On Saturday, 16 March 2019 at 03:47:43 UTC, Murilo wrote:
Does anyone know if when I create a variable inside a scope as
in
{int a = 10;}
it disappears complete from the memory when the scope finishes?
Or does it remain in some part of the memory? I am thinking of
using scopes to make optimized programs that consume less
memory.
Others have made good points in this thread, but what is missing
is that indeed scopes _can_ be used beneficially to reduce memory
footprint.
I recommend playing with this code on d.godbolt.org:
```
void func(ref int[10] a); // important detail: pointer
void foo()
{
{
int[10] a;
func(a);
} {
int[10] b;
func(b);
}
}
```
Because the variable is passed by reference (pointer), the
optimizer cannot merge the storage space of `a` and `b` _unless_
scope information is taken into account. Without taking scope
into account, the first `func` call could store the pointer to
`a` somewhere for later use in the second `func` call for
example. However, because of scope, using `a` after its scope has
ended is UB, and thus variables `a` and `b` can be used.
GDC uses scope information for variable lifetime optimization,
but LDC and DMD both do not.
For anyone interested in working on compilers: adding variable
scope lifetime to LDC (not impossibly hard) would be a nice
project and be very valuable.
-Johan
