llvm dev - Nov 2020 - RFC: [SmallVector] Adding SVec<T> and Vec<T> convenience wrappers.

On Mon, Nov 16, 2020 at 12:55 PM David Blaikie <dblaikie at gmail.com>
wrote:
> I will say I'm not a huge fan of adding even more names for things in
> this fairly core/common use case (now we'll have even more vector
> names to pick from) - can we use default template arguments so we can
> write SmallVector<T> instead of SmallVector<T, N> and would
that
> address some of the use cases proposed here?
>
I won't claim it is perfect, but the added names are a compromise over
rounds of reviews with the folks in the revision. In particular I'm quite
concerned that a default value for N on the SmallVector does not carry the
intent the same way, and is too easy to miss in review (or while reading
code). To me the drawbacks are outweighing the benefits too much.
Also, `SmallVector<LargeType>` would end-up with N==0 implicitly, without
an easy way to figure it out that there is no actual inline storage while
reading the code. An alternative was to reserve the default to only "small
object" so that N isn't zero, but there isn't a platform
independent way of
doing that and keep the code portable I believe. So `SVec<T>` is really
saying: "I am willing to pay for some limite inline storage if possible but
I don't have a N in mind".

Finally the simple `llvm::Vector` case to replace `SmallVector<T, 0>` is
because it is frequently preferable to `std::vector` but still isn't
readable or immediately intuitive and so is rarely used in practice (see
https://www.llvm.org/docs/ProgrammersManual.html#llvm-adt-smallvector-h for
the documented points on N=0).

I think someone (JYKnight, perhaps) mentioned in the code
review
> (always difficult fragmenting the discussion between code review and
> RFC, unfortunately - not sure there's a great solution to that - some
> way to lock comments on a Phab review might be nice) that there are
> cases where you do want a small inline buffer even when you're nested
> inside another data structure and/or heap allocated (like tail
> allocations).
>
Yes: I think it is misleading to formulate anything about heap, I see a
SmallVector inside a heap allocated object is akin to a trailing allocation
optimization.

> Got any sense of the total value here? Major savings to be had?
>
> (if SmallVector<T> can do what your proposed SVec<T> does, that
leaves
> the Vec<T> - could you expound on the benefits of SmallVector<T,
0>
> over std::vector<T>? I guess the SmallVectorImpl generic algorithm
> opportunities? Though that's rarely needed compared to ArrayRef.)
> If SmallVector<T> would suffice then maybe Vec<T> could be
> ZeroSmallVector<T>? Not sure.
>
ZeroSmallVector<T> does not really address your "more vector names to
pick
from" concerns, and it is longer than `SmallVector<T, 0>`:
shouldn't we aim
for the "default case" to be the easiest to reach / most intuitive to
pick?
`llvm::Vec` looks like "just a vector".

