cmd/internal/disasm: correct instruction length handling for riscv64
disasm_riscv64 currently always returns an instruction length of four,
which is not correct if compressed instructions are in use. Return the
length of the decoded instruction, defaulting to two bytes if the
instruction is unknown.
With this change it is possible to correctly objdump a binary that is
written in C and includes compressed instructions:
$ go tool objdump ./hello
TEXT _start(SB)
:0 0x5b0
ef002002 CALL 8(PC)
:0 0x5b4 aa87 ADD X10, X0, X15
:0 0x5b6
17250000 AUIPC $2, X10
:0 0x5ba
033525a3 MOV -1486(X10), X10
:0 0x5be 8265 MOV (X2), X11
:0 0x5c0 3000 ADDI $8, X2, X12
...
Fixes #71102
Change-Id: Ia99eb114a98c6d535de872ce8a526cd5e6203fff
Reviewed-on: https://go-review.googlesource.com/c/go/+/639995
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Jorropo <jorropo.pgm@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>