inlining...

[Manu]

So, I'm constantly running into issues with not having control over inline.
I've run into it again doing experiments in preparation for my dconf talk...

I have identified 2 cases which come up regularly:
 1. A function that should always be inline unconditionally (std.simd is
effectively blocked on this)
 2. A particular invocation of a function should be inlined for this call
only

The first case it just about having control over code gen. Some functions
should effectively be macros or pseudo-intrinsics (ie, intrinsic wrappers
in std.simd, beauty wrappers around asm code, etc), and I don't ever want
to see a symbol appear in the binary.

My suggestion is introduction of __forceinline or something like it. We
need this.


The second case is interesting, and I've found it comes up a few times on
different occasions.
In my current instance, I'm trying to build generic framework to perform
efficient composable data processing, and a basic requirement is that the
components are inlined, such that the optimiser can interleave the work
properly.

Let's imagine I have a template which implements a work loop, which wants
to call a bunch of work elements it receives by alias. The issue is, each
of those must be inlined, for this call instance only, and there's no way
to do this.
I'm gonna draw the line at stringified code to use with mixin; I hate that,
and I don't want to encourage use of mixin or stringified code in
user-facing API's as a matter of practise. Also, some of these work
elements might be useful functions in their own right, which means they can
indeed be a function existing somewhere else that shouldn't itself be
attributed as __forceinline.

What are the current options to force that some code is inlined?

My feeling is that an ideal solution would be something like an enhancement
which would allow the 'mixin' keyword to be used with regular function
calls.
What this would do is 'mix in' the function call at this location, ie,
effectively inline that particular call, and it leverages a keyword and
concept that we already have. It would obviously produce a compile error of
the code is not available.

I quite like this idea, but there is a potential syntactical problem; how
to assign the return value?

int func(int y) { return y*y+10; }

int output = mixin func(10); // the 'mixin' keyword seems to kinda 'get in
the way' if the output
int output = mixin(func(10)); // now i feel paren spammy...
mixin(int output = func(10)); // this doesn't feel right...

My feeling is the first is the best, but I'm not sure about that
grammatically.


The other thing that comes to mind is that it seems like this might make a
case for AST macros... but I think that's probably overkill for this
situation, and I'm not confident we're ever gonna attempt to crack that
nut. I'd like to see something practical and unobjectionable preferably.


This problem is fairly far reaching; phobos receives a lot of lambdas these
days, which I've found don't reliably inline and interfere with the
optimisers ability to optimise the code.
There was some discussion about a code unrolling API some time back, and
this would apply there (the suggested solution used string mixins! >_<).
Debug build performance is a problem which would be improved with this
feature.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.puremagic.com/pipermail/digitalmars-d/attachments/20140314/711e7123/attachment-0001.html>