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>