Cheers,

-- 
Mehdi


>
> On Fri, Nov 13, 2020 at 2:06 PM Sean Silva via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> >
> > We've pretty happy now with a patch that adds two wrappers around
> SmallVector that make it 1) more convenient to use and 2) will tend to
> mitigate misuse of SmallVector. We think it's ready for wider
discussion:
> https://reviews.llvm.org/D90884
> >
> > SVec<T> is a convenience alias for SmallVector<T, N> with
N chosen
> automatically to keep its size under 64 Bytes (that heuristic is easy to
> change though). The recommendation in the patch is to use this "on the
> stack, where a "small" number of elements are expected".
> >
> > Vec<T> is a convenience alias for SmallVector<T, 0>. It
lets us get the
> (little-known?) benefits of SmallVector even when it has no inline elements
> (see https://llvm.org/docs/ProgrammersManual.html#llvm-adt-smallvector-h).
> The recommendation in the patch is to use this when the SmallVector is on
> the heap.
> >
> > A lot of this is boiled out from the discussion in
> https://groups.google.com/g/llvm-dev/c/q1OyHZy8KVc/m/1l_AasOLBAAJ?pli=1
> >
> > The goals here are twofold:
> >
> > 1. convenience: not having to read/write "N", or do an extra
> edit/recompile cycle if you forgot it
> >
> > 2. avoiding pathological cases: The choice of N is usually
> semi-arbitrary in our experience, and if one isn't careful, can result
in
> sizeof(SmallVector) becoming huge, especially in the case of nested
> SmallVectors. This patch avoids pathological cases in two ways:
> >   A. SVec<T>'s heuristic keeps sizeof(SVec<T>)
bounded, which prevents
> pathological size amplifications like in `SmallVector<struct
> {SmallVector<T, 4> a, b; }, 4>`, where the small sizes effectively
multiply
> together. Of course, users can always write SmallVector<T, N>
explicitly to
> bypass this, but the need for that seems rare.
> >   B. SmallVector<T, 0> feels "weird to write" for most
folks, even
> though it is frequently the right choice. Vec<T> mitigates that by
"looking
> natural".
> >
> > I'm surfacing this as an RFC to get feedback on a couple
higher-level
> points:
> > - does everybody agree that SVec<T> and Vec<T> are useful
to have?
> > - get wider consensus around suggesting these as "defaults"
(see my
> updates to ProgrammersManual.rst in the patch)
> > - how much we want to bulk migrate code vs let it organically grow.
> Replacing SmallVector<T, 0> with Vec<T> should be completely
mechanical.
> Replacing SmallVector<T, N> for general N would be a lot more work.
> > - of course: naming. SVector/Vector were floated in the patch as well
> and seem ok. SmallVec was rejected as it was a prefix of SmallVector
> (messes up autocomplete).
> >
> > Looking forward to a world with fewer guessed SmallVector sizes,
> >
> > -- Sean Silva
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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On Mon, Nov 16, 2020 at 1:55 PM Mehdi AMINI <joker.eph at gmail.com>
wrote:
> On Mon, Nov 16, 2020 at 12:55 PM David Blaikie <dblaikie at
gmail.com> wrote:
>>
>> I will say I'm not a huge fan of adding even more names for things
in
>> this fairly core/common use case (now we'll have even more vector
>> names to pick from) - can we use default template arguments so we can
>> write SmallVector<T> instead of SmallVector<T, N> and would
that
>> address some of the use cases proposed here?
>
>
> I won't claim it is perfect, but the added names are a compromise over
rounds of reviews with the folks in the revision. In particular I'm quite
concerned that a default value for N on the SmallVector does not carry the
intent the same way, and is too easy to miss in review (or while reading code).
I'm not quite following here - what sort of problems do you anticipate
by readers missing the default value for N?
>  To me the drawbacks are outweighing the benefits too much.
> Also, `SmallVector<LargeType>` would end-up with N==0 implicitly,
without an easy way to figure it out that there is no actual inline storage
while reading the code.
Why would it be necessary to figure that out? If we generally feel the
right default inline storage happens to be zero in that case and that
SmallVector will pick a good default (including possibly 0) that seems
like a good thing to me. (why would we call out zero specially, if we
want to avoid calling out other values explicitly)
> An alternative was to reserve the default to only "small object"
so that N isn't zero, but there isn't a platform independent way of
doing that and keep the code portable I believe. So `SVec<T>` is really
saying: "I am willing to pay for some limite inline storage if possible but
I don't have a N in mind".
That quoted statement still sounds like it could encapsulate the
possibility of 0 too, though. "limited inline storage if possible"
could be "and the answer to that constraint is zero in this case - but
if you happen to make the T smaller, it could become non-zero
organically/without the need to touch this code" (as the N could vary
organically between non-zero values as struct sizes change too)
> Finally the simple `llvm::Vector` case to replace `SmallVector<T, 0>`
is because it is frequently preferable to `std::vector` but still isn't
readable or immediately intuitive and so is rarely used in practice (see
https://www.llvm.org/docs/ProgrammersManual.html#llvm-adt-smallvector-h for the
documented points on N=0).
I'm not sure llvm::Vector/Vec would necessarily make it more often
used in practice. Maybe a bunch of cleanup and more specific wording
in the style guide that would ban std::vector might help more there.

Though I agree that it may be suitable to have a clear name for
SmallVector<T, 0> since the "Small" is slightly misleading
(though not
entirely - it's important for the "SmallVectorImpl" benefits of
SmallVector - so renaming it to Vector and still using it via
SmallVectorImpl<T>& might also be confusing).
>> Got any sense of the total value here? Major savings to be had?
>>
>> (if SmallVector<T> can do what your proposed SVec<T> does,
that leaves
>> the Vec<T> - could you expound on the benefits of
SmallVector<T, 0>
>> over std::vector<T>? I guess the SmallVectorImpl generic
algorithm
>> opportunities? Though that's rarely needed compared to ArrayRef.)
>> If SmallVector<T> would suffice then maybe Vec<T> could be
>> ZeroSmallVector<T>? Not sure.
>
>
> ZeroSmallVector<T> does not really address your "more vector
names to pick from" concerns,
Somewhat - if SmallVector<T> can be used instead of SVec<T> then we
have one fewer name and less convention to base the Vec<T> on (since
it won't have the SVec<T> sibling), so picking a name closer to the
existing SmallVector might be more viable/helpful.
> and it is longer than `SmallVector<T, 0>`: shouldn't we aim for
the "default case" to be the easiest to reach / most intuitive to
pick? `llvm::Vec` looks like "just a vector".
Somewhat - but Vec is an abbreviation that isn't really used otherwise
(if we consider SVec as well, I'd still push back on the introduction
of the abbreviation for both cases) and llvm::Vector would be only a
case separation away form a standard name (when considering the
unqualified name - which is likely to come up a fair bit, as we'll see
"Vector" used unqualified a lot).

