[llvm-dev] Which pass should be propagating memory copies

Thu May 18 08:47:38 PDT 2017

Hi Peter,

thank you for concern and advice. Since we both write the compiler and
design the language, we are not particularly
bound by any pre-existing spec. The concerns about multi-threaded data
races are very relevant of course
and we're well aware of the implications. In the particular case where this
comes up, language semantics
generally guarantee that this is unobservable both in single-threaded and
multi-threaded contexts (though
we generally do allow the user to shoot themselves in the foot if they want
to, the primary concern here
is not really observability, but what the programmer expects from the
semantics of the language). For what
it's worth, this isn't exactly CICO. Our calling convention is generally by
reference. However, we do have
notions of semantic immutability, which is where this particular pattern
arises (in cases where a new immutable
gets created by taking an existing field and modifying it in one place
only). Because of these semantic
guarantees, we know that there's no aliasing of the kind that would be
problematic (and expose this
information to LLVM through the various AA mechanisms). Now, similar issues
of course arise with mutable
memory locations as well. However, in such cases the data race would be
explicitly present in the source
program, so we don't have a problem with the compiler making this
optimization. FWIW, our multi-threading
programming model is in the early stages, and we're considering various
language level constraints
on concurrent data modification to mostly disallow that situation unless
explicitly opted in to by the user,
but that's a bit off.

>From my perspective, I don't see a reason why GVN shouldn't be doing this
(which is why I sent the original
email in the first place). It would of course be very possible for us to
write our own pass that pattern matches
this and performs the transformation that we want. However, we generally
tend to prefer working with the
community to put the optimizations in the best possible place such that
others may automatically take advantage.
It sounds like the community consensus is that GVN should be able to do
this kind of optimization (and thanks
to Daniel for providing some guidance on implementation!). If people feel
strongly that memcpyopt (or a new pass)
with appropriate pattern matching would be a better place, I'd be happy to
go that way as well of course.

Keno

On Wed, May 17, 2017 at 1:28 PM, Peter Lawrence <peterl95124 at sbcglobal.net>
wrote:

> Keno,
>          Hmmm, seems like you are saying “copy-in-copy-out” argument
> semantics are required by the language,
> Otherwise you would be using “by-reference" argument semantics,
> And that CICO is easiest for you to represent with memcpy.
>
> Usually there are some very subtle issues with CICO and the memory model,
> Typically the original object isn’t supposed to be modified until the
> function returns,
> IE multiple stores, especially of different values, to a field in the
> original object should not be visible, only the final store,
> This is clearly “observable" in multithreaded programs, but can also be
> observable in a single threaded program
> If the same object is visible from within the called function for example
> as a global variable
> Which would be seen to have its internal values change multiple times,
> even though the
> Intent of the language using CICO is to try to ensure all-at-once state
> changes (at least for single-threaded programs)
>
>
> My advice, if the above applies to you, is to add a new pass to the
> compiler that figures out if
> The transformation from memcpy to explicit multiple load/store is actually
> legal (won’t produce intermediate
> State changes before the exit of the function which would violate the
> strict CICO calling convention),
> And also profitable (I don’t view the code explosion of [1000000 x double]
> as profitable!),
> Or if the transformation from “CICO" to pure “by-reference” is both legal,
> and profitable.
>
> (Also, don’t forget to check what the language spec says about this
> function passing the object,
> Or parts of it, to other functions before or after making modifications)
>
>
> My advice regarding teaching GVN about memcpy is not to. It would be one
> thing if the memcpy
> Were copying in/out a single variable, in that case the memcpy can and
> should be viewed as a load / store pair,
> But in your case it isn’t being used that way, it is being used to copy
> multiple values, and the only
> Logical thing that GVN could do is expand those out to multiple individual
> loads and stores. GVN should not
> Be doing this, instead your new pass (that first checks to see if it is
> legal w.r.t. calling convention) is
> The place to do this, or if should convert to pure “by-reference” if
> legal, which also shouldn’t be done in GVN.
>
>
> —Peter Lawrence.
>
>
>
> On May 17, 2017, at 8:55 AM, Keno Fischer <keno at juliacomputing.com> wrote:
>
> Well, mostly I want to hoist the store to the stack and transform it into
> a store to the heap. After that the memcpys are essentially trivially dead,
> so instcombine or dse will delete them for me. If the memcpys were made of
> individual stores instead, there'd have to be some sort of exponential
> search somewhere in the compiler to figure that out. For the extreme case
> consider [100000000 x double]. The same optimization can apply here, but if
> it tried to do 100000000 stores instead, I wouldn't expect the compiler to
> really figure that out. What I meant was that I think the memcpys are the
> correct representation of this from the frontend, it's just that I'd like
> more optimization to happen here.
>
>
> On Wed, May 17, 2017 at 11:48 AM, Peter Lawrence <
> peterl95124 at sbcglobal.net> wrote:
>
>> Keno,
>>           "No, I very much want the memcpys there” seems like a
>> contradiction,
>> Aren’t you trying to optimize away the memcpys.
>> Peter Lawrence
>>
>>
>>
>>
>>
>>
>> On May 17, 2017, at 8:22 AM, Keno Fischer <keno at juliacomputing.com>
>> wrote:
>>
>>
>> On Wed, May 17, 2017 at 12:09 AM, Peter Lawrence via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>>
>>> Keno,
>>>           Perhaps you can view the problem to be the memcpys themselves,
>>> We humans can look at the memcpys and see loads and stores
>>> but to almost all optimizer passes they aren’t what it is looking for,
>>> They instead see function calls which they mostly don’t touch,
>>>
>>> If these memcpys were inlined into plain old loads and stores
>>> The redundant loads and stores should be deleted by existing opts
>>>
>>> A question I have for you is, because this looks like “copy-in-copy-out”
>>> argument semantics,
>>> Which to me looks more like Ada than C, what was the source language ?
>>>
>>>
>>> Peter Lawrence.
>>>
>>
>> No, I very much want the memcpys there. With individual stores I'd give
>> up hope that the optimizer can figure out what's going on here, esp. if it
>> gets beyond a few bytes, but I with memcpys it does seem doable. As for
>> which frontend produced this, we're considering adding language semantics
>> that would produce lots of code like this to julia, so we're looking into
>> getting the optimizer to fold the extra copies away.
>>
>> Keno
>>
>>
>>
>
>
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