[llvm-dev] [RFC] Aggreate load/store, proposed plan

Thu Aug 20 23:24:04 PDT 2015

I've done some investigation. The proposed approach seems to work well on
struct with no padding, but the optimizer gets confused when the struct has
some. That's a bummer but that is already improving things, so I'd rather
have that than nothing.

Alternatively, I think I have, in theory only so far, that may be better.
The idea is to create an alloca, memcpy the aggregate in it, and transform
extract values into loads, and insertvalues into an optional memecy into a
new alloca (if the old value is still alive after the insertvalue) + a
store. There is some extra trickery one need to do for call arguments, ret
and phi nodes, but overall, I think this can work. This approach would rely
on SROA to generate something sensible. As it is going to do massive
substitutions, I'm not sure InstCombine would be the right place to do it.
Maybe its own pass ?

That wouldn't work for atomic/volatile, but I think that would work nicely
for regular load/stores.

How does that sound ?

2015-08-20 16:38 GMT-07:00 Hal Finkel <hfinkel at anl.gov>:

> ----- Original Message -----
> > From: "deadal nix" <deadalnix at gmail.com>
> > To: "Hal Finkel" <hfinkel at anl.gov>
> > Cc: "Mehdi Amini" <mehdi.amini at apple.com>, "llvm-dev" <
> llvm-dev at lists.llvm.org>
> > Sent: Thursday, August 20, 2015 4:09:17 PM
> > Subject: Re: [llvm-dev] [RFC] Aggreate load/store, proposed plan
> >
> > Problem :
> >
> > Many languages define aggregates and some way to manipulate them.
> > LLVM define aggregates types (arrays and structs) to handle them.
> > However, when aggregate are loaded or stored, LLVM will simply
> > ignore these up to the legalization in the backend. This lead to
> > many misoptimizations. Most frontend are using a set of trick to
> > work around this limtation, but that an undesirable situation as it
> > increase the work required to write a front end. Ideally that work
> > should be done once by LLVM instead of every time by each frontend.
> >
> > In previous discussion on the subject, many LLVM user have expressed
> > interest in being able to use aggregate memory access. In addition,
> > it is likely that it would have reduced the workload of some
> > existing frontends.
> >
> > The proposed solution is to transform aggregate loads and stores to
> > something that the LLVM toolcahin already understand and is able to
> > work with.
> >
> > The proposed solution will use InstCombine to do the
> > transformation as it is done early and will allow subsequent passes
> > to work with something familiar (basically, canonicalization).
> >
> >
> > Proposed solution :
> >
> >
> > Aggregate load and store are turned into aggregate load and store of
> > a scalar of the same size and alignement. Binary manipulation, like
> > mask and shift, are used to build the aggregate from the scalar
> > after loading and the aggregate to a scalar when storing.
> >
> >
> > For instance, the following IR (extracted from a D frontend) :
> >
> > %B__vtbl = type { i8*, i32 (%B*)* }
> > @B__vtblZ = constant %B__vtbl { i8* null, i32 (%B*)* @B.foo }
> >
> > %0 = tail call i8* @allocmemory(i64 32)
> >
> > %1 = bitcast i8* %0 to %B*
> > store %B { %B__vtbl* @B__vtblZ, i32 42 }, %B* %1, align 8
> >
> >
> > Would be canonized into :
> > %0 = tail call i8* @allocmemory(i64 32)
> > %1 = bitcast i8* %0 to i128*
> > store i128 or (i128 zext (i64 ptrtoint (%B__vtbl* @B__vtblZ to i64)
> > to i128), i128 774763251095801167872), i128* %1, align 8
> >
>
> As I've said before, we really should introduce some useful
> canonicalization for these, and so I support the investigations in this
> area.
>
> Have you verified that InstCombine that undo this transformation
> reasonably? Specifically, if you have a bunch of insertvalues, and an
> aggregate store in one function, and a load in another function with a
> bunch of extractvalues, which gets inlined into the function with the
> store, does everything get combined appropriately to forward the values
> directly regardless of how many of the members are extracted and in what
> order?
>
>  -Hal
>
> >
> >
> > Which the rest of the LLVM pipeline can work with.
> >
> >
> >
> > Limitations :
> >
> >
> > 1/ This solution will not handle properly large (tens of kilobytes)
> > aggregates. It is an accepted limitation, both for this proposal and
> > other part of the pipeline that handle aggregates. Optionally,
> > checks can be added both for this canonicalization and SROA to
> > disable them on very large aggregates as to avoid wasting work that
> > won't yield good codegen at the end anyway. This limitation should
> > not be a blocker as most aggregate are fairly small. For instance,
> > some language make heavy use of fat pointers, and would greatly
> > benefit from this canonicalization.
> >
> >
> > 2/ This solution will generate loads and stores of value that may not
> > be natively supported by the hardware. The hardware do not natively
> > support aggregate to begin with, so both original IR and canonized
> > IR will require optimization. This is not ideal, but the
> > canonicalization is a plus for 2 reasons:
> > - A subset of these memory access won't need canonicalization
> > anymore.
> >
> > - Other passes in LLVM will be able to work with these load and
> > perform adequate transformations.
> >
> >
> >
> > Possible alternatives :
> >
> >
> > In order to mitigate 1/ it is possible to gate the canonicalization
> > to aggregate under a certain size. This essentially avoiding to do
> > work that will lead to bad codegen no matter what.
> >
