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[IR][SVE] Add new llvm.experimental.stepvector intrinsic

Authored by david-arm on Feb 8 2021, 7:46 AM.

Description

[IR][SVE] Add new llvm.experimental.stepvector intrinsic

This patch adds a new llvm.experimental.stepvector intrinsic,
which takes no arguments and returns a linear integer sequence of
values of the form <0, 1, ...>. It is primarily intended for
scalable vectors, although it will work for fixed width vectors
too. It is intended that later patches will make use of this
new intrinsic when vectorising induction variables, currently only
supported for fixed width. I've added a new CreateStepVector
method to the IRBuilder, which will generate a call to this
intrinsic for scalable vectors and fall back on creating a
ConstantVector for fixed width.

For scalable vectors this intrinsic is lowered to a new ISD node
called STEP_VECTOR, which takes a single constant integer argument
as the step. During lowering this argument is set to a value of 1.
The reason for this additional argument at the codegen level is
because in future patches we will introduce various generic DAG
combines such as

mul step_vector(1), 2 -> step_vector(2)
add step_vector(1), step_vector(1) -> step_vector(2)
shl step_vector(1), 1 -> step_vector(2)
etc.

that encourage a canonical format for all targets. This hopefully
means all other targets supporting scalable vectors can benefit
from this too.

I've added cost model tests for both fixed width and scalable
vectors:

llvm/test/Analysis/CostModel/AArch64/neon-stepvector.ll
llvm/test/Analysis/CostModel/AArch64/sve-stepvector.ll

as well as codegen lowering tests for fixed width and scalable
vectors:

llvm/test/CodeGen/AArch64/neon-stepvector.ll
llvm/test/CodeGen/AArch64/sve-stepvector.ll

See this thread for discussion of the intrinsic:
https://lists.llvm.org/pipermail/llvm-dev/2021-January/147943.html

Details

Committed
david-armMar 23 2021, 3:43 AM
Parents
rG94470777af8b: [CMAKE] Fix cross-compilation build
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Event Timeline

mdchen added a subscriber: mdchen.Apr 21 2021, 12:34 AM

@david-arm Hi David, there's a tiny issue of the commit that the llvm.experimental.stepvector intrinsic is not well indexed in the changes to Langref.rst, and now it resides under llvm.experimental.vector.splice. I guess it's not intended and leave a message here FYI:)

@david-arm Hi David, there's a tiny issue of the commit that the llvm.experimental.stepvector intrinsic is not well indexed in the changes to Langref.rst, and now it resides under llvm.experimental.vector.splice. I guess it's not intended and leave a message here FYI:)

Hi @mdchen, yeah I'm aware of that as another colleague pointed it out to me yesterday! It's just a formatting issue and I'll fix it today. :)

@david-arm Glad to know that. Thank you for the work!