[Bug c++/101361] Bogus -Wstringop-overread warning with -O3

["mail+gnu at tzik dot jp" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101361

--- Comment #13 from Taiju Tsuiki <mail+gnu at tzik dot jp> ---
I saw a similar inaccurate -Wstringop-overread warning with a smaller
reproducer (attached).
https://wandbox.org/permlink/EPjH0ZPoA4EWky0e

Reproducing gcc was tip of trunk (
https://github.com/gcc-mirror/gcc/commits/33fb1625992ba8180b42988e714460bcab08ca0f
),
and compile options and the error were:
$ g++ -Werror -O3 -std=c++20 -c foo.cc
In function ‘auto operator<=>(const V&, const V&)’,
    inlined from ‘Node* foo(Node*, const V&)’ at minify2/foo.cc:33:17:
minify2/foo.cc:20:36: error: ‘int __builtin_memcmp(const void*, const void*,
long unsigned int)’ specified bound [9223372036854775808, 18446744073709551615]
exceeds maximum object size 9223372036854775807 (9) 
[-Werror=stringop-overread]
   20 |     const auto c = __builtin_memcmp(x.beg, y.beg, len) <=> 0;
      |                    ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~
minify2/foo.cc:17:15: note: source object allocated here
   17 |   }(x.end - x.beg, y.end - y.beg);
      |             ~~^~~

$ cat foo.cc
#include <compare>

struct Res {
  long min;
  std::strong_ordering cmp;
};

struct V {
  const unsigned char* beg = nullptr;
  const unsigned char* end = nullptr;
};

auto operator<=>(const V& x, const V& y) {
  const auto [len, lencmp] = [&](long len_x, long len_y) {
    auto c = len_x <=> len_y;
    return Res{c > 0 ? len_y : len_x, c};
  }(x.end - x.beg, y.end - y.beg);

  if (len) {
    const auto c = __builtin_memcmp(x.beg, y.beg, len) <=> 0;
    if (c != 0)
      return c;
  }
  return lencmp;
}

struct Node {
  Node* next = nullptr;
  V v;
};

Node* foo(Node* x, const V& k) {
  while (x && k < x->v)
    x = x->next;
  return x;
}