inlining...

[Ola Fosheim Grøstad" <ola.fosheim.grostad+dlang at gmail.com>]

On Thursday, 20 March 2014 at 02:08:16 UTC, Manu wrote:
> The problem is upside down. If you want to inline multiple 
> levels, you
> start from the leaves and move downwards, not from the root 
> moving upwards

Yes, that is true in cases where leaves are frequently visited. 
Good point. I am most interested in full inlining, but the 
heuristics should probably start with the leaves for people not 
interested in that. Agree.

Anyway, in the case of ray tracing (or any search structure) I 
could see the value of having the opposite in combination with 
CTFE/partial evaluation.

Example: Define a static scene (of objects) and let the compiler 
turn it into "a state machine" of code.

Another example: Define an array of data, use partial evaluation 
to turn it into a binary tree, then turn the binary tree into 
code.

> Inlining should be strictly deliberate, there's nothing to say 
> that every
> function called in a tree should be inlined. There's a high 
> probability
> there's one/some that shouldn't be among a few that should.

In the case of a long running loop it does not really matter. 
What it does get you is a chance to use generic code (or 
libraries) and then do a first-resort optimization. I basically 
see it as a time-saving feature (programmers time). A tool for 
cutting development costs.

> Remember too, that call-site inlining isn't the only method, 
> there would
> also be always-inline...

Yes, that is the first. I have in another thread some time ago 
suggested a solution that use weighted inlining to aid compiler 
heuristics:

http://forum.dlang.org/thread/szjkyfpnachnnyknnfwp@forum.dlang.org#post-szjkyfpnachnnyknnfwp:40forum.dlang.org

As you can see I also suggested call-site inlining, so I am fully 
behind you in this. :-) Lack of inlining and GC are my main 
objections to D.

> I think always-inline is what you want for some
> decidedly trivial functions (although these will probably be 
> heuristically
> inlined anyway), not call-site inlining.

I agree. Compiler heuristics can change. It is desirable to be 
able to express intent no matter what the current heuristics are.

> I just don't see how recursive
> call-site inlining is appropriate, considering that call trees 
> are often
> complex, subject to change, and may even call functions that 
> you don't have
> source for.

You should not use it blindly.

> You can cascade the mixin keyword if you want to, that's very 
> simple.

Not if you build the innerloop using generic components. I want 
this

inline_everything while(conditon){
statement;
statement;
}

> I'd be highly surprised if you ever encountered a call tree 
> where
> you wanted to inline everything (and the optimiser didn't do it 
> for you).

Not if you move to high-level programming using prewritten code 
and only go low level after profiling.

> As soon as you encounter a single function in the tree that 
> shouldn't be
> inlined, then you'll be forced to do it one level at a time 
> anyway.

But then you have to change the libraries you are using!?

Nothing prevents you to introduce exceptions as an extension 
though. I want inline(0.5) as default, but also be able to write 
inline(1) for inline always and inline(0) for inline never.

func1(){} // implies inline(0.5) weighting
inline func2(){} // same as inline(1) weighting, inline always
inline(0.75) fun31(){} // increase the heuristics weighting
inline(0) func4(){} // never-ever inline

Ola.