[Bug c++/70834] Incorrect warning for placement new when conditionally used
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70834
frankhb1989 at gmail dot com changed:
What|Removed |Added
CC||frankhb1989 at gmail dot com
--- Comment #8 from frankhb1989 at gmail dot com ---
(In reply to Martin Sebor from comment #7)
> No change in GCC 11. This false positive too will be avoided by running
> -Wplacement-new later, when the program is in SSA form, rather than in the
> front end.
Actually worse in GCC 11. Try this case:
#include
#include
#include
#include
#include
struct hdr_t
{
void* p_block;
};
constexpr auto offset = sizeof(hdr_t) > alignof(hdr_t) ? sizeof(hdr_t) :
alignof(hdr_t);
void* operator new(std::size_t bytes, std::size_t alignment)
{
auto space(offset + bytes + alignment);
auto ptr(static_cast(std::malloc(space)));
void* p([offset]);
if(std::align(alignment, bytes, p, space))
{
(::new(static_cast(static_cast(p) - offset))
hdr_t)->p_block = ptr;
// (::new(ptr + (std::size_t(static_cast(p) - ptr)) -
offset) hdr_t)->p_block = ptr;
return p;
}
throw std::bad_alloc();
}
int main()
{}
This is warned in G++ 11.1, but not G++ 10.2.
[Bug c++/70834] Incorrect warning for placement new when conditionally used
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70834 Martin Sebor changed: What|Removed |Added Last reconfirmed|2017-01-03 00:00:00 |2021-5-3 Blocks||100399 Known to fail|8.0 |11.1.0, 12.0, 8.4.0 --- Comment #7 from Martin Sebor --- No change in GCC 11. This false positive too will be avoided by running -Wplacement-new later, when the program is in SSA form, rather than in the front end. Referenced Bugs: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100399 [Bug 100399] bogus/missing -Wplacement-new
[Bug c++/70834] Incorrect warning for placement new when conditionally used
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70834 Martin Sebor changed: What|Removed |Added Known to fail||6.4.0, 7.2.0, 8.0 --- Comment #6 from Martin Sebor --- No change in 8.0.
[Bug c++/70834] Incorrect warning for placement new when conditionally used
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70834
--- Comment #5 from Martin Sebor ---
As a workaround until the warning is made smarter, partial specialization can
be used to avoid the compile-time conditional, for example like so:
struct Test {
char buf[4];
template
struct Ctor {
static T *construct(void*) {
return new T; // else make it on the heap
}
};
template T* construct () {
return Ctor::construct (buf);
}
};
template
struct Test::Ctor {
static T* construct (void *p) {
return new (p) T;
}
};
[Bug c++/70834] Incorrect warning for placement new when conditionally used
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70834 Martin Sebor changed: What|Removed |Added CC||msebor at gcc dot gnu.org --- Comment #4 from Martin Sebor --- I'm hoping to move -Wplacement-new to the middle end in GCC 8 and have it make use of object size checking similarly to -Wstringop-overflow.
[Bug c++/70834] Incorrect warning for placement new when conditionally used
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70834 Manuel López-Ibáñez changed: What|Removed |Added Keywords||diagnostic Status|UNCONFIRMED |NEW Last reconfirmed||2017-01-03 CC||manu at gcc dot gnu.org Ever confirmed|0 |1 --- Comment #3 from Manuel López-Ibáñez --- The warning comes from the FE, which knows nothing about control-flow. We have many open PRs regarding this (see PR23577) but it is not something that will change in the near future. It may work if you use ?: since that is optimised very early (but perhaps not before this warning, I don't know).
[Bug c++/70834] Incorrect warning for placement new when conditionally used
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70834 --- Comment #2 from Matt Godbolt --- if constexpr is of course the right solution here, for certain. I appreciate the compiler determining which if() branch to take in the non-constexpr case is tricky.
[Bug c++/70834] Incorrect warning for placement new when conditionally used
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70834 --- Comment #1 from Roel Standaert --- I guess you can't really expect the compiler to correctly infer the value of the if statement at compile time? If std::enable_if is used instead of an if statement, GCC won't generate a warning: https://godbolt.org/g/GO7Ao7 Also, if you use C++17's "if constexpr", it's fine: https://godbolt.org/g/H58gBI
