Thoughts about D

[Nicholas Wilson]

On Friday, 1 December 2017 at 11:07:32 UTC, Walter Bright wrote:
> On 11/30/2017 8:34 PM, Nicholas Wilson wrote:
>> What I meant in terms of icache pollution is with the 'cold' 
>> is instead of generating:
>> 
>> if(!cond)
>>      _d_assert(__FILE__, __LINE__,message);
>> //rest of code
>> 
>> it should actually generate,
>> 
>> if (!cond)
>>      goto failed;
>> //rest of code
>> 
>> failed:
>>       _d_assert(__FILE__, __LINE__,message);//call is cold & 
>> out of line. no icache pollution
>> 
>> I'm not sure that it does that given the triviality of the 
>> example, but it looks like it doesn't.
>
> You're right, it would be better to generate code that way. But 
> it currently does not (I should fix that).

Great!

> It's not completely correct that icache isn't polluted.

True.

> Functions that are tightly coupled can be located adjacent for 
> better cache performance, and the various asserts would push 
> them apart.

Does DMD optimise for locality?
I would hope co-located functions are either larger than cache 
lines by a reasonable amount or, if they are small enough, 
inlined so that the asserts can be aggregated. It is also 
possible (though I can't comment on how easy it would be to 
implement) if you are trying to optimise for co-location to have 
the asserts be completely out of line so that you have

function1
function2
function3
call asserts of function1
call asserts of function2
call asserts of function3

such that the calls to the asserts never appear in the icache at 
all apart from overlap of e.g. function1's asserts after the end 
of function3, or one of the the asserts fail.

> Also, the conditional jumps may need to be the longer variety 
> due to the longer distance, rather than the 2 byte one.

Then it becomes a tradeoff, one that I'm glad the compiler is 
doing for me.