I wouldn't entirely object to SmallVector<T> getting a smart default
buffer size, and llvm::Vector being an alias for SmallVector<T, 0> - I
don't feel /super/ great about it, but yeah, if we're going to push
the Programmer's Manual encouragement further in terms of
reducing/removing use of std::vector, then maybe llvm::Vector isn't
the worst way to do that.

(it might be that this would benefit from being two separate
discussions, though - one on how to provide smart defaults for
SmallVector<T> and one on if/how to push std::vector deprecation in
favor of SmallVector<T, 0> (semantically speaking - no matter how it's
spelled) further along)
>
> Cheers,
>
> --
> Mehdi
>
>
>>
>>
>> On Fri, Nov 13, 2020 at 2:06 PM Sean Silva via llvm-dev
>> <llvm-dev at lists.llvm.org> wrote:
>> >
>> > We've pretty happy now with a patch that adds two wrappers
around SmallVector that make it 1) more convenient to use and 2) will tend to
mitigate misuse of SmallVector. We think it's ready for wider discussion:
https://reviews.llvm.org/D90884
>> >
>> > SVec<T> is a convenience alias for SmallVector<T, N>
with N chosen automatically to keep its size under 64 Bytes (that heuristic is
easy to change though). The recommendation in the patch is to use this "on
the stack, where a "small" number of elements are expected".
>> >
>> > Vec<T> is a convenience alias for SmallVector<T, 0>.
It lets us get the (little-known?) benefits of SmallVector even when it has no
inline elements (see
https://llvm.org/docs/ProgrammersManual.html#llvm-adt-smallvector-h). The
recommendation in the patch is to use this when the SmallVector is on the heap.
>> >
>> > A lot of this is boiled out from the discussion in
https://groups.google.com/g/llvm-dev/c/q1OyHZy8KVc/m/1l_AasOLBAAJ?pli=1
>> >
>> > The goals here are twofold:
>> >
>> > 1. convenience: not having to read/write "N", or do an
extra edit/recompile cycle if you forgot it
>> >
>> > 2. avoiding pathological cases: The choice of N is usually
semi-arbitrary in our experience, and if one isn't careful, can result in
sizeof(SmallVector) becoming huge, especially in the case of nested
SmallVectors. This patch avoids pathological cases in two ways:
>> >   A. SVec<T>'s heuristic keeps sizeof(SVec<T>)
bounded, which prevents pathological size amplifications like in
`SmallVector<struct {SmallVector<T, 4> a, b; }, 4>`, where the small
sizes effectively multiply together. Of course, users can always write
SmallVector<T, N> explicitly to bypass this, but the need for that seems
rare.
>> >   B. SmallVector<T, 0> feels "weird to write" for
most folks, even though it is frequently the right choice. Vec<T>
mitigates that by "looking natural".
>> >
>> > I'm surfacing this as an RFC to get feedback on a couple
higher-level points:
>> > - does everybody agree that SVec<T> and Vec<T> are
useful to have?
>> > - get wider consensus around suggesting these as
"defaults" (see my updates to ProgrammersManual.rst in the patch)
>> > - how much we want to bulk migrate code vs let it organically
grow. Replacing SmallVector<T, 0> with Vec<T> should be completely
mechanical. Replacing SmallVector<T, N> for general N would be a lot more
work.
>> > - of course: naming. SVector/Vector were floated in the patch as
well and seem ok. SmallVec was rejected as it was a prefix of SmallVector
(messes up autocomplete).
>> >
>> > Looking forward to a world with fewer guessed SmallVector sizes,
>> >
>> > -- Sean Silva
>> > _______________________________________________
>> > LLVM Developers mailing list
>> > llvm-dev at lists.llvm.org
>> > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

