Re: [PATCH v4 1/5] target/riscv: add vector unit stride load and store instructions

[Richard Henderson]

On 2/27/20 5:50 PM, LIU Zhiwei wrote:
>> This is not what I had in mind, and looks wrong as well.
>>
>>     int idx = (index * mlen) / 64;
>>     int pos = (index * mlen) % 64;
>>     return (((uint64_t *)v0)[idx] >> pos) & 1;
>>
>> You also might consider passing log2(mlen), so the multiplication could be
>> strength-reduced to a shift.
> I don't think so. For example, when mlen is 8 bits and index is 0, it will
> reduce to
> 
> return (((uint64_t *)v0)[0]) & 1
> 
> And it's not right.
> 
> The right bit is first bit in vector register 0. And in host big endianess,
> it will be  the first bit of the seventh byte.

You've forgotten that we've just done an 8-byte big-endian load, which means
that we *are* looking at the first bit of the byte at offset 7.

It is right.

>> You don't need to pass mlen, since it's
> Yes.

I finally remembered all of the bits that go into mlen and thought I had
deleted that sentence -- apparently I only removed half.  ;-)


r~