Optimisation possibilities: current, future and enhancements

[Basile B. via Digitalmars-d]

On Friday, 26 August 2016 at 14:12:24 UTC, Basile B. wrote:
> 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;
>>     }
>> }
>>
>> import std.stdio;
>>
>> void main()
>> {
>>     auto t = new Test();
>>     writeln(t.getN()); //Prints 0
>>     t.setN(1);
>>     writeln(t.getN()); //Prints 1
>> }
>
> That's another story. Of course the optimization pass for this 
> should check that **ALL** the calls to Test in a sub program 
> (or in this scope if you want) are const... Which is not the 
> case here.

void foo(Bar bar)
{
     foreach(i; 0 .. bar.length) // bar.length can be evaluated 
once
     {
        stuffA.a = bar[i].propertyA // bar[i] can be cached
        stuffB.b = bar[i].propertyB // to get propertyB
     }
}

with

interface Bar
{
     size_t length() pure const;
     Stuff opIndex(size_t) pure const;
}