On Mon, Nov 16, 2020 at 2:12 PM David Blaikie <dblaikie at gmail.com>
wrote:
> On Mon, Nov 16, 2020 at 1:55 PM Mehdi AMINI <joker.eph at gmail.com>
wrote:
> > On Mon, Nov 16, 2020 at 12:55 PM David Blaikie <dblaikie at
gmail.com>
> wrote:
> >>
> >> I will say I'm not a huge fan of adding even more names for
things in
> >> this fairly core/common use case (now we'll have even more
vector
> >> names to pick from) - can we use default template arguments so we
can
> >> write SmallVector<T> instead of SmallVector<T, N> and
would that
> >> address some of the use cases proposed here?
> >
> >
> > I won't claim it is perfect, but the added names are a compromise
over
> rounds of reviews with the folks in the revision. In particular I'm
quite
> concerned that a default value for N on the SmallVector does not carry the
> intent the same way, and is too easy to miss in review (or while reading
> code).
>
> I'm not quite following here - what sort of problems do you anticipate
> by readers missing the default value for N?
>
Reading code `SmallVector<SomeType>` does not express that it is
*intentional* to leave of the value for N. It can easily be just forgotten,
and it could easily be implicitly `0`, and as a reviewer (or code reader
later) I don't know if this is was intentional or not. This is why I am
quite opposed to "loosen" the current requirement on SmallVector: a
different name for a different intent is better fitting to me.

>
> >  To me the drawbacks are outweighing the benefits too much.
> > Also, `SmallVector<LargeType>` would end-up with N==0
implicitly,
> without an easy way to figure it out that there is no actual inline storage
> while reading the code.
>
> Why would it be necessary to figure that out? If we generally feel the
> right default inline storage happens to be zero in that case and that
> SmallVector will pick a good default (including possibly 0) that seems
> like a good thing to me. (why would we call out zero specially, if we
> want to avoid calling out other values explicitly)
>
I think this is an API that is "easy to misuse", I don't see any
advantage
to it while it has drawbacks: why would we do that?

>
> > An alternative was to reserve the default to only "small
object" so that
> N isn't zero, but there isn't a platform independent way of doing
that and
> keep the code portable I believe. So `SVec<T>` is really saying:
"I am
> willing to pay for some limite inline storage if possible but I don't
have
> a N in mind".
>
> That quoted statement still sounds like it could encapsulate the
> possibility of 0 too, though. "limited inline storage if
possible"
> could be "and the answer to that constraint is zero in this case - but
> if you happen to make the T smaller, it could become non-zero
> organically/without the need to touch this code" (as the N could vary
> organically between non-zero values as struct sizes change too)
>
Yes the quoted statement says exactly that! This is why I don't like having
this behavior on SmallVector: this is a different semantics / intent
that the programmers have and this is something that I like being able to
read differently.
 SVec does not accept a `N`: if I read SVec<Type> there can't be a
possible
accidental omission of N here. It has to be a choice between
`SmallVector<SomeType, 4>` and `SVec<SomeType>` but not
`SmallVector<SomeType>`.

> Finally the simple `llvm::Vector` case to replace `SmallVector<T, 0>`
is
> because it is frequently preferable to `std::vector` but still isn't
> readable or immediately intuitive and so is rarely used in practice (see
> https://www.llvm.org/docs/ProgrammersManual.html#llvm-adt-smallvector-h
> for the documented points on N=0).
>
> I'm not sure llvm::Vector/Vec would necessarily make it more often
> used in practice. Maybe a bunch of cleanup and more specific wording
> in the style guide that would ban std::vector might help more there.
>
> Though I agree that it may be suitable to have a clear name for
> SmallVector<T, 0> since the "Small" is slightly misleading
(though not
> entirely - it's important for the "SmallVectorImpl" benefits
of
> SmallVector - so renaming it to Vector and still using it via
> SmallVectorImpl<T>& might also be confusing).
>
> >> Got any sense of the total value here? Major savings to be had?
> >>
> >> (if SmallVector<T> can do what your proposed SVec<T>
does, that leaves
> >> the Vec<T> - could you expound on the benefits of
SmallVector<T, 0>
> >> over std::vector<T>? I guess the SmallVectorImpl generic
algorithm
> >> opportunities? Though that's rarely needed compared to
ArrayRef.)
> >> If SmallVector<T> would suffice then maybe Vec<T>
could be
> >> ZeroSmallVector<T>? Not sure.
> >
> >
> > ZeroSmallVector<T> does not really address your "more
vector names to
> pick from" concerns,
>
> Somewhat - if SmallVector<T> can be used instead of SVec<T>
then we
> have one fewer name and less convention to base the Vec<T> on (since
> it won't have the SVec<T> sibling), so picking a name closer to
the
> existing SmallVector might be more viable/helpful.
>
> > and it is longer than `SmallVector<T, 0>`: shouldn't we aim
for the
> "default case" to be the easiest to reach / most intuitive to
pick?
> `llvm::Vec` looks like "just a vector".
>
> Somewhat - but Vec is an abbreviation that isn't really used otherwise
> (if we consider SVec as well, I'd still push back on the introduction
> of the abbreviation for both cases)

One aspect of the naming is to avoid one not being a prefix of the other
(IDE completion, etc.) or them being too close to differentiate.

