[llvm-dev] [RFC] Generalize llvm.memcpy / llvm.memmove intrinsics.

Fri Aug 21 14:57:13 PDT 2015

----- Original Message -----
> From: "Lang Hames" <lhames at gmail.com>
> To: "Hal Finkel" <hfinkel at anl.gov>
> Cc: "Mehdi Amini" <mehdi.amini at apple.com>, "LLVM Developers Mailing List" <llvm-dev at lists.llvm.org>, "Philip Reames"
> <listmail at philipreames.com>, "Peter Cooper" <peter_cooper at apple.com>, "Gerolf Hoflehner" <ghoflehner at apple.com>
> Sent: Friday, August 21, 2015 1:02:18 AM
> Subject: Re: [llvm-dev] [RFC] Generalize llvm.memcpy / llvm.memmove intrinsics.
> 
> Hi Hal
> 
> > By this I assume you mean some new 'nooverlap' metadata? I don't
> > think we have any existing metadata with the correct semantics.
> 
> 
> I was thinking we could just use the existing noalias metadata.
> Implicitly, the current llvm.memcpy semantics are "src and dst
> overlap perfectly or not at all" (perhaps we should update the docs
> to reflect this if we plan to rely on it?

If we're going to do that, we certainly should.

>). Attaching noalias
> metadata to the source and destination would capture the extra
> information that the pointers really do not overlap, when we can
> figure that out (e.g. when lowering a libc memcpy call).

If you attach noalias metadata to the memcpy call, it will apply to both the source and destination; we don't have a way to differentiate. It might be true that if you attach both noalias and alias.scope metadata to the call, then querying the call against itself will return NoModRef, but that's really hacky (and, in part, wrong, because the destination still alias with itself).

> 
> It does seem odd that we would rely on the documented behaviour of
> libc memcpy (dst/src should not alias at all) to attach the noalias
> metadata, while simultaneously relying on the undocumented behaviour
> of libc memcpy (perfect aliasing works in practice) to lower to
> llvm.memcpy for struct copies. The clang struct copy code should
> probably carry a warning: Do what we say, not what we do.
> 

I agree. Chatting with Chandler offline, he suggested that it might be better to have Clang emit a pointer-quality check and branch around the memcpy when the pointers are equal. This might be faster than the self-copies anyway, and we might often be able to statically prove the result of the comparison. I think this is worth experimenting with.

