[llvm-dev] Proposal: Introduce memory comparison intrinsics

Chris Lattner via llvm-dev
Tue, 23 Nov 2021 07:29:00 -0000

> On Nov 22, 2021, at 6:22 AM, Dmitry Makogon via llvm-dev <llvm-dev at
lists.llvm.org> 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).

Could you elaborate on the specific signatures these intrinsics would have?

llvm.memcpy and friends exist because we want to capture additional semantics
beyond what the memcpy signature does - notably alignment information.  What is
all the additional information are you planning to capture for these?

-Chris


>
> 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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