[PATCH] [AArch64] FP load balancing pass for Cortex-A57

[David Blaikie]

On Fri, Aug 8, 2014 at 8:39 AM, James Molloy <james at jamesmolloy.co.uk> wrote:
> Hi David,
>
> Partly this may be to do with my unfamiliarity with the unique_ptr idiom,
> and may be using it incorrectly. We're talking about pointers to "class
> Chain", which reside in two containers:
>
> std::set<Chain*> AllChains;
> std::map<unsigned, Chain*> ActiveChains;
>
> AllChains obviously contains all the chains, and ActiveChains may contain a
> subset of AllChains (as values); there may be duplicates too.

There may be duplicates in ActiveChains, that is? (AllChains is a set,
so it's safe/unique)

So ActiveChains both contains not all chains and duplicate chains - so
it's not directly a candidate for single ownership, at least.

> To make this
> work, I'd need to change AllChains to be a std::set<std::unique_ptr<Chain>>,
> and would have to use "::get()" to get the unmanaged version of the pointer
> before it could be passed into ActiveChains, as I'm effectively duplicating
> the pointer. This seems to hack around the intended semantics (shared_ptr
> might be better here?).

I don't think this is hacking around the intended semantics,
necessarily. The names (ActiveChains and AllChains) actually give a
pretty good indicator to me, the unknowing reader, that ActiveChains
is a subset of AllChains (I wouldn't've guessed that ActiveChains
might have duplicates, but that's not too important) and seeing
AllChains have unique_ptr and ActiveChains have raw (non-owning)
pointer would help cement that for me. "Oh, I see, AllChains owns the
Chains and ActiveChains has non-owning pointers to those same Chains"
is how I'd read that.

>
> Secondly the loops become more clunky. From:
>
>   for (auto *I : AllChains)
>     EC.insert(I);
>
>   for (auto *I : AllChains) {
>     for (auto *J : AllChains) {
>       if (I != J && I->rangeOverlapsWith(J))
>         EC.unionSets(I, J);
>     }
>   }
>
>
> to:
>
>   for (auto &I : AllChains)
>     EC.insert(I.get());

This one's just a necessary thing - I don't think it's overly clunky.
(I could think of some other things to do, but they wouldn't really be
better)

>   for (auto &I : AllChains) {
>     for (auto &J : AllChains) {
>       if (&*I != &*J && I->rangeOverlapsWith(&*J))
>         EC.unionSets(&*I, &*J);

This bit I /think/ is just your lack of familiarity, and perhaps a few
other things. std::unique_ptr has operator== so you can just do "I ==
J" as you would with raw pointers.

As for rangeOverlapsWith and unionSets - I've been using unique_ptr as
a great excuse to make more functions take references rather than
pointers. If unionSets and rangeOverlapsWith both took Chain& instead
of Chain*, then the call sites with unique_ptr or raw pointer would be
the same:

rangeOverlapsWith(*J)
unionSets(*I, *J)

And their contracts are clearer - the functions can't take null/non-values, etc.

>     }
>   }
>
> (where I've written &* I could have used .get()).
>
> Am I misusing the idiom? What would you recommend?
>
> Cheers,
>
> James
>
>
>
> On 8 August 2014 16:27, David Blaikie <dblaikie at gmail.com> wrote:
>>
>> On Fri, Aug 8, 2014 at 5:43 AM, James Molloy <james at jamesmolloy.co.uk>
>> wrote:
>> > Thanks Tim,
>> >
>> > Using unique_ptr ended up making the code harder to read, so I simply
>> > added
>> > a deletion loop which I think is cleaner.
>>
>> I haven't looked at this patch in detail, but is there a reasonable
>> summary of why unique_ptr made the code harder to read?
>>
>> I'm really interested in making/encouraging ownership to be more
>> explicit/clear and would sometimes even encourage it when it hurts
>> (superficial (which might not be the case here)) readability. It's
>> sometimes the case that ownership looks awkward when you make it
>> explicit because it's an awkward ownership scheme and making it
>> implicit just hinds that complexity (in the not good way of hiding -
>> making it a trap/difficult for others to correctly modify the code in
>> the future).
>>
>> - David
>>
>> >
>> > Committed in r215199, thankyou to all reviewers!
>> >
>> > Cheers,
>> >
>> > James
>> >
>> >
>> > On 8 August 2014 10:42, Tim Northover <t.p.northover at gmail.com> wrote:
>> >>
>> >> Just added llvm-commits.
>> >>
>> >> ================
>> >> Comment at: lib/Target/AArch64/AArch64A57FPLoadBalancing.cpp:318
>> >> @@ +317,3 @@
>> >> +  // "link" register between each inst in the chain.
>> >> +  std::map<unsigned, Chain*> ActiveChains;
>> >> +  std::set<Chain*> AllChains;
>> >> ----------------
>> >> Tim Northover wrote:
>> >> > Aren't these chains leaked? It *looks* like you could make this (or
>> >> > the
>> >> > set) a unique_ptr<Chain>.
>> >> Also, this should definitely be the set. I hadn't noticed things get
>> >> removed from ActiveChains when I wrote this. Should have been obvious,
>> >> in
>> >> retrospect.
>> >>
>> >> http://reviews.llvm.org/D4791
>> >>
>> >>
>> >>
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>> >
>> >
>> >
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>
>