Re: [RFC PATCH v2 30/44] target/loongarch: Implement vclo vclz

[Richard Henderson]

On 4/7/23 00:40, gaosong wrote:
> 在 2023/4/2 上午11:34, Richard Henderson 写道:
> > On 3/27/23 20:06, Song Gao wrote:
> > > +#define DO_CLO_B(N)  (clz32((uint8_t)~N) - 24)
> > > +#define DO_CLO_H(N)  (clz32((uint16_t)~N) - 16)
> >
> > I think this is wrong. 
> It is wried,  the result is always right. :-\
> and  (clz32(~N) - 24)  or (clz32((uint32_t)~N) - 24) is wrong.
> > You *want* the high bits to be set, so that they are ones, and included in the count, 
> > which you then subtract off.  You want the "real" count to start after the 24th leading 1.
> Yes,
> and  we use clz32(),   how about the following way?
>
> #define DO_CLO_B(N)  (clz32( ~N & 0xff) -24)
> #define DO_CLO_H(N)  (clz32( ~N & 0xffff) -16)

Ah yes, I see.  My mistake.  Either old or new formulation is fine.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>


r~