DIP60: @nogc attribute

[Kapps via Digitalmars-d]

On Thursday, 17 April 2014 at 09:46:23 UTC, bearophile wrote:
> Walter Bright:
>
>> http://wiki.dlang.org/DIP60
>>
>> Start on implementation:
>>
>> https://github.com/D-Programming-Language/dmd/pull/3455
>
> If I have this program:
>
> __gshared int x = 5;
> int main() {
>     int[] a = [x, x + 10, x * x];
>     return a[0] + a[1] + a[2];
> }
>
>
> If I compile with all optimizations DMD produces this X86 asm, 
> that contains the call to __d_arrayliteralTX, so that main 
> can't be @nogc:
>
> But if I compile the code with ldc2 with full optimizations the 
> compiler is able to perform a bit of escape analysis, and to 
> see the array doesn't need to be allocated, and produces the 
> asm:
>
> Now there are no memory allocations.
>
> So what's the right behavour of @nogc? Is it possible to 
> compile this main with a future version of ldc2 if I compile 
> the code with full optimizations?
>
> Bye,
> bearophile

That code is not @nogc safe, as you're creating a dynamic array 
within it. The fact that LDC2 at full optimizations doesn't 
actually allocate is simply an optimization and does not affect 
the design of the code.

If you wanted it to be @nogc, you could use:
int main() @nogc {
     int[3] a = [x, x + 10, x * x];
     return a[0] + a[1] + a[2];
}