Optimisation possibilities: current, future and enhancements

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On Friday, 26 August 2016 at 10:51:15 UTC, Johan Engelen wrote:
> On Thursday, 25 August 2016 at 14:42:28 UTC, Basile B. wrote:
>> 
>> I'll add
>>
>> * create temporaries based on the const function attribute.
>
> Struct method constness (as in your example) does not mean that 
> the return value is constant when calling it twice in a row. As 
> pointed out by others, the function needs to be pure. Dlang 
> pureness is not a strong enough guarantee.
> For example, this is explicitly allowed by the spec:
> ```
> pure int foo()
> {
>     debug writeln("in foo()"); // ok, impure code allowed in 
> debug statement
>     return 1;
> }
> ```
> That makes it illegal to transform `foo()+foo()` to `a=foo(); 
> a+a`, at least in debug builds.
>
> David discusses your proposed optimization, and why it cannot 
> be done in general (!) on Dlang pure functions.
> http://klickverbot.at/blog/2012/05/purity-in-d/
>
> -Johan

Here's an example that doesn't even need to use debug statements, 
and is perfectly legal.

class Test
{
     int n;

     void setN(int val) pure
     {
         n = val;
     }

     int getN() const pure
     {
         return n;
     }
}

import std.stdio;

void main()
{
     auto t = new Test();
     writeln(t.getN()); //Prints 0
     t.setN(1);
     writeln(t.getN()); //Prints 1
}