From: lkcl Date: Mon, 10 Apr 2023 09:37:15 +0000 (+0100) Subject: (no commit message) X-Git-Tag: opf_rfc_ls012_v1~39 X-Git-Url: https://git.libre-soc.org/?a=commitdiff_plain;h=d31d0d80fa171810232b01fb5981fe7b872e7c25;p=libreriscv.git --- diff --git a/openpower/sv/rfc/ls012.mdwn b/openpower/sv/rfc/ls012.mdwn index 7107185c7..2a4e5c910 100644 --- a/openpower/sv/rfc/ls012.mdwn +++ b/openpower/sv/rfc/ls012.mdwn @@ -102,6 +102,23 @@ include `pcdec` and the Galois Field arithmetic operations. From a purely practical perspective due to the quantity the lower-priority instructions were simply left out. However they remain in the Libre-SOC resources. +Some of these SFFS instructions appear to be duplicates of VSX. +A frequent argument comes up that if instructions +are in VSX already they should not be added to SFFS, especially if +they are nominally the same. The logic that this effectively damages +performance of an SFFS-only implementation was raised earlier, however +there is a more subtle reason why the instructions are needed. + +Future versions of SVP64 and SVP64Single are expected to be developed +by future Power ISA Stakeholders on top of VSX. The decisions made +there about the meaning of Prefixed Vectorised VSX may be **completely** +different from those made for Prefixed SFFS instructions. At which +point the lack of SFFS equivalents would penalise SFFS implementors +in a much more severe way, effectively expecting them and SFFS programmers +to work with a non-orthogonal paradigm, to their detriment. +The solution is to give the SFFS Subset the space and respect that it deserves +and allow it to be stand-alone on its own merits. + ## SVP64 Management instructions These without question have to go in EXT0xx. Future extended variants,