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

[David Blaikie]

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>
>> >
>> >
>> > _______________________________________________
>> > llvm-commits mailing list
>> > llvm-commits at cs.uiuc.edu
>> > http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
>>
>>
>> _______________________________________________
>> llvm-commits mailing list
>> llvm-commits at cs.uiuc.edu
>> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
>>
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20150122/bf690608/attachment.html>