cmd/compile: optimise subtraction with const on riscv64

Convert subtraction from const to a negated ADDI with negative const
value, where possible. At worst this avoids a register load and uses
the same number of instructions. At best, this allows for further
optimisation to occur, particularly where equality is involved.

For example, this sequence:

   li      t0,-1
   sub     t1,t0,a0
   snez    t1,t1

Becomes:

   addi    t0,a0,1
   snez    t0,t0

Removes more than 2000 instructions from the Go binary on linux/riscv64.

Change-Id: I68f3be897bc645d4a8fa3ab3cef165a00a74df19
Reviewed-on: https://go-review.googlesource.com/c/go/+/426263
Reviewed-by: Meng Zhuo <mzh@golangcn.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Joel Sing <joel@sing.id.au>

0 files changed, 0 insertions, 0 deletions