target is an embedded system with a fixed memory layout; I have been usi=
ng
constexpr uintptr_t/size_ts's to describe the memory regions, and
correspondingly calls to set or copy memory regions are now emitting bogus
warnings.

I have made an example minimal test case here (also attached):
https://godbolt.org/z/WPaGY8eaz

Relevant errors (compiling with -O -Werror):

void StringopOverread() {
    // error: 'void* memset(void*, int, size_t)' writing 16 bytes into a re=
gion
of size 0 overflows the destination [-Werror=3Dstringop-overflow=3D]
    std::memset(reinterpret_cast<void *>(0xCAFEBABE), 0xCC, 0x10);
}

void StringopOverflow2(const void *src) {
    // error: 'void* memcpy(void*, const void*, size_t)' writing 16 bytes i=
nto
a region of size 0 overflows the destination [-Werror=3Dstringop-overflow=
=3D]
    std::memcpy(reinterpret_cast<void *>(0xCAFEBABE), src, 0x10);
}

void StringopOverread(void *dst) {
    // error: 'void* memcpy(void*, const void*, size_t)' reading 16 bytes f=
rom
a region of size 0 [-Werror=3Dstringop-overread]
    std::memcpy(dst, reinterpret_cast<void *>(0xCAFEBABE), 0x10);
}=