[Bug pending/103875] New: Dead writes are not optimized out
lh_mouse at 126 dot com
gcc-bugzilla@gcc.gnu.org
Fri Dec 31 13:08:59 GMT 2021
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103875
Bug ID: 103875
Summary: Dead writes are not optimized out
Product: gcc
Version: 11.2.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: pending
Assignee: unassigned at gcc dot gnu.org
Reporter: lh_mouse at 126 dot com
Target Milestone: ---
I was reading
<https://quuxplusone.github.io/blog/2018/04/17/downsides-of-omitting-trivial-destructor-calls/>,
which I think is a great article. But the issue that was mentioned in that
article seems to be specific to `std::vector`.
Specifically, for this hand-written 'static vector', neither GCC or Clang is
able to optimize out any dead writes (godbolt [1]):
```
struct foo
{
using value_type = int;
unsigned long size;
value_type data[10];
value_type&
back()
{
return this->data[this->size-1];
}
void
pop()
{
this->size --;
this->data[this->size].~value_type(); // the last element is
destroyed here
}
};
void
use_foo(foo& f)
{
f.back() *= 42; // this write should be dead
f.pop();
}
```
```
use_foo(foo&):
mov rdx, QWORD PTR [rdi]
lea rax, [rdx-1]
imul edx, DWORD PTR [rdi+4+rdx*4], 42
mov DWORD PTR [rdi+8+rax*4], edx # dead write is here
mov QWORD PTR [rdi], rax
ret
```
Making `value_type` non-trivial as suggested does not help (godbolt [2]):
```
struct foo
{
struct value_type
{
int val = 0;
int& operator*=(int x) { return this->val *= x; }
~value_type() { } // non-trivial
};
unsigned long size;
value_type data[10];
value_type&
back()
{
return this->data[this->size-1];
}
void
pop()
{
this->size --;
this->data[this->size].~value_type();
}
};
void
use_foo(foo& f)
{
f.back() *= 42;
f.pop();
}
```
```
use_foo(foo&):
mov rdx, QWORD PTR [rdi]
lea rax, [rdx-1]
imul edx, DWORD PTR [rdi+4+rdx*4], 42
mov DWORD PTR [rdi+8+rax*4], edx # dead write is still here
mov QWORD PTR [rdi], rax
ret
```
[1] https://gcc.godbolt.org/z/f9xx3c6Y7
[2] https://gcc.godbolt.org/z/Ysss5GMjj
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