Adding AVX512 path to Base64 encoding/Decoding #92241

DeepakRajendrakumaran · 2023-09-18T19:16:43Z

Overview

This PR implements an AVX512 code path for Base64 encoding/Decoding. This is based on the work by Wojciech Muła and Daniel Lemire. There is a fast AVX512VBMI path and the fallback uses AVX512F/BW - I'll refer to these as VBMI_AVAILABLE and VBMI_UNAVAILABLE here on. For performance purposes, this will be compared to an AVX2 implementation which will be referred to as BASE_Version
Reference for the algorithm:

https://arxiv.org/pdf/1910.05109.pdf

This version uses intrinsics directly and not generic vector libraries due to lack of current support in JIt/Vector libraries to produce optimal code. Some additional support which would be required in order to use generic vector library for implementing this would be

Add ShuffleUnsafe for Vector512
Extend Vector512.Shuffle() to lower to intrinsics instead of going to fallback for more cases.
Expand Vector512 surface area to incorporate more high level functions

Even the current implementation can be further optimized by adding the multishift() implementation. This is a further optimization

Generated code

(Will be focusing on the actual encoding/decoding code within the loop only)

Encoding

VBMI_AVAILABLE

VBMI_UNAVAILABLE

BASE_VERSION

Decoding

VBMI_AVAILABLE

VBMI_UNAVAILABLE

BASE_VERSION

Performance

ON ICX -

BASE_VERSION vs VBMI_UNAVAILABLE

BASE_VERSION vs VBMI_AVAILABLE

DeepakRajendrakumaran · 2023-09-18T19:17:56Z

@BruceForstall @tannergooding @dotnet/avx512-contrib

src/libraries/System.Private.CoreLib/src/System/Buffers/Text/Base64Decoder.cs

src/libraries/System.Private.CoreLib/src/System/Buffers/Text/Base64Encoder.cs

danmoseley · 2023-09-19T01:05:32Z

Do we need a new entry in the third party notices file?

EgorBo · 2023-09-19T09:53:07Z

Do we need a new entry in the third party notices file?

We should already have them if I am not mistaken (unless avx512 uses a different article)

DeepakRajendrakumaran · 2023-09-19T16:01:12Z

Do we need a new entry in the third party notices file?

We should already have them if I am not mistaken (unless avx512 uses a different article)

I'm not sure about this-

The original sse and avx implementations use a similar algorithm and this is the reference provided - https://github.com/dotnet/runtime/blob/main/src/libraries/System.Private.CoreLib/src/System/Buffers/Text/Base64Encoder.cs#L13-L14.

This implementation(the VBMI version) uses a modified version based on this(https://github.com/dotnet/runtime/pull/92241/files#diff-db463201901c2d83d2b563871ae11fafee9d5afe94e4d014b77212996b25f770R635) - https://arxiv.org/pdf/1910.05109.pdf

There is an AVX512 based implementation in there - https://github.com/aklomp/base64/tree/master/lib/arch/avx512. But it has only the encode and it requires 'multishift' which we do not currently support

danmoseley · 2023-09-19T20:06:46Z

We generally acknowledge significant reuse in the TPN file even if there's a link from the sources. I see

runtime/THIRD-PARTY-NOTICES.TXT

Line 345 in 17b60e3

License notice for vectorized base64 encoding / decoding

but I'm not sure it points to that pdf (eg it doesn't include Lemire in the list)

but, whatever @EgorBo recommends..

EgorBo · 2023-09-19T20:24:52Z

I am not an expert in THIRD-PARTY-NOTICES.TXT 🙂 but seems like the link to that article is worth adding here https://github.com/dotnet/runtime/blob/main/THIRD-PARTY-NOTICES.TXT#L345

There is an AVX512 based implementation in there - https://github.com/aklomp/base64/tree/master/lib/arch/avx512.

Do we use any of the code from that repo? I see it has BSD-2 license

DeepakRajendrakumaran · 2023-09-19T20:39:44Z

I am not an expert in THIRD-PARTY-NOTICES.TXT 🙂 but seems like the link to that article is worth adding here https://github.com/dotnet/runtime/blob/main/THIRD-PARTY-NOTICES.TXT#L345

There is an AVX512 based implementation in there - https://github.com/aklomp/base64/tree/master/lib/arch/avx512.

