cmd/internal/obj/riscv: riscv64 compressed instructions

[4a6f656c]

The RISC-V architecture has support for compressed instructions - these are effectively a subset of the standard 4 byte long instructions, which fit in 2 bytes. These compressed instructions have limited functionality, however require half the code size.

Adding support to the assembler is relatively straight forward and from a code generation perspective, things work as expected and the binary runs. However, I keep hitting problems relating to the pcdatavalue tables while building toolchain3 - this is in the form of deadlock loops while generating tracebacks for inline functions, "runtime: invalid pc-encoded table" from symtab.go while handling async pre-empt, etc.

These all seems to suggest that the various PCDATA tables are wrong/broken, however as far as I can tell p.Pc is being set correctly in cmd/internal/obj/riscv. I would also expect that we handle variable instruction sizes (and variable p.Pc steps), given CISC architectures like x86.

Overall it seems like I'm missing something obvious - should things work if p.Pc is set correctly on a per *obj.Prog basis (even with multiple machine instructions with various widths being generated)?

Any suggestions regarding what else I should check?

(will mail out the work in progress code shortly)

[4a6f656c]

/cc @cherrymui

[gopherbot]

Change https://golang.org/cl/340151 mentions this issue: cmd/internal/obj/riscv: compressed instruction support for riscv64

[randall77]

You probably want to look at MinLC in cmd/internal/sys/arch.go. The PCs in pcln tables are expressed as multiples of this constant.

[4a6f656c]

@randall77 thanks - turns out it was MinLC and PCQuantum - adjusting those, along with a change to runtime.goexit (smaller NOP slots) and we have working code.

[gopherbot]

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