[PATCH][ADT] Add move-construction and move-assignment from SmallVectorImpl<T> to SmallVector<T,N>.

[Lang Hames]

Hi Guys,

Thanks for the patch review. I've committed the move operations in r226899.

Cheers,
Lang.


On Thu, Jan 22, 2015 at 8:05 PM, Lang Hames <lhames at gmail.com> wrote:

> Hi Dave,
>
> an O(N) move for SmallVector would be a performance bug.
>
>
> I'm inclined to argue that that's distinct from a correctness bug (in the
> sense I was using the term), but either way it seems reasonable to test it
> here. ;)
>
> several of the tests in this file call "reset()" after "makeSequence"
>
>
> I missed that when I was flicking through the tests - I withdraw my
> objection. Still, I'm going to commit this with my suggested approach. If
> you think the other approach seems more reasonable feel free to switch it.
>
> (I noticed the copy operations went missing from the original patch - were
> they already committed? Did you want to commit them? They seem
> reasonable/handy/etc - just curious)
>
>
> They don't have any users (or even potential users) yet, so I left them
> out to keep this change minimal. I have no objection to them going in
> though. If you think they're warranted I'll add them in a follow-up patch
> (if you don't beat me to it).
>
> Cheers,
> Lang.
>
> On Jan 22, 2015, at 6:52 PM, David Blaikie <dblaikie at gmail.com> wrote:
>
>
>
> On Thu, Jan 22, 2015 at 6:24 PM, Lang Hames <lhames at gmail.com> wrote:
>
>> Hi Guys,
>>
>> I was attempting to pre-empt this with  "I'm still just testing
>> correctness, rather than implementation specifics". ;)
>>
>
> For std::vector at least, this is an important guarantee because it means
> std::vector's move operations don't require move/copy operations from its
> elements (you can have an immovable object in a std::vector and still move
> the vector around). That's not the case for SmallVector owing to the
> small-portion, but I think this is more than implementation specifics - an
> O(N) move for SmallVector would be a performance bug.
>
>
>> If we want to verify the implementation behavior for head-allocated
>> source vectors in this test, then my inclination would be to use something
>> like:
>>
>> Constructable *OrigData = Source.data();
>> // Do move...
>> EXPECT_TRUE(Source.isSmall() || Dest.data() == OrigData);
>>
>
> Fair enough - makes the test generic over the different big/small pairs. I
> kind of like the "no ctors were called in the making of this move" idea,
> but that's just me.
>
>
>> If that is true it implies that nothing was moved, but it doesn't require
>> a counter-reset in the middle of the function to zero out the construction
>> count, which would cause me to do a double take if I came across it.
>>
>
> Can't say I've looked - I thought a few tests either reset or saved
> counter state between each step to test that certain operations had certain
> effects (or not). A quick skim seems to indicate this is actually common -
> several of the tests in this file call "reset()" after "makeSequence" which
> seems expected/reasonable.
>
>
>> How does that sound?
>>
>
> All good.
>
> (I noticed the copy operations went missing from the original patch - were
> they already committed? Did you want to commit them? They seem
> reasonable/handy/etc - just curious)
>
>
>>
>> Cheers,
>> Lang.
>>
>> On Jan 22, 2015, at 5:41 PM, David Blaikie <dblaikie at gmail.com> wrote:
>>
>>
>>
>> On Thu, Jan 22, 2015 at 5:14 PM, Duncan P. N. Exon Smith <
>> dexonsmith at apple.com> wrote:
>>
>>> > On 2015-Jan-22, at 17:11, Duncan P. N. Exon Smith <
>>> dexonsmith at apple.com> wrote:
>>> >
>>> > This tests that `Constructable` was moved, but it still doesn't test
>>> > whether the heap allocation was moved (as opposed to individual
>>> > elements being moved to a new allocation).  You need something like
>>> > this to test that:
>>> >
>>> >     SmallVector<int, 4> V1(8), V2;
>>> >     auto First = V1.begin();
>>> >     V2 = std::move(V1);
>>> >     EXPECT_EQ(First, V2.begin());
>>>
>>> Or maybe you could check that no constructors had been called at all
>>> in the move?
>>>
>>
>> This. That's the guarantee I expect from moving a container with dynamic
>> storage - no operations on my elements should be performed at all.
>>
>>
>>>
>>> >
>>> > Of course, you only want to check this when you expect the whole
>>> > allocation to move over: when the source vector is in big mode.
>>> >
>>> >> On 2015-Jan-22, at 16:43, Lang Hames <lhames at gmail.com> wrote:
>>> >>
>>> >> Hi Guys,
>>> >>
>>> >> Thanks for catching that - I've attached an updated patch. I'm still
>>> just testing correctness, rather than implementation specifics, but it does
>>> at least verify that it's move-constructing rather than copy-constructing
>>> now.
>>> >>
>>> >> Cheers,
>>> >> Lang.
>>> >>
>>> >>
>>> >> On Thu, Jan 22, 2015 at 4:19 PM, David Blaikie <dblaikie at gmail.com>
>>> wrote:
>>> >>
>>> >>
>>> >> On Thu, Jan 22, 2015 at 4:17 PM, Duncan P. N. Exon Smith <
>>> dexonsmith at apple.com> wrote:
>>> >>
>>> >>> On 2015-Jan-22, at 16:04, David Blaikie <dblaikie at gmail.com> wrote:
>>> >>>
>>> >>> This looks like it's only adding copy, not move - did I miss
>>> something?
>>> >>>
>>> >>> On Thu, Jan 22, 2015 at 3:54 PM, Lang Hames <lhames at gmail.com>
>>> wrote:
>>> >>> Hi All,
>>> >>>
>>> >>> The attached patch adds move-construction and move-assignment from
>>> SmallVectorImpl<T> to SmallVector<T, N>. It enables us to move between
>>> SmallVectors of different types, which will allow me to fix one of the most
>>> commented-upon warts from the new JIT APIs.
>>> >>>
>>> >>> Unit-test included.
>>> >>>
>>> >>> Cheers,
>>> >>> Lang.
>>> >>>
>>> >>
>>> >> Yeah, the tests don't actually confirm that the thing was moved.  In
>>> >> particular, you want to check that whenever the source is in big-mode,
>>> >> the destination points at the same memory that the source used to
>>> (well,
>>> >> assuming that's the behaviour you want, which the naive implementation
>>> >> will give you IIUC).
>>> >>
>>> >> There's also a "big mode -> small mode" funny state thing that'll
>>> happen
>>> >> with the naive implementation.  I'm not sure it's *wrong*, but...
>>> >>
>>> >> Given:
>>> >>
>>> >>    SmallVector<int, 1> InBigMode(2);
>>> >>    SmallVector<int, 2> InBigModeToo = std::move(InBigMode);
>>> >>
>>> >> I think this will pass:
>>> >>
>>> >>    assert(!InBigModeToo.isSmall());
>>> >>
>>> >> This a new state -- a SmallVector can be in "big" mode with a heap
>>> >> allocation, even though its capacity would fit in its "small" mode
>>> >> stack allocation.
>>> >>
>>> >> While weird, I don't really have a problem with it (but maybe others
>>> >> do?).  Might be worth testing a few operations in this untested state
>>> >> though.
>>> >>
>>> >> It's probably valuable that this is the outcome - it'd be wasteful to
>>> throw away the dynamic allocation (going down to small mode) - better to
>>> keep it even though it's < small_size. So I'd be in favor of this.
>>> >>
>>> >>
>>> >> <SmallVectorMoveAssignmentFromSmallVectorImpl-2.patch>
>>> >
>>> >
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>>>
>>>
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>>
>>
>
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