https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109442
--- Comment #19 from Jan Hubicka <hubicka at gcc dot gnu.org> ---
Note that the testcase from PR115037 also shows that we are not able to
optimize out dead stores to the vector, which is another quite noticeable
problem.
void
test()
{
std::vector<int> test;
test.push_back (1);
}
We alocate the block, store 1 and immediately delete it.
void test ()
{
int * test$D25839$_M_impl$D25146$_M_start;
struct vector test;
int * _61;
<bb 2> [local count: 1073741824]:
_61 = operator new (4);
<bb 3> [local count: 1063439392]:
*_61 = 1;
operator delete (_61, 4);
test ={v} {CLOBBER};
test ={v} {CLOBBER(eol)};
return;
<bb 4> [count: 0]:
<L1>:
test ={v} {CLOBBER};
resx 2
}
So my understanding is that we decided to not optimize away the dead stores
since the particular operator delete does not pass test:
/* If the call is to a replaceable operator delete and results
from a delete expression as opposed to a direct call to
such operator, then we can treat it as free. */
if (fndecl
&& DECL_IS_OPERATOR_DELETE_P (fndecl)
&& DECL_IS_REPLACEABLE_OPERATOR (fndecl)
&& gimple_call_from_new_or_delete (stmt))
return ". o ";
This is because we believe that operator delete may be implemented in an insane
way that inspects the values stored in the block being freed.
I can sort of see that one can write standard conforming code that allocates
some data that is POD and inspects it in destructor.
However for std::vector this argument is not really applicable. Standard does
specify that new/delete is used to allocate/deallocate the memory but does not
say how the memory is organized or what happens before deallocation.
(i.e. it is probably valid for std::vector to memset the block just before
deallocating it).
Similar argument can IMO be used for eliding unused memory allocations. It is
kind of up to std::vector implementation on how many allocations/deallocations
it does, right?
So we need a way to annotate the new/delete calls in the standard library as
safe for such optimizations (i.e. implement clang's
__bulitin_operator_new/delete?)
How clang manages to optimize this out without additional hinting?