Thanks again,
Hal

> 
> Cheers,
> Lang.
> 
> 
> 
> 
> 
> On Thu, Aug 20, 2015 at 4:17 PM, Hal Finkel < hfinkel at anl.gov >
> wrote:
> 
> 
> ----- Original Message -----
> > From: "Lang Hames" < lhames at gmail.com >
> > To: "Gerolf Hoflehner" < ghoflehner at apple.com >
> > Cc: "Mehdi Amini" < mehdi.amini at apple.com >, "LLVM Developers
> > Mailing List" < llvm-dev at lists.llvm.org >, "Hal Finkel"
> > < hfinkel at anl.gov >, "Philip Reames" < listmail at philipreames.com >,
> > "Peter Cooper" < peter_cooper at apple.com >
> > Sent: Thursday, August 20, 2015 4:26:20 PM
> > Subject: Re: [llvm-dev] [RFC] Generalize llvm.memcpy / llvm.memmove
> > intrinsics.
> > 
> > 
> > Pete - That patch sounds great!
> > 
> > 
> > Philip, Hal, Medhi, Gerolf - Thanks very much for the feedback.
> > 
> > 
> > So how about this:
> > (1) We drop llvm.memcpy's alignment argument and use Pete's
> > alignment-via-metadata patch (whatever version of it passes
> > review).
> > (2) llvm.memcpy retains its current semantics, but we teach clang,
> > SimplifyLibCalls, etc. to add noalias metadata where we know it's
> > safe.
> 
> By this I assume you mean some new 'nooverlap' metadata? I don't
> think we have any existing metadata with the correct semantics.
> 
> > 
> > Dropping the alignment argument will still change the signature of
> > llvm.memcpy / llvm.memmove, so I guess there's one other issue
> > worth
> > discussing: Should we also split 'isVolatile' into 'isSrcVolatile'
> > and 'isDstVolatile' ?
> 
> Yes. We should be able to specify all relevant properties of the
> source and destination separately. I see no reason not to do this.
> 
> -Hal
> 
> 
> 
> > Nobody has asked for this as far as I know,
> > but I believe it would improve codegen in some cases. E.g.:
> > 
> > 
> > typedef struct {
> > unsigned X[8];
> > } S;
> > 
> > 
> > unsigned foo(volatile S* s) {
> > S t = *s;
> > return t.X[4];
> > }
> > 
> > If the frontend lowers the struct copy to a volatile memcpy we'll
> > have to copy the whole struct before reading part of 't'. If we
> > could mark only the source as volatile then we could discard the
> > stores to 't'.
> > 
> > Again - nobody has asked for this, but if there's interest now
> > would
> > be a good time to look at it.
> > 
> > Cheers,
> > Lang.
> > 
> > 
> > 
> > 
> > On Wed, Aug 19, 2015 at 1:56 PM, Gerolf Hoflehner via llvm-dev <
> > llvm-dev at lists.llvm.org > wrote:
> > 
> > 
> > 
> > 
> > 
> > 
> > 
> > 
> > 
> > 
> > On Aug 19, 2015, at 12:54 PM, Mehdi Amini via llvm-dev <
> > llvm-dev at lists.llvm.org > wrote:
> > 
> > 
> > 
> > 
> > On Aug 19, 2015, at 12:01 PM, Hal Finkel via llvm-dev <
> > llvm-dev at lists.llvm.org > wrote:
> > 
> > ----- Original Message -----
> > 
> > 
> > From: "Philip Reames via llvm-dev" < llvm-dev at lists.llvm.org >
> > To: "Pete Cooper" < peter_cooper at apple.com >, "Lang Hames" <
> > lhames at gmail.com >
> > Cc: "LLVM Developers Mailing List" < llvm-dev at lists.llvm.org >
> > Sent: Wednesday, August 19, 2015 12:14:19 PM
> > Subject: Re: [llvm-dev] [RFC] Generalize llvm.memcpy / llvm.memmove
> > intrinsics.
> > 
> > On 08/19/2015 09:35 AM, Pete Cooper via llvm-dev wrote:
> > 
> > 
> > Hey Lang
> > 
> > 
> > On Aug 18, 2015, at 6:04 PM, Lang Hames via llvm-dev
> > < llvm-dev at lists.llvm.org > wrote:
> > 
> > Hi All,
> > 
> > I'd like to float two changes to the llvm.memcpy / llvm.memmove
> > intrinsics.
> > 
> > 
> > (1) Add an i1 <mayPerfectlyAlias> argument to the llvm.memcpy
> > intrinsic.
> > 
> > When set to '1' (the auto-upgrade default), this argument would
> > indicate that the source and destination arguments may perfectly
> > alias (otherwise they must not alias at all - memcpy prohibits
> > partial overlap). While the C standard says that memcpy's
> > arguments can't alias at all, perfect aliasing works in practice,
> > and clang currently relies on this behavior: it emits
> > llvm.memcpys for aggregate copies, despite the possibility of
> > self-assignment.
> > 
> > Going forward, llvm.memcpy calls emitted for aggregate copies
> > would have mayPerfectlyAlias set to '1'. Other uses of
> > llvm.memcpy (including lowerings from memcpy calls) would have
> > mapPerfectlyAlias set to '0'.
> > 
> > This change is motivated by poor optimization for small memcpys on
> > targets with strict alignment requirements. When a user writes a
> > small, unaligned memcpy we may transform it into an unaligned
> > load/store pair in instcombine (See