Do we use any of the code from that repo? I see it has BSD-2 license

Not directly but the logic(including shuffle constants are similar). But this implementation uses '_mm512_multishift_epi64_epi8' and that's not the one I'm using. This(https://github.com/WojciechMula/base64simd/blob/master/encode/encode.avx512vbmi.cpp) in particular is probably worth mentioning in hindsight(Since I used the non multishift version in my implementation with VBMI)

I used the below as resources to understand available implementations and available options(But they are all related). Which brings up the question which of these exactly I should be referencing

esp this for understanding : http://0x80.pl/notesen/2016-04-03-avx512-base64.html#id29

https://github.com/WojciechMula/base64simd/tree/master
https://arxiv.org/pdf/1910.05109.pdf
https://github.com/lemire/fastbase64/blob/master/src/fastavx512bwbase64.c
https://github.com/aklomp/base64/tree/master/lib/arch/avx512

ghost · 2023-09-20T10:59:34Z

Tagging subscribers to this area: @dotnet/area-system-buffers
See info in area-owners.md if you want to be subscribed.

Issue Details

Overview

This PR implements an AVX512 code path for Base64 encoding/Decoding. This is based on the work by Wojciech Muła and Daniel Lemire. There is a fast AVX512VBMI path and the fallback uses AVX512F/BW - I'll refer to these as VBMI_AVAILABLE and VBMI_UNAVAILABLE here on. For performance purposes, this will be compared to an AVX2 implementation which will be referred to as BASE_Version
Reference for the algorithm:

https://arxiv.org/pdf/1910.05109.pdf

This version uses intrinsics directly and not generic vector libraries due to lack of current support in JIt/Vector libraries to produce optimal code. Some additional support which would be required in order to use generic vector library for implementing this would be

Add ShuffleUnsafe for Vector512
Extend Vector512.Shuffle() to lower to intrinsics instead of going to fallback for more cases.
Expand Vector512 surface area to incorporate more high level functions

Even the current implementation can be further optimized by adding the multishift() implementation. This is a further optimization

Generated code

(Will be focusing on the actual encoding/decoding code within the loop only)

Encoding

VBMI_AVAILABLE

VBMI_UNAVAILABLE

BASE_VERSION

Decoding

VBMI_AVAILABLE

VBMI_UNAVAILABLE

BASE_VERSION

Performance

ON ICX -

BASE_VERSION vs VBMI_UNAVAILABLE

BASE_VERSION vs VBMI_AVAILABLE

Author:	DeepakRajendrakumaran
Assignees:	-
Labels:	`area-System.Buffers`, `community-contribution`, `needs-area-label`
Milestone:	-

DeepakRajendrakumaran · 2023-09-20T23:20:03Z

I am not an expert in THIRD-PARTY-NOTICES.TXT 🙂 but seems like the link to that article is worth adding here https://github.com/dotnet/runtime/blob/main/THIRD-PARTY-NOTICES.TXT#L345

There is an AVX512 based implementation in there - https://github.com/aklomp/base64/tree/master/lib/arch/avx512.

Do we use any of the code from that repo? I see it has BSD-2 license

Not directly but the logic(including shuffle constants are similar). But this implementation uses '_mm512_multishift_epi64_epi8' and that's not the one I'm using. This(https://github.com/WojciechMula/base64simd/blob/master/encode/encode.avx512vbmi.cpp) in particular is probably worth mentioning in hindsight(Since I used the non multishift version in my implementation with VBMI)

I used the below as resources to understand available implementations and available options(But they are all related). Which brings up the question which of these exactly I should be referencing

esp this for understanding : http://0x80.pl/notesen/2016-04-03-avx512-base64.html#id29

https://github.com/WojciechMula/base64simd/tree/master https://arxiv.org/pdf/1910.05109.pdf https://github.com/lemire/fastbase64/blob/master/src/fastavx512bwbase64.c https://github.com/aklomp/base64/tree/master/lib/arch/avx512

@EgorBo I modified the reference to point to https://github.com/WojciechMula/base64simd/tree/master. Which has the closest versions to the implementation I went with. Will this require me adding anything to notice?

DeepakRajendrakumaran · 2023-09-21T18:06:24Z

I am not an expert in THIRD-PARTY-NOTICES.TXT 🙂 but seems like the link to that article is worth adding here https://github.com/dotnet/runtime/blob/main/THIRD-PARTY-NOTICES.TXT#L345

There is an AVX512 based implementation in there - https://github.com/aklomp/base64/tree/master/lib/arch/avx512.

Do we use any of the code from that repo? I see it has BSD-2 license

Not directly but the logic(including shuffle constants are similar). But this implementation uses '_mm512_multishift_epi64_epi8' and that's not the one I'm using. This(https://github.com/WojciechMula/base64simd/blob/master/encode/encode.avx512vbmi.cpp) in particular is probably worth mentioning in hindsight(Since I used the non multishift version in my implementation with VBMI)
I used the below as resources to understand available implementations and available options(But they are all related). Which brings up the question which of these exactly I should be referencing
esp this for understanding : http://0x80.pl/notesen/2016-04-03-avx512-base64.html#id29
https://github.com/WojciechMula/base64simd/tree/master https://arxiv.org/pdf/1910.05109.pdf https://github.com/lemire/fastbase64/blob/master/src/fastavx512bwbase64.c https://github.com/aklomp/base64/tree/master/lib/arch/avx512

@EgorBo I modified the reference to point to https://github.com/WojciechMula/base64simd/tree/master. Which has the closest versions to the implementation I went with. Will this require me adding anything to notice?

