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[AArch64] Add updated FEAT_SVE_B16B16 and begin replacement of 'b16b16' flag (#101480)
This patch adds FeatureSVEB16B16 to the AArch64 backend in order to
represent the new behavior of FEAT_SVE_B16B16 (as described in the
latest [Armv9.4 extensions
documentation](https://developer.arm.com/documentation/109697/0100/Feature-descriptions/The-Armv9-4-architecture-extension?lang=en#md461-the-armv94-architecture-extension__FEAT_SVE_B16B16))
as well as a 'sve-b16b16' flag to enable it.
The predication of non-widening SVE BFloat16 instructions has changed to
require this feature, instead of the previously required and
soon-to-be-removed FeatureB16B16 which is enabled by the 'b16b16' flag.
Therefore, this change weakens the 'b16b16' flag in favour of
'sve-b16b16'. Existing tests that are effected by this have been
modified to use and/or expect 'sve-b16b16', and new tests have been
added to verify the behavior and implementation of 'sve-b16b16'.
This patch is in response to the response to the following changes.
The architecture features previously enabled by FEAT_SVE_B16B16 have
been relaxed such that it now implements:
- With FEAT_SVE2 : SVE non-widening BFloat16 instructions in
Non-streaming SVE mode
- With FEAT_SME2: SVE non-widening BFloat16 instructions when the
PE is in Streaming SVE mode and SME
Z-targeting multi-vector non-widening BFloat16 instructions.
- **It no longer implements** SME ZA-targeting non-widening
BFloat16 instructions.
The SME ZA-targeting non-widening BFloat16 instructions are implemented
by the new FEAT_SME_B16B16, **this patch does not change how this
architecture feature is enabled** ('+b16b16+sme2'). Only those that are
implemented by FEAT_SVE_B16B16 have been changed to require 'sve-b16b16'
instead of 'b16b16'.
New flags must be created to represent FEAT_SVE_B16B16 and
FEAT_SME_B16B16:
- 'sve-b16b16' enables the updated FEAT_SVE_B16B16 (described
here)
- 'sme-b16b16' will enable the new FEAT_SME_B16B16
- **This patch includes 'sve-b16b16' only**
A future patch will add 'sme-b16b16', SME ZA-targeting non-widening
BFloat16 instructions would then be guarded by '+sme-b16b16+sme2', and
'b16b16' can be removed.
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