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Re: RFC: using clz for comparing to zero on the tree level.
Richard Henderson wrote:
> On Thu, May 08, 2003 at 12:36:35PM -0400, David Edelsohn wrote:
>> Not Alpha and not ARM V5, which have single instruction clz?
>
> [...]
>
> ARM:
> cmp in, #0
> movne out, #0
> moveq out, #1
>
> Hmm, so it might be a win for ARM. You'll have to work out
> with Richard Earnshaw how you want to arrange these costs.
> At the moment arm_rtx_costs doesn't tell you that the SEQ
> operation takes 3 instructions.
This isn't directly relevant to the clz issue, but I notice that the
above sequence isn't optimal for any ARM - this is better:
rsbs out, in, #1
movcc out, #0
This is essentially: out = 1 - in; if ((unsigned)in > 1) out = 0;
Cheers,
Olly