Have updated THIRD PARTY NOTICE based on conversation with @tannergooding . Removing fallback avx512Bw path meant I had to add only 2 references.

EgorBo

Thanks! Looks way simpler now

DeepakRajendrakumaran · 2023-10-09T18:26:06Z

@tannergooding @BruceForstall Any comments on this? I'd be great if we can move this forward this week,

BruceForstall · 2023-10-10T02:30:48Z

I'm not the right person to review this. If @tannergooding can't review, maybe @stephentoub can review (or pick an appropriate reviewer).

tannergooding · 2023-10-24T15:32:23Z

src/libraries/System.Private.CoreLib/src/System/Buffers/Text/Base64Decoder.cs

+
+            // This algorithm requires AVX512VBMI support.
+            // Vbmi was first introduced in CannonLake and is avaialable from IceLake on.
+            // This makes it okay to use Vbmi instructions since Vector512.IsHardwareAccelerated returns True only from IceLake on.


This comment isn't quite accurate.

Vector512.IsHardwareAccelerated can be made to return true for Skylake-X and up to before IceLake via an environment variable. This is why the caller has the check for Vector512.IsHardwareAccelerated && Avx512Vbmi.IsSupported and why this function has [CompExactlyDependsOn(typeof(Avx512Vbmi))]

We're fine with it not being usable pre IceLake since those often incur heavier downclocking and its unnecessary complexity for a non-default scenario.

Removed line 667

src/libraries/System.Private.CoreLib/src/System/Buffers/Text/Base64Decoder.cs

tannergooding · 2023-10-24T15:35:52Z

src/libraries/System.Private.CoreLib/src/System/Buffers/Text/Base64Decoder.cs

+                str = Avx512Vbmi.PermuteVar64x8(multiAdd2.AsByte(), vbmiPackedLanesControl).AsSByte();
+
+                AssertWrite<Vector512<sbyte>>(dest, destStart, destLength);
+                Vector512.Store(str.AsByte(), dest);


nit: Most other places in the JIT we do str.Store(dest) since its an extension method and can be accessed using instance syntax.

I'm not sure I fully understand how this works

It's likely conflicting because dest is a byte* while str is a Vector512<sbyte> and so it can't resolve

str.Store((sbyte*)dest) should fix it, or str.AsByte().Store(dest). The former is less IL, most notably.

Ah..I messed up and was using str.AsSbyte().Store(dest) It's fixed now. Thank you

tannergooding

LGTM. Just a request to cleanup a couple minor things.

Signed-off-by: Deepak Rajendrakumaran <[email protected]>

src/libraries/System.Private.CoreLib/src/System/Buffers/Text/Base64Decoder.cs

src/libraries/System.Private.CoreLib/src/System/Buffers/Text/Base64Encoder.cs

DeepakRajendrakumaran · 2023-10-25T16:32:19Z

LGTM. Just a request to cleanup a couple minor things.

I've committed the clean-up changes. Please let me know if you want me to make any other changes

* Adding AVX512 path to Base64 encoding/Decoding * Addressing review Comments. Signed-off-by: Deepak Rajendrakumaran <[email protected]> * Removing fallback path. * Updating Third Party Notice. * Addressing review comments --------- Signed-off-by: Deepak Rajendrakumaran <[email protected]>

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DeepakRajendrakumaran force-pushed the encoding branch from 8b593f6 to d29d4d0 Compare September 19, 2023 00:14

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DeepakRajendrakumaran added 4 commits October 24, 2023 09:57

Adding AVX512 path to Base64 encoding/Decoding

911b517

Addressing review Comments.

7b351ad

Signed-off-by: Deepak Rajendrakumaran <[email protected]>

Removing fallback path.

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Updating Third Party Notice.

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Adding AVX512 path to Base64 encoding/Decoding #92241

Adding AVX512 path to Base64 encoding/Decoding #92241

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Overview

Generated code

Encoding

Decoding

Performance

ON ICX -

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Adding AVX512 path to Base64 encoding/Decoding #92241

Adding AVX512 path to Base64 encoding/Decoding #92241

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Overview

Generated code

Encoding

Decoding

Performance

ON ICX -

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