Optimisation possibilities: current, future and enhancements

[Patrick Schluter via Digitalmars-d]

On Friday, 26 August 2016 at 14:03:13 UTC, Meta wrote:
> 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;
>     }
> }

getN() is not pure, simple as that (and an ideal compiler should 
complain in that case). A function is pure if it depends only of 
the state passed as parameter. If it touches memory that is set 
outside its scope it is NOT PURE!!!

>
> import std.stdio;
>
> void main()
> {
>     auto t = new Test();
>     writeln(t.getN()); //Prints 0
>     t.setN(1);
>     writeln(t.getN()); //Prints 1

It has to, as getN() is not pure.

> }

In case you didn't get it, getN() is not pure. Marking it as such 
is a BUG!.