From 73dbadecd5aa76f73f947e25954bd54a0ff3474e Mon Sep 17 00:00:00 2001 From: Luke Kenneth Casson Leighton Date: Mon, 1 Oct 2018 03:12:01 +0100 Subject: [PATCH] add HINT section --- simple_v_extension/specification.mdwn | 34 ++++++++++++++++++++------- 1 file changed, 25 insertions(+), 9 deletions(-) diff --git a/simple_v_extension/specification.mdwn b/simple_v_extension/specification.mdwn index 5ca195208..2bf0f6a5a 100644 --- a/simple_v_extension/specification.mdwn +++ b/simple_v_extension/specification.mdwn @@ -8,19 +8,20 @@ # Summary and Background: Rationale Simple-V is a uniform parallelism API for RISC-V hardware that has several -unplanned side-effects including code-size reduction. The reason for +unplanned side-effects including code-size reduction, expansion of +HINT space and more. The reason for creating it is to provide a manageable way to turn a pre-existing design into a parallel one, in a step-by-step incremental fashion, allowing the implementor to focus on adding hardware where it is needed and necessary. -**No new instructions are added**. The parallelism (if any is implemented) -is implicitly added by tagging *standard* scalar registers for redirection. -When such a tagged register is used in any instruction, it indicates -that the PC shall **not** be incremented; instead a loop is activated -where *multiple* instructions are issued to the pipeline (as determined -by a length CSR), with contiguously incrementing register numbers starting -from the tagged register. Thus Simple-V effectively sits (slots) *in between* -the instruction decode phase and the ALU(s). +Critically: **No new instructions are added**. The parallelism (if any +is implemented) is implicitly added by tagging *standard* scalar registers +for redirection. When such a tagged register is used in any instruction, +it indicates that the PC shall **not** be incremented; instead a loop +is activated where *multiple* instructions are issued to the pipeline +(as determined by a length CSR), with contiguously incrementing register +numbers starting from the tagged register. Thus Simple-V effectively sits +(slots) *in between* the instruction decode phase and the ALU(s). The barrier to entry with SV is therefore very low. The minimum is software-emulation (traps), requiring only the CSRs and CSR tables, and that @@ -727,6 +728,21 @@ Similar rules apply to the destination register. * Throw an exception. Whether that actually results in spawning threads as part of the trap-handling remains to be seen. +# Hints + +A "HINT" is an operation that has no effect on architectural state, +where its use may, by agreed convention, give advance notification +to the microarchitecture: branch prediction notification would be +a good example. Usually HINTs are where rd=x0. + +With Simple-V being capable of issuing *parallel* instructions where +rd=x0, the space for possible HINTs is expanded considerably. VL +could be used to indicate different hints. In addition, if predication +is set, the predication register itself could hypothetically be passed +in as a *parameter* to the HINT operation. + +No specific hints are yet defined in Simple-V + # Under consideration ## Retro-fitting Predication into branch-explicit ISA -- 2.30.2