[llvm-dev] Proposal: Introduce memory comparison intrinsics

Dmitry Makogon via llvm-dev
Mon, 06 Dec 2021 13:30:11 -0000

Our main motivation is to have an atomic variant of memcmp. We discussed
extending the pattern matching and we may do it sometime later, but it's not
the first on our list.

Regarding non-standard signatures. I think it'd be worth to have an
'isVolatile' flag for non-atomic memcmp (like for memcpy). We cannot
pass this flag to a lib function call, so your suggested approach with
introducing new libfuncs wouldn't work.
Also introducing the memcmp intrinsic would make the code more consistent and
clearer to understand. memcpy, memset, memmove already have intrinsics and we
can share some intrinsics optimization. E.g., we could hoist memcpy and memcmp
of invariant arrays out of the loop using a generalized code for memory
intrinsics.

Dmitry

От: Philip Reames <listmail at philipreames.com>
Отправлено: Thursday, November 25, 2021 2:16 AM
Кому: Dmitry Makogon <dmakogon at azul.com>; llvm-dev at lists.llvm.org
Тема: Re: [llvm-dev] Proposal: Introduce memory comparison intrinsics


Glancing at the in-tree usage, it looks like we have decent support for
optimizing and lowering existing calls to bcmp/memcpy, but very little in the
way of pattern matching formation.  Are you planning on extending the matching
pieces?  Or is the primary intent to be able to share lowering code for the
atomic invariants?

One thing I note is that glancing at existing code, it looks like not all
targets support bcmp or memcmp.  Given that, any intrinsic formation is going to
have to remain dependent on the appropriate TLI checks.  That's slightly
odd, but not a show stopper.

I would find this proposal more compelling if you could show benefit to the
existing lowering/transformations by introducing the non-standard signatures.  I
don't see any obvious ways to do so, but maybe give that some thought?

The major alternative to this proposal would be to simply add two new libfuncs
for the atomic variants of bcmp/memcpy, and then configure them to be
not-present on most targets.  This would allow you to reuse the lowering code -
which I do think is entirely reasonable for upstream - without the need for the
intrinsics.

Overall, I think this proposal is reasonable.  I'm not strongly in support
given the ease of the libfunc approach, but I don't really see any serious
downsides to it either.

Philip
On 11/22/21 8:22 AM, Dmitry Makogon via llvm-dev wrote:
Hello everyone.

I would like to introduce new intrinsics for memory comparison:

memcmp - an equivalent of libc' memcmp,
bcmp - an equivalent of libc' bcmp,
memcmp.element.unordered.atomic - an element atomic version of memcmp,
bcmp.element.unordered.atomic - an element atomic version of bcmp.

Currently there exist some optimizations for memcmp/bcmp libc calls.
We would like to have these optimizations for element atomic comparisons
(atomicity permitted).

I suggest we rewrite the existing optimizations to work with on new intrinsics
and transform
memcmp/bcmp libc calls to the corresponding intrinsics. This is similar to what
we do with
memcpy library calls.

Having these optimizations work on intrinsics and not on recognized libc calls
will allow us to share some existing transforms between atomic and non-atomic
variants.

I propose the following plan for introducing the new intrinsics:
1. Introduce non-atomic memcmp and bcmp intrinsics.
2. Reimplement existing transforms for non-atomic memcmp intrinsic,
the same way as it's done for memcpy.
3. Introduce atomic intrinsics and reuse the optimizations.

Please express your concerns about this.

Dmitry



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