[llvm] r215343 - In LVI(Lazy Value Info), originally value on a BB can only be caculated once,
Daniel Berlin
dberlin at dberlin.org
Tue Sep 30 12:27:00 PDT 2014
FWIW: You don't just lose the performance and memory guarantees if you go
back, you actually lose the guarantee that it will terminate
In fact, when someone did something similar to what is being proposed here
in a similar GCC algorithm, it caused infinite ping-ponging in some cases
until we tracked down what had been committed. While there is a limit in
Jiangning's patch, that will just cause it to decide to stop on whatever
the current ping pong value is, and completely unrelated changes may cause
it to ping pong to the other value.
On Tue, Sep 30, 2014 at 12:10 PM, Nick Lewycky <nicholas at mxc.ca> wrote:
> Jiangning Liu wrote:
>
>> Hi Dan,
>>
>> 2014-09-25 6:01 GMT+08:00 Daniel Berlin <dberlin at dberlin.org
>> <mailto:dberlin at dberlin.org>>:
>>
>>
>>
>> On Wed, Sep 24, 2014 at 12:12 AM, Jiangning Liu
>> <liujiangning1 at gmail.com <mailto:liujiangning1 at gmail.com>> wrote:
>>
>> Hi Dan,
>>
>> I consider your question again, and now I think the lattice
>> lowering order in this algorithm should be "overdefined ->
>> constant/constant_range".
>>
>> At first look, following the text book it seems the lattice
>> value TOP should be "undefined", and BOTTOM should be
>> "overdefined". But this is not true for the specific
>> implementation in this LLVM algorithm.
>>
>>
>> Why?
>>
>>
>> See my comments below.
>>
>>
>> In this algorithm, the lowering order is
>>
>> Overdefined(TOP) -> constant/constant_range -> Undefined(BOTTOM).
>>
>>
>> What does overdefined and undefiend mean then?
>>
>>
>> I think "overdefined" means it is a value that can't be known as a
>> constant at compile time, so it might be any value. "undefined" means it
>> is a value we don't care about at all until we evaluate it, so before
>> the algorithm evaluate it, it's value is unknown.
>>
>
> It's a bit stronger than that. Undefined means that it has no value. An
> uninitialized variable is a good way of thinking about it.
>
> We then collect possible definitions only from statements we've proven are
> reachable (according to our particular model) and add those definitions. If
> there are too many definitions, it's overdefined.
>
>
>>
>> In order to easily implement the algorithm, originally the BBLV
>> is initialized to be BOTTOM, and this doesn't mean the lowering
>> start point is "Undefined", and it should still be "Overdefined"
>> instead.
>>
>>
>> If it's initiatlized to bottom, then that is the TOP of the lattice ;)
>>
>> If we never touch it in the algorithm, it will be kept as it is.
>>
>>
>> Right.
>>
>> This is why once we visit a value at a specific BB, the
>> algorithm will change Undefined to be Overdefined immediately at
>> the very beginning of each lowering procedure.
>>
>>
>> It should either be lowered to constant or overdefined.
>> if lowered to constant, it may be lowered again later to overdefined.
>> It should not be lowered to overdefined and then raised back to
>> constant.
>>
>>
>> If it is lowering to overdefined too quickly, you should make it
>> *not lower* to overdefined*. When I read the patch, and saw the
>> implementation, it looks like you take *overdefined* values and
>> raise them to *constant* sometimes.
>> Did i misread it?
>>
>>
>> I don't think it is to raise to constant, but lower to constant. I'd
>> like to say changing "undefined" to "overdefined" in current algorithm
>> is not a lowering, but an initialization to "overdefined" only.
>>
>
> No, you initialize to "I haven't shown that any definitions are reachable
> yet" aka. undefined then add more definitions. I think the problem here is
> when you started trying to connect "undefined" and "overdefined" to "top"
> and "bottom". There is no standard convention for what "top" and "bottom"
> mean (or more accurately, there are both conventions). This causes an
> enormous amount of confusion for academic papers for a start, where even
> the symbols ⊤ and ⊥ will mean opposite things in different papers, despite
> being consistently pronounced 'top' and 'bottom' respectively. Just stick
> with the terms undefined and overdefined. As far as I know, those have
> crisp meanings.
>
> If a
>
>> value is never visited by the algorithm, it will be "undefined" forever,
>> but this is a meaningless value, and the algorithm never return it. The
>> algorithm can only return either overdefined or constant/constant_range.
>>
>
> It means it's safe to convert into llvm undef, just like an uninitialized
> variable.
>
> If you look into the implementation details of this algorithm,
>> you may find originally the lower ordering is like that.
>> Originally the algorithm will return "overdefined"(TOP) forever
>> for a specific (Val, BB) pair.
>>
>>
>> This sounds like either a buggy or conservative implementationt hen,
>> and i would fix *this* issue not by raising overdefined
>> occasionally, but by stopping it from getting to overdefined in the
>> first place.
>>
>>
>> Again, Instead, I think it is not raising overdefined, but lowering
>> overdefined. So I don't think it is a bug. I admit there might be some
>> misleading implementation in the algorithm that is not perfect, but
>> conceptually, I personally think it is acceptable, although it doesn't
>> strictly follow the text book.
>>
>
> Whether you call it up or down or left or right doesn't matter. The point
> is that once you get to overdefined, it doesn't make sense to go back.
>
> Your variables all start with "no definitions", then you only add possible
> definitions when you show that they're reachable. There's no reason to go
> back and say "oh wait, just kidding, that one really wasn't possible". That
> may be correct but it's no longer a lattice driven algorithm and you lose
> all the performance and memory guarantees that it comes with.
>
> Nick
>
>
>> This could miss some optimization opportunities as I described
>> in the comment. My patch is trying to increase the number of
>> lowering this "overdefined"(TOP) value.
>>
>>
>>
>> This is my current understanding, but maybe I'm wrong.
>>
>> Thanks,
>> -Jiangning
>>
>>
>> 2014-09-23 11:08 GMT+08:00 Jiangning Liu
>> <liujiangning1 at gmail.com <mailto:liujiangning1 at gmail.com>>:
>>
>>
>> Hi Dan,
>>
>>
>> So can you explain how you aren't doing this?
>>
>> It looksl ike you think the lattice goes
>> undefined
>> overdefined
>> constant
>>
>> That should 100% not be the case
>> the lattice order should be
>> undefined
>> constant
>> overdefined
>>
>> undefined means you don't know
>> constant means it has one value.
>> overdefined means it has too many values
>>
>> This is a traditional value range lattice (though
>> sometimes there are more things in the middle).
>> There is no way you should go from overdefined back to
>> constant.
>>
>> Ah, I see. Thanks for your explanation! I think you are
>> absolutely correct, and I misunderstood lattice value
>> 'overdefined'. The original code is like this,
>>
>> if ((!BBLV.isUndefined() {
>> ...
>> return;
>> }
>>
>> // Otherwise, this is the first time we're seeing this
>> block. Reset the
>> // lattice value to overdefined, so that cycles will
>> terminate and be
>> // conservatively correct.
>> BBLV.markOverdefined();
>>
>> So this algorithm is really conservative. I think the
>> solution might be removing this lowering or adding threshold
>> control for this lowering. Do you have any suggestions?
>>
>> Thanks,
>> -Jiangning
>>
>>
>>
>>
>>
>>
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>> llvm-commits at cs.uiuc.edu
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>>
>
>
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