proposal: change GORISCV64=rva20u64 to include compressed instructions

[4a6f656c]

Proposal Details

When the proposal for GORISCV64 was being discussed, the situation regarding compressed instructions for RISC-V was somewhat unclear and there were discussions about potentially reusing the encoding space. As such, the decision was to make the default GORISCV64 value be rva20u64 but explicitly exclude compressed instructions, effectively giving rv64g:

#61476 (comment)

There has since been a decision regarding the future of the C extension, with it remaining a mandatory part of RVA20 and RVA22:

https://lists.riscv.org/g/tech-profiles/topic/rvi_bod_decision_regarding/102522954

This means that all general purpose RISC-V hardware that Go will run on must support compressed instructions.

This proposal is to change Go's meaning of GORISCV64=rva20u64 to include compressed instructions, instead of continuing to prohibit them.

[4a6f656c]

/cc @markdryan

[gabyhelp]

Related Issues

• pkg/cmd/pr/update/update.go #68505 (closed)
• all: add GORISCV64 environment variable #61476 (closed)

Related Code Changes

• cmd/go: add rva23u64 as a valid value for GORISCV64
• cmd/go: add GORISCV64 environment variable

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[gopherbot]

Change https://go.dev/cl/523477 mentions this issue: cmd/internal/obj/riscv: add support for compressed instructions

[gopherbot]

Change https://go.dev/cl/523478 mentions this issue: cmd/compile/internal/riscv64: use CNOP for compiler inserted NOPs

[gopherbot]

Change https://go.dev/cl/523476 mentions this issue: cmd/internal/obj/riscv: add encoding for compressed riscv64 instructions

[gopherbot]

Change https://go.dev/cl/639656 mentions this issue: cmd/obj/riscv: add support for the Compressed extension

[gopherbot]

Change https://go.dev/cl/639657 mentions this issue: cmd/obj/riscv: compress ADD instructions

[gopherbot]

Change https://go.dev/cl/639658 mentions this issue: cmd/obj/riscv: compress NOP instructions

[ianlancetaylor]

CC @golang/riscv64