cmd/compile: slicing can be improved on ARM

[rasky]

This code:

func slice(s []int64, idx int) []int {
    return s[idx:]
}

generates this code on ARM:

CMP R0, R7           // bound check
B.GE panic
SUB R7, R0, R0       // len
SUB R7, R4, R4       // cap
RSB $0, R4, R9       // mask(cap)
ASR $31, R9, R9      // mask(cap)
AND R7<<$3, R9, R9   // delta&mask(cap)
ADD R5, R9, R5       // ptr+delta&mask(cap)

but it could be optimized this way:

SUBS R7, R0, R0
B.LS panic
SUBS R7, R4, R4
ADD.NE R7<<$3, R9, R5

which is much shorter and faster.

[rasky]

Moving the bound check into the slice operation sounds hard to me, but the other part should probably be easier. Not sure if it's better to special-case the code generation within (*state).slice() or trying to do the magic as a rule.

/cc @cherrymui @benshi001

[benshi001]

Currently code is frozen, and only bug fix is allowed. We can try the optimization in go1.11.

[rasky]

@benshi001 did you try this optimization? I think it can be beneficial for slides on Arm

[benshi001]

@rasky

I have tried to optimize arm code with ADD.S/SUB.S, if both the flags and the result are further used. But unfortunately the go1 benchmark shows some regression, which I need more tuning work.