cmd/compile: 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

[markdryan]

I'd previously +1'ed this proposal but thought it best to also add my support with some actual words.

All current riscv64 devices support the compressed instruction set. We now know that all future riscv64 devices supporting a standard profile will also have compressed instructions. Compressed instructions consume 75% of the current RISC-V instruction encoding space, so it's a real waste not to use them. The use of compressed instructions in Go should reduce binary size and improve instruction cache utilisation. To quote from the The RISC-V Instruction Set Manual Volume I (Version 20240411)

The philosophy of RVC is to reduce code size for embedded applications and to improve performance and energy-efficiency for all applications due to fewer misses in the instruction cache. Waterman shows that RVC fetches 25%-30% fewer instruction bits, which reduces instruction cache misses by 20%-25%, or roughly the same performance impact as doubling the instruction cache size.

Note that we have had some compressed instruction patches pending for quite a while. These patches were put on hold when there was some doubt about the future of compressed instructions in the standard RISC-V profiles, e.g.,

https://go-review.googlesource.com/c/go/+/523477/3

If you look at the comments of this and other related PRs, you will see Russ Cox wrote

This would require compressed instruction support in the riscv port, so we need an accepted proposal for that.

This is the requested proposal to add their support to Go and unblock the pending patches.