From: lkcl Date: Mon, 26 Oct 2020 17:23:35 +0000 (+0000) Subject: (no commit message) X-Git-Tag: convert-csv-opcode-to-binary~1939 X-Git-Url: https://git.libre-soc.org/?a=commitdiff_plain;h=5ad509d982bfd6986bebbeaa2732105901d36897;p=libreriscv.git --- diff --git a/openpower/sv/predication.mdwn b/openpower/sv/predication.mdwn index 3360052b8..1c0c80ce3 100644 --- a/openpower/sv/predication.mdwn +++ b/openpower/sv/predication.mdwn @@ -131,7 +131,7 @@ Implementation-wise just like in the CR-based case a special regfile port could The disadvantages appear on closer analysis: -* Unlike the "full" CR port (which reads 8x CRs CR0-7 in one hit) trying the same trick on the scalar integer regfile, to obtain 8 predicate bits, would require a whopping 8x64bit set of reads to the INT regfile instead of a scant 1x32bit read. Resource-wise, then, this idea is expensive. +* Unlike the "full" CR port (which reads 8x CRs CR0-7 in one hit) trying the same trick on the scalar integer regfile, to obtain just 8 predicate bits (each being an LSB of a given 64 bit scalar int), would require a whopping 8x64bit set of reads to the INT regfile instead of a scant 1x32bit read. Resource-wise, then, this idea is expensive. * With predicate bits being distributed out amongst 64 bit scalar registers, scalar bitmanipulation operations that can be performed after transferring Vectors of CMP operations from CRs to INTs (vectorised-mfcr) are more challenging and costly. Rather than use vectorised mfcr, complex transfers of the LSBs into a single scalar int are required. In a "normal" Vector ISA this would be solved by adding opcodes that perform the kinds of bitmanipulation operations normally needed for predicate masks, as specialist operations *on* those masks. However for SV the rule has been set: "no unnecessary additional Vector Instructions" because it is possible to use existing PowerISA scalar bitmanip opcodes to cover the same job.