https://gcc.gnu.org/bugzilla/show_bug.cgi?id=122083
Bug ID: 122083
Summary: [Regression 15/16] Bogus -Wnonnull for call to memcmp
with length determined using <=> operator
Product: gcc
Version: 15.2.1
Status: UNCONFIRMED
Keywords: diagnostic
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: redi at gcc dot gnu.org
Blocks: 95507
Target Milestone: ---
Reduced from 25_algorithms/lexicographical_compare_three_way/1.cc in the
libstdc++ testsuite, compile with -std=gnu++20 -O2 -Wall
#include <compare>
template<typename Tp>
constexpr auto
min_cmp(Tp x, Tp y)
{
struct Res {
Tp M_min;
decltype(x <=> y) M_cmp;
};
auto c = x <=> y;
if (c > 0)
return Res{y, c};
return Res{x, c};
}
template<typename InputIter1, typename InputIter2>
auto
lexicographical_compare_three_way(InputIter1 first1,
InputIter1 last1,
InputIter2 first2,
InputIter2 last2)
-> decltype(*first1 <=> *first2)
{
const auto [len, lencmp] =
min_cmp(last1 - first1, last2 - first2);
if (len)
{
const auto blen = len * sizeof(*first1);
const auto c
= __builtin_memcmp(&*first1, &*first2, blen) <=> 0;
if (c != 0)
return c;
}
return lencmp;
}
auto
test03()
{
unsigned char a[2] = { 1, 2 };
unsigned char* p = nullptr;
return lexicographical_compare_three_way(p, p, a, a+2);
}
In function ‘decltype (((* first1) <=> (* first2)))
lexicographical_compare_three_way(InputIter1, InputIter1, InputIter2,
InputIter2) [with InputIter1 = unsigned char*; InputIter2 = unsigned char*]’,
inlined from ‘auto test03()’ at lexcmp.cc:44:43:
lexcmp.cc:32:29: warning: argument 1 null where non-null expected because
argument 3 is nonzero [-Wnonnull]
32 | = __builtin_memcmp(&*first1, &*first2, blen) <=> 0;
| ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~
lexcmp.cc:32:29: note: in a call to built-in function ‘int
__builtin_memcmp(const void*, const void*, long unsigned int)’
The call to __builtin_memcmp only happens if min(last1 - first1, last2 -
first2) != 0 so there is no call with a null argument.
Referenced Bugs:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95507
[Bug 95507] [meta-bug] bogus/missing -Wnonnull