> and llvm::Vector would be only a
> case separation away form a standard name (when considering the
> unqualified name - which is likely to come up a fair bit, as we'll see
> "Vector" used unqualified a lot).
>
Note: this is why we converged with llvm::Vec and not llvm::Vector in the
revision


I wouldn't entirely object to SmallVector<T> getting a smart
default
> buffer size, and llvm::Vector being an alias for SmallVector<T, 0> -
I
> don't feel /super/ great about it, but yeah, if we're going to push
> the Programmer's Manual encouragement further in terms of
> reducing/removing use of std::vector, then maybe llvm::Vector isn't
> the worst way to do that.
>
> (it might be that this would benefit from being two separate
> discussions, though - one on how to provide smart defaults for
> SmallVector<T> and one on if/how to push std::vector deprecation in
> favor of SmallVector<T, 0> (semantically speaking - no matter how
it's
> spelled) further along)
>
For historical purpose: the review was actually originally only adding a
default for SmallVector and nothing else :)
We only converged to the current proposal after a few rounds of reviews
trying to tradeoff amongst folks in the review.




>
> >
> > Cheers,
> >
> > --
> > Mehdi
> >
> >
> >>
> >>
> >> On Fri, Nov 13, 2020 at 2:06 PM Sean Silva via llvm-dev
> >> <llvm-dev at lists.llvm.org> wrote:
> >> >
> >> > We've pretty happy now with a patch that adds two
wrappers around
> SmallVector that make it 1) more convenient to use and 2) will tend to
> mitigate misuse of SmallVector. We think it's ready for wider
discussion:
> https://reviews.llvm.org/D90884
> >> >
> >> > SVec<T> is a convenience alias for SmallVector<T,
N> with N chosen
> automatically to keep its size under 64 Bytes (that heuristic is easy to
> change though). The recommendation in the patch is to use this "on the
> stack, where a "small" number of elements are expected".
> >> >
> >> > Vec<T> is a convenience alias for SmallVector<T,
0>. It lets us get
> the (little-known?) benefits of SmallVector even when it has no inline
> elements (see
> https://llvm.org/docs/ProgrammersManual.html#llvm-adt-smallvector-h). The
> recommendation in the patch is to use this when the SmallVector is on the
> heap.
> >> >
> >> > A lot of this is boiled out from the discussion in
> https://groups.google.com/g/llvm-dev/c/q1OyHZy8KVc/m/1l_AasOLBAAJ?pli=1
> >> >
> >> > The goals here are twofold:
> >> >
> >> > 1. convenience: not having to read/write "N", or do
an extra
> edit/recompile cycle if you forgot it
> >> >
> >> > 2. avoiding pathological cases: The choice of N is usually
> semi-arbitrary in our experience, and if one isn't careful, can result
in
> sizeof(SmallVector) becoming huge, especially in the case of nested
> SmallVectors. This patch avoids pathological cases in two ways:
> >> >   A. SVec<T>'s heuristic keeps
sizeof(SVec<T>) bounded, which
> prevents pathological size amplifications like in `SmallVector<struct
> {SmallVector<T, 4> a, b; }, 4>`, where the small sizes effectively
multiply
> together. Of course, users can always write SmallVector<T, N>
explicitly to
> bypass this, but the need for that seems rare.
> >> >   B. SmallVector<T, 0> feels "weird to write"
for most folks, even
> though it is frequently the right choice. Vec<T> mitigates that by
"looking
> natural".
> >> >
> >> > I'm surfacing this as an RFC to get feedback on a couple
higher-level
> points:
> >> > - does everybody agree that SVec<T> and Vec<T>
are useful to have?
> >> > - get wider consensus around suggesting these as
"defaults" (see my
> updates to ProgrammersManual.rst in the patch)
> >> > - how much we want to bulk migrate code vs let it organically
grow.
> Replacing SmallVector<T, 0> with Vec<T> should be completely
mechanical.
> Replacing SmallVector<T, N> for general N would be a lot more work.
> >> > - of course: naming. SVector/Vector were floated in the patch
as well
> and seem ok. SmallVec was rejected as it was a prefix of SmallVector
> (messes up autocomplete).
> >> >
> >> > Looking forward to a world with fewer guessed SmallVector
sizes,
> >> >
> >> > -- Sean Silva
> >> > _______________________________________________
> >> > LLVM Developers mailing list
> >> > llvm-dev at lists.llvm.org
> >> > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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