[GSoC] 'Replace Runtime Hooks with Templates' progress and update thread

[Seb]

On Thursday, 30 May 2019 at 19:20:36 UTC, rikki cattermole wrote:
> On 31/05/2019 4:45 AM, H. S. Teoh wrote:
>> On Thu, May 30, 2019 at 12:15:55PM +0000, Johannes Pfau via 
>> Digitalmars-d wrote:
>> [...]
>>> Now regarding this specific GSoC project: I don't think we 
>>> have to
>>> worry that this will affect compilation speed a lot. The 
>>> compiler code
>>> will be mostly identical, only instead of calling an extern(C)
>>> function, it calls a template. Simple template instantiations 
>>> are not
>>> particularily slow though, so this should not cause much of a 
>>> slow
>>> down.
>> 
>> That depends on how the template is implemented, doesn't it?  
>> If there
>> are too many recursive templates, for example, the slowdown 
>> can be quite
>> dramatic.
>> 
>> 
>>> There is however one side-effect: Unlike the extern(C) 
>>> functions used
>>> before, template instances will be visible to the optimizer. 
>>> This
>>> means that parts of these hooks can be inlined and optimized 
>>> to
>>> improve performance. This however might affect compile time a 
>>> bit.
>> [...]
>> 
>> There's also the issue of code bloat, which should at least be
>> considered, even if actually solving it might complicate 
>> things a bit
>> too much for GSoC.  Previously copying an array from A to B 
>> involved
>> only a single runtime function; now we're looking at O(n) 
>> template
>> instantiations, which potentially translates to O(n) functions 
>> that,
>> ostensibly, do the exact same copying of some number of bytes 
>> from A to
>> B.
>
> Agreed, I'm worried about template bloat.
>
> But at the same time, I hope we can force inlining. If we can, 
> that will solve that problem.

FWIW we have been doing this for years now (have a look at the 
various templates in object.d, e.g. __cmp or __switch) and the 
fact that no one noticed nor complained, shows that this is no 
concern.

tl;dr: the compiler already generates a special template 
everytime you do e.g. a switch, a cast, a comparison between 
classes etc.

Examples:
- https://run.dlang.io/is/gsCSO1
- https://run.dlang.io/is/ti7rlu

There are many more.