inlining...

Manu turkeyman at gmail.com
Thu Mar 20 05:31:05 PDT 2014


On 20 March 2014 18:35,
<7d89a89974b0ff40.invalid at internationalized.invalid>wrote:

> 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.
>

I'm sorry. I really can't support any of these wildly complex ideas. I just
don't feel they're useful, and they're not very well founded.
A numeric weight? What scale is it in? I'm not sure of any
'standard-inline-weight-measure' that any programmer would be able to
intuitively gauge the magic number against. That will simply never be
agreed by the devs.
It also doesn't make much sense... different platforms will assign very
different weights and different heuristics at the inliner. It's not a
numeric quantity; it's a complex determination whether a function is a good
candidate or not.
The value you specify is likely highly context sensitive and probably not
portable. Heuristic based Inlining should be left to the optimiser to
decide.

And I totally object to recursive inlining. It has a kind of absolute
nature that removes control all the way down the call tree, and I don't
feel it's likely that you would often (ever?) want to explicitly inline an
entire call tree.
If you want to inline a second level, then write mixin in the second level.
Recurse.
You are talking about generic code as if this isn't appropriate, but I
specifically intend to use this in generic code very similar to what you
suggest; so I don't see the incompatibility.
I think you're saying like manually specifying it all the way down the call
tree is inconvenient, but I would argue that manually specifying
*exclusions* throughout the call tree after specifying a recursive inline
is even more inconvenient. It requires more language (a feature to mark an
exclusion), has a kind of obtuse double-negative logic about it, and it's
equally invasive to your code.

If you can prove that single level call-site inlining doesn't satisfy your
needs at some later time, make a proposal then, along with your real-world
use cases. But by throwing it in this thread right now, you're kinda just
killing the thread, and making it very unlikely that anything will happen
at all, which is annoying, because I REALLY need this (I've been trying to
motivate inline support for over 3 years), and I get the feeling you're
just throwing hypotheticals around.

You're still fairly new here, but be aware that feature requests will
become exponentially less likely to be accepted with every degree of
complexity added. By making this seem hard, you're also making it almost
certain not to happen, which isn't in either of our interest.

My OP suggestion is the simplest solution I can conceive which will
definitely satisfy all the real-world use cases that I've ever encountered.
Is predictable, portable, simple.
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