llvm dev - Jan 2018 - Suggestions on code generation for SIMD

Hi everyone,

I'm quite new to LLVM, but am working on a project that might need to
generate some SIMD code using LLVM. The SIMD code will be using INTEL MIC
intrinsics and I'm not sure about the
steps and tool set that I need to use to generate those.

I also have a confusion on the following problems:

   1. Do people usually generate SIMD code at source code level, using
   __m512?
   2. If not, does LLVM have corresponding IR instructions for the SIMD
   registers and instructions?


Since I'm new, I would appreciate any help that could give me some
directions at any level. Some references would also help. Thanks in advance!

-- 
Sincerely,
Linchuan
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> On 6 Jan 2018, at 00:26, Linchuan Chen via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
> Hi everyone,
> 
> I'm quite new to LLVM, but am working on a project that might need to
generate some SIMD code using LLVM. The SIMD code will be using INTEL MIC
intrinsics and I'm not sure about the
> steps and tool set that I need to use to generate those.
> 
> I also have a confusion on the following problems:
> Do people usually generate SIMD code at source code level, using __m512?
> If not, does LLVM have corresponding IR instructions for the SIMD registers
and instructions?
> 
> Since I'm new, I would appreciate any help that could give me some
directions at any level. Some references would also help. Thanks in advance!

Hi Linchuan,

I believe clang supports Intel AVX512 intrinsics so it should be possible to
generate vector code using that.

For 2), LLVM has first class vector types such as <4 x i32> and can do the
usual things on those types, including masking. The vectoriser is where most of
the vector code that LLVM generates will originate from. These types aren’t
target specific however, and there are no notions of vector “registers” at the
IR level.

Cheers,
Amara
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Thanks Amara so much for the info!

One more question: what do people usually do if they want to generate
vectorized code for some existing c/c++ code?
Do they usually do C/C++ source level transformation, or do at LLVM's IR
level?

I know clang supports auto vectorizations, such as loop vectorization and
SLP, but they are not flexible enough if we
want to do more custom vectorizations or handle more complex cases, for
example, SLP might not be able to handle
branches in the code (or may be latest version already can handle branches
using mask).

On Mon, Jan 8, 2018 at 11:30 AM, Amara Emerson via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> On 6 Jan 2018, at 00:26, Linchuan Chen via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
> Hi everyone,
>
> I'm quite new to LLVM, but am working on a project that might need to
> generate some SIMD code using LLVM. The SIMD code will be using INTEL MIC
> intrinsics and I'm not sure about the
> steps and tool set that I need to use to generate those.
>
> I also have a confusion on the following problems:
>
>    1. Do people usually generate SIMD code at source code level, using
>    __m512?
>    2. If not, does LLVM have corresponding IR instructions for the SIMD
>    registers and instructions?
>
>
> Since I'm new, I would appreciate any help that could give me some
> directions at any level. Some references would also help. Thanks in
advance!
>
>
>
> Hi Linchuan,
>
> I believe clang supports Intel AVX512 intrinsics so it should be possible
> to generate vector code using that.
>
> For 2), LLVM has first class vector types such as <4 x i32> and can
do the
> usual things on those types, including masking. The vectoriser is where
> most of the vector code that LLVM generates will originate from. These
> types aren’t target specific however, and there are no notions of vector
> “registers” at the IR level.
>
> Cheers,
> Amara
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>

-- 
Sincerely,
Linchuan
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