Why does the ARM provide a reverse subtract instruction RSB r0,r1,r2, that implements [r0] = [r2] ? [r1], when the normal subtraction instruction SUB r0,r2,r1, will do exactly the same job?
What will be an ideal response?
In principle, you don’t need a reverse subtract and most other processors don’t provide it. However, it is useful with literal operands. SUB r0,r1,#4 is a typical subtraction with a literal that allows you to calculate [r0] = [r1] ? 4. However, RSB r0,r1,#4 lets you calculate r0 = 4 ? [r1]. This saves you an instruction swapping the literal round.
You can also use RSB to negate a number; for example RSB r0,r0,#0 calculates [r0] = 0 ? [r0] = ?[r0].
Moreover, because ARM has a shifter in only one operand path, the reverse subtraction makes it possible to shift either operand. For example, SUB r0,r1,LSL #2 and RSB r1,r0,LSL#2.
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