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| author | Mark Ryan <markdryan@rivosinc.com> | 2025-11-10 10:07:51 +0100 |
|---|---|---|
| committer | Gopher Robot <gobot@golang.org> | 2025-11-25 12:25:24 -0800 |
| commit | 4ca048cc326bf310f873315dfb1e2644ec243299 (patch) | |
| tree | 25eb36dfd3bd8a18e427e7dd3a912327e6efb445 /src/cmd | |
| parent | a572d571fae7b269b8b7e3b8c6ec41c01cd0f25b (diff) | |
| download | go-4ca048cc326bf310f873315dfb1e2644ec243299.tar.xz | |
cmd/internal/obj/riscv: document compressed instructions
We update the RISC-V assembler documentation to describe how
the RISC-V compressed instruction set is implemented by the
assembler and how compressed instructions can be disabled.
Change-Id: Ic7b1cb1586e6906af78adb8ff5fa10f5fbfde292
Reviewed-on: https://go-review.googlesource.com/c/go/+/719221
Reviewed-by: Mark Freeman <markfreeman@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Joel Sing <joel@sing.id.au>
Auto-Submit: Joel Sing <joel@sing.id.au>
Reviewed-by: Meng Zhuo <mengzhuo1203@gmail.com>
Diffstat (limited to 'src/cmd')
| -rw-r--r-- | src/cmd/internal/obj/riscv/doc.go | 27 |
1 files changed, 27 insertions, 0 deletions
diff --git a/src/cmd/internal/obj/riscv/doc.go b/src/cmd/internal/obj/riscv/doc.go index 365bedd299..898a45b0e6 100644 --- a/src/cmd/internal/obj/riscv/doc.go +++ b/src/cmd/internal/obj/riscv/doc.go @@ -289,6 +289,33 @@ the constant literal is 0.0, MOVF and MOVD will be encoded as FLW and FLD instructions that load the constant from a location within the program's binary. +# Compressed instructions + +The Go assembler converts 32 bit RISC-V instructions to compressed +instructions when generating machine code. This conversion happens +automatically without the need for any direct involvement from the programmer, +although judicious choice of registers can improve the compression rate for +certain instructions (see the [RISC-V ISA Manual] for more details). This +behaviour is enabled by default for all of the supported RISC-V profiles, i.e., +it is not affected by the value of the GORISCV64 environment variable. + +The use of compressed instructions can be disabled via a debug flag, +compressinstructions: + + - Use -gcflags=all=-d=compressinstructions=0 to disable compressed + instructions in Go code. + - Use -asmflags=all=-d=compressinstructions=0 to disable compressed + instructions in assembly code. + +To completely disable automatic instruction compression in a Go binary both +options must be specified. + +The assembler also permits the use of compressed instructions in hand coded +assembly language, but this should generally be avoided. Note that the +compressinstructions flag only prevents the automatic compression of 32 +bit instructions. It has no effect on compressed instructions that are +hand coded directly into an assembly file. + [RISC-V ISA Manual]: https://github.com/riscv/riscv-isa-manual [rva20u64]: https://github.com/riscv/riscv-profiles/blob/main/src/profiles.adoc#51-rva20u64-profile [rva22u64]: https://github.com/riscv/riscv-profiles/blob/main/src/profiles.adoc#rva22u64-profile |