March 21
On Friday, 19 March 2021 at 17:10:27 UTC, Ali Çehreli wrote:

> Jon mentioned how PR 7678 reduced the performance of std.regex.matchOnce. After analyzing the code we realized that the performance loss must be due to two delegate context allocations:
>
> https://github.com/dlang/phobos/pull/7678/files#diff-269abc020de3a951eaaa5b8eca5a0700ba8b298767c7a64f459e74e1531a80aeR825
>
> One delegate is 'matchOnceImp' and the other one is the anonymous delegate created on the return expression.
>
> We understood that 'matchOnceImp' could not be a nested function because of an otherwise useful rule: the name of the nested function alone would *call* that function instead of being a symbol for it. That is not the case for a local delegate variable, so that's why 'matchOnceImp' exists as a delegate variable there.
>
> Then there is the addition of the 'pure' attribute to it. Fine...
>
> After tinkering with the code, we realized that the same effect can be achieved with a static member function of a static struct, which would not allocate any delegate context. I add @nogc to the following code to prove that point. The following code is even simpler than Jon and I came up with yesterday.
>
> [... Code snippet removed ...]
>
> There: we injected @trusted code inside a @nogc @safe function.
>
> Question to others: Did we understand the reason for the convoluted code in that PR fully? Is the above method really a better solution?

I submitted PR 7902 (https://github.com/dlang/phobos/pull/7902) to address this. I wasn't able to use the version Ali showed in the post, but the PR does use what is essentially the same idea identified at the D Meetup. It is a performance regression, and is a bit more nuanced than would be ideal. Comments and review would be appreciated.

--Jon


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