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[docs] Add documentation on AArch64 SME for feature detection

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Give a brief explanation of the Scalable Matrix Extension and where we
believe it will be beneficial, in line with the existing documentation
for Neon and SVE.

Change-Id: I477b7f293c00740ce8346a96a9a0ad133f4ef1c2
Reviewed-on: https://chromium-review.googlesource.com/c/libyuv/libyuv/+/5587508
Reviewed-by: Frank Barchard <fbarchard@chromium.org>
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George Steed 2024-05-22 10:58:33 +01:00 committed by Frank Barchard
parent 214b4a25c7
commit cc823114a1
1 changed files with 22 additions and 1 deletions
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docs/feature_detection.md
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@ -18,7 +18,7 @@ Neon is available and mandatory in AArch64 from the base Armv8.0-A
architecture. Neon can be used even if later extensions like the Scalable architecture. Neon can be used even if later extensions like the Scalable
Vector Extension (SVE) are also present. The exception to this is if the CPU is Vector Extension (SVE) are also present. The exception to this is if the CPU is
currently operating in streaming mode as introduced by the Scalable Matrix currently operating in streaming mode as introduced by the Scalable Matrix
Extension, which is not currently used in libyuv. Extension, described later.
There are also a couple of architecture extensions present for Neon that we can There are also a couple of architecture extensions present for Neon that we can
take advantage of in libyuv: take advantage of in libyuv:
@ -64,6 +64,27 @@ Armv8.6-A or Armv9.1-A, however there is no micro-architecture at time of
writing where SVE2 is implemented without all previously-mentioned features writing where SVE2 is implemented without all previously-mentioned features
also being implemented. also being implemented.
### The Scalable Matrix Extension (SME)
The Scalable Matrix Extension (SME) is an optional feature introduced from
Armv9.2-A. SME exists alongside SVE and introduces new execution modes for
applications performing extended periods of data processing. In particular SME
introduces a few new components of interest:
* Access to a scalable two-dimensional ZA tile register and new instructions to
interact with rows and columns of the ZA tiles. This can be useful for data
transformations like transposes.
* A streaming SVE (SSVE) mode, during which the SVE vector length matches the
ZA tile register width. In typical systems where the ZA tile register width
is longer than the core SVE vector length, SSVE processing allows for faster
data processing, even if the ZA tile register is unused. While the CPU is
executing in streaming mode, Neon instructions are unavailable.
* When both SSVE and the ZA tile registers are enabled there are additional
outer-product instructions accumulating into a whole ZA tile, suitable for
accelerating matrix arithmetic. This is likely less useful in libyuv.
## Linux and Android ## Linux and Android
On AArch64 running under Linux and Android, features are detected by inspecting On AArch64 running under Linux and Android, features are detected by inspecting