[Bug c++/103843] New: Direct call to Desctructor is optimized out
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103843
Bug ID: 103843
Summary: Direct call to Desctructor is optimized out
Product: gcc
Version: 10.2.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: georgii.sha...@be-tse.de
Target Milestone: ---
cat ./d.cpp
#include
class S {
public:
S() = default;
~S() { i=10; }
void foo() { this->~S(); }
int getI() const { return i; }
private:
int i{0};
};
int main()
{
S s;
do {
std::cout << "Before foo: " << s.getI();
s.foo();
std::cout << "; After: " << s.getI() << std::endl;
} while (false);
return 0;
}
g++ -O0 ./d.cpp
$./a.out
Before foo: 0; After: 10
g++ -O3 ./d.cpp
georgii@ltgscosvm:~/prj/test$./a.out
Before foo: 0; After: 0
gcc --version
gcc (GCC) 10.2.1 20210130 (Red Hat 10.2.1-11)
Copyright (C) 2020 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
hostnamectl
Static hostname: ltgscosvm.be-tse01.de
Icon name: computer-vm
Chassis: vm
Machine ID: fb944a0ffb46449f9b639e589d00b598
Boot ID: 433e59a4db5c419a9081cc6968e4e590
Virtualization: microsoft
Operating System: CentOS Linux 7 (Core)
CPE OS Name: cpe:/o:centos:centos:7
Kernel: Linux 3.10.0-1160.45.1.el7.x86_64
Architecture: x86-64
uname -a
Linux ltgscosvm.be-tse01.de 3.10.0-1160.45.1.el7.x86_64 #1 SMP Wed Oct 13
17:20:51 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux
[Bug c++/103843] Direct call to Desctructor is optimized out
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103843
--- Comment #1 from Georgii.Shagov ---
>From experiments, I would guess that in case of -O3 the call to destructor was
substituted by initializer:
This works fine:
cat ./d.cpp
#include
class S {
public:
S() : i{1} {}
~S() { i=0; }
void foo() { this->~S(); }
int getI() const { return i; }
private:
int i{0};
};
int main()
{
S s;
do {
std::cout << "Before foo: " << s.getI();
s.foo();
std::cout << "; After: " << s.getI() << std::endl;
} while (false);
return 0;
}
g++ ./d.cpp
$./a.out
Before foo: 1; After: 0
$g++ -O3 ./d.cpp
$./a.out
Before foo: 1; After: 0
g++ --version
g++ (GCC) 10.2.1 20210130 (Red Hat 10.2.1-11)
[Bug c++/103843] Direct call to Desctructor is optimized out
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103843 Georgii.Shagov changed: What|Removed |Added Resolution|INVALID |FIXED --- Comment #3 from Georgii.Shagov --- (In reply to Andrew Pinski from comment #2) > This is undefined code. The object is officially not live after you call the > deconstructor so GCC is able to remove the store from the deconstructor as > being dead. I appreciate your reply. But this is confusing. The object was NOT released (de-allocated). In essence Destructor is just a function. Why the content of the class had been re-initialized? IMU: there should be not such obvious difference between optimized and non-optimized code thnx in advance.
[Bug c++/103843] Direct call to Desctructor is optimized out
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103843
Georgii.Shagov changed:
What|Removed |Added
Resolution|INVALID |FIXED
--- Comment #5 from Georgii.Shagov ---
> No, the destructor is not just a function in C++, it terminates the object
> being alive (not de-allocated though). You can then reuse the space for
> another object, either the same type or a different type by using the
> inplacement new.
>
> > Why the content of the class had been re-initialized?
>
> It was not re-initialized rather the store was removed.
>
> > IMU: there should be not such obvious difference between optimized and
> > non-optimized code
>
> why not, it is undefined code.
I really appreciate and value your Reply, Andrew.
I have modified the code a little bit:
#include
class S {
public:
S() = default;
~S() { i=10; }
void foo() { this->~S(); }
int getI() const { return i; }
private:
int i{100};
};
int main()
{
S s;
do {
std::cout << "Before foo: " << s.getI();
s.foo();
std::cout << "; After: " << s.getI() << std::endl;
} while (false);
return 0;
}
$g++ -O3 ./d.cpp
$./a.out
Before foo: 100; After: 0
$g++ -O0 ./d.cpp
$./a.out
Before foo: 100; After: 10
> No, the destructor is not just a function in C++, it terminates the object
> being alive (not de-allocated though).
I do understand your point, yet I would not feel comfortable in sharing one.
- It is not clear: what function/code has reset S::i value to 0 after a Direct
call to Destructor?
- Why 0? :-?
- In order to avoid misleading I have allocated the object on the stack as you
can see. So, no-memory mgmt is not involved
- Does it mean gcc implants some code, being called after Destructor call,
resetting the content of the class Instance to 0? ::memset??
- Is there any agreement unto this? I would be happy to learn more about this.
Thnx in advance,
Yours truly, George