> > InstCombine::SimplifyMemTransfer), which is then broken up into
> > an unwieldy series of smaller loads and stores during
> > legalization. I have a fix for this issue which tags the pointers
> > for unaligned load/store pairs with noalias metadata allowing
> > CodeGen to produce better code during legalization, but it's not
> > safe to apply while clang is emitting memcpys with pointers that
> > may perfectly alias. If the 'mayPerfectlyAlias' flag were
> > introduced, I could inspect that and add the noalias tag only if
> > mayPerfectlyAlias is '0'.
> > 
> > Note: We could also achieve the desired effect by adding a new
> > intrinsic (llvm.structcpy?) with semantics that match the current
> > llvm.memcpy ones (i.e. perfect-aliasing or non-aliasing, but no
> > partial), and then reclaim llvm.memcpy for non-aliasing pointers
> > only. I floated this idea with David Majnemer on IRC and he
> > suggested that adding a flag to llvm.memcpy might be less
> > disruptive and easier to maintain - thanks for the suggestion
> > David!
> > Given there's a semantically conservative interpretation and a more
> > optimistic one, this really sounds like a case for metadata not
> > another
> > argument to the function. Our memcpy could keep it's current
> > semantics,
> > and we could add a piece of metadata which says none of the
> > arguments
> > to
> > the call alias.
> > 
> > We could add some "memcpy-allows-self-copies" metadata, and have
> > Clang tag its associated aggregate copies with it. That would also
> > work.
> > 
> > Isn’t introducing an instruction wise “correctness” related
> > metadata?
> > Shouldn’t it be the opposite for correctness, i.e.
> > “memcpy-disallows-self-copies”?
> > (correctness in the sense that dropping the metadata does not break
> > anything).
> > 
> > 
> > 
> > 
> > 
> > 
> > 
> > 
> > 
> > Actually, can't we already get this interpretation by marking both
> > argument points as noalias? Doesn't that require that they don't
> > overlap at all? I think we just need the ability to specify noalias
> > at
> > the callsite for each argument. I don't know if that's been tried,
> > but
> > it should work in theory. There are some issues with control
> > dependence
> > of call site attributes though that we'd need to watch out for/fix.
> > 
> > But that's not quite what we want. We want to say: These can't
> > alias,
> > unless they're exactly equal. noalias either means that it does not
> > alias at all, nor do any derived pointers, and obviously the lack
> > of
> > it says nothing.
> > 
> > This we can still make aliasing assumptions if can prove that src
> > !=
> > destination, which is often easier than proving things accounting
> > for overlaps.
> > 
> > Is this limited to the memcpy case or are these other use-cases so
> > that it would be worth having another attribute than noalias that
> > would carry this semantic (“nooverlap”)?
> > 
> > 
> > I was wondering about that, too. This looks like information either
> > the user has or the compiler could derive. Would it be best to
> > condense the properties into *alias* and *align* attributes that
> > are
> > also user visible?
> > 
> > 
> > 
> > 
> > —
> > 
> > 
> > Mehdi
> > 
> > 
> > 
> > 
> > 
> > 
> > 
> > 
> > 
> > 
> > 
> > 
> > 
> > 
> > (2) Allow different source and destination alignments on both
> > llvm.memcpy / llvm.memmove.
> > 
> > Since I'm talking about changes to llvm.memcpy anyway, a few
> > people asked me to float this one. Having separate alignments for
> > the source and destination pointers may allow us to generate
> > better code when one of the pointers has a higher alignment.
> > 
> > The auto-upgrade for this would be to set both source and
> > destination alignment to the original 'align' value.
> > FWIW, I have a patch for this lying around. I can dig it up. I
> > use alignment attributes to do it as there’s no need for alignment
> > to be its own argument any more.
> > This would be a nice cleanup in general. +1
> > 
> > I agree, this sounds useful.
> > 
> > -Hal
> > 
> > 
> > 
> > 
> > 
> > 
> > Cheers,
> > Pete
> > 
> > 
> > 
> > 
> > Any thoughts?
> > 
> > Cheers,
> > Lang.
> > 
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> > 
> > 
> > --
> > Hal Finkel
> > Assistant Computational Scientist
> > Leadership Computing Facility
> > Argonne National Laboratory
> > _______________________________________________
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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