> > In order to mitigate 2/, it is possible to slice aggregates loads and
> > stores according to the target's data layout. This CANNOT be
> > implemented for atomic/volatile as it would change semantic, but can
> > be done for regulars ones, which are the most commons.
> >
> >
> >
> >
> > Do that looks better as an RFC ?
> >
> >
> >
> >
> >
> > 2015-08-19 22:11 GMT-07:00 Hal Finkel < hfinkel at anl.gov > :
> >
> >
> > ----- Original Message -----
> > > From: "Mehdi Amini via llvm-dev" < llvm-dev at lists.llvm.org >
> > > To: "deadal nix" < deadalnix at gmail.com >
> > > Cc: "llvm-dev" < llvm-dev at lists.llvm.org >
> > > Sent: Wednesday, August 19, 2015 7:24:28 PM
> > > Subject: Re: [llvm-dev] [RFC] Aggreate load/store, proposed plan
> > >
> > > Hi,
> > >
> > > To be sure, because the RFC below is not detailed and assume
> > > everyone
> > > knows about all the emails from 10 months ago,
> >
> > I agree. The RFC needs to summarize the problems and the potential
> > solutions.
> >
> > > is there more to do
> > > than what is proposed in http://reviews.llvm.org/D9766 ?
> > >
> > > So basically the proposal is that *InstCombine*
> >
> > I think that fixing this early in the optimizer makes sense
> > (InstCombine, etc.). This seems little different from any other
> > canonicalization problem. These direct aggregate IR values are valid
> > IR, but not our preferred canonical form, so we should transform the
> > IR, when possible, into our preferred canonical form.
> >
> > -Hal
> >
> >
> >
> > > turns aggregate
> > > load/store into a load/store using an integer of equivalent size
> > > and
> > > insert the correct bitcast before/after, right?
> > >
> > > Example is:
> > >
> > > %0 = tail call i8* @allocmemory(i64 32)
> > > %1 = bitcast i8* %0 to %B*
> > > store %B { %B__vtbl* @B__vtblZ, i32 42 }, %B* %1, align 8
> > >
> > > into:
> > >
> > > store i128 or (i128 zext (i64 ptrtoint (%B__vtbl* @B__vtblZ to i64)
> > > to i128), i128 774763251095801167872), i128* %1, align 8
> > >
> > > Where the aggregate is:
> > >
> > > %B__vtbl = type { i8*, i32 (%B*)* }
> > > @B__vtblZ = constant %B__vtbl { i8* null, i32 (%B*)* @B.foo }
> > >
> > >
> > > Thanks,
> > >
> > > —
> > > Mehdi
> > >
> > >
> > > > On Aug 19, 2015, at 5:02 PM, deadal nix via llvm-dev
> > > > < llvm-dev at lists.llvm.org > wrote:
> > > >
> > > > It is pretty clear people need this. Let's get this moving.
> > > >
> > > > I'll try to sum up the point that have been made and I'll try to
> > > > address them carefully.
> > > >
> > > > 1/ There is no good solution for large aggregates.
> > > > That is true. However, I don't think this is a reason to not
> > > > address smaller aggregates, as they appear to be needed.
> > > > Realistically, the proportion of aggregates that are very large
> > > > is
> > > > small, and there is no expectation that such a thing would map
> > > > nicely to the hardware anyway (the hardware won't have enough
> > > > registers to load it all anyway). I do think this is reasonable
> > > > to
> > > > expect a reasonable handling of relatively small aggregates like
> > > > fat pointers while accepting that larges ones will be
> > > > inefficient.
> > > >
> > > > This limitation is not unique to the current discussion, as SROA
> > > > suffer from the same limitation.
> > > > It is possible to disable to transformation for aggregates that
> > > > are
> > > > too large if this is too big of a concern. It should maybe also
> > > > be
> > > > done for SROA.
> > > >
> > > > 2/ Slicing the aggregate break the semantic of atomic/volatile.
> > > > That is true. It means slicing the aggregate should not be done
> > > > for
> > > > atomic/volatile. It doesn't mean this should not be done for
> > > > regular ones as it is reasonable to handle atomic/volatile
> > > > differently. After all, they have different semantic.
> > > >
> > > > 3/ Not slicing can create scalar that aren't supported by the
> > > > target. This is undesirable.
> > > > Indeed. But as always, the important question is compared to what
> > > > ?
> > > >
> > > > The hardware has no notion of aggregate, so an aggregate or a
> > > > large
> > > > scalar ends up both requiring legalization. Doing the
> > > > transformation is still beneficial :
> > > > - Some aggregates will generate valid scalars. For such
> > > > aggregate,
> > > > this is 100% win.
> > > > - For aggregate that won't, the situation is still better as
> > > > various optimization passes will be able to handle the load in a
> > > > sensible manner.
> > > > - The transformation never make the situation worse than it is to
> > > > begin with.
> > > >
> > > > On previous discussion, Hal Finkel seemed to think that the
> > > > scalar
> > > > solution is preferable to the slicing one.
> > > >
> > > > Is that a fair assessment of the situation ? Considering all of
> > > > this, I think the right path forward is :
> > > > - Go for the scalar solution in the general case.
> > > > - If that is a problem, the slicing approach can be used for non
> > > > atomic/volatile.
> > > > - If necessary, disable the transformation for very large
> > > > aggregates (and consider doing so for SROA as well).
> > > >
> > > > Do we have a plan ?
> > > >
> > > > _______________________________________________
> > > > LLVM Developers mailing list
> > > > llvm-dev at lists.llvm.org
> > > >
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> > >
> >
> > --
> > Hal Finkel
> > Assistant Computational Scientist
> > Leadership Computing Facility
> > Argonne National Laboratory
> >
> >
>
> --
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory
>
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