Re: [PATCH V14] VECT: Add decrement IV iteration loop control by variable amount support

[Richard Sandiford <richard.sandiford@arm.com>]

钟居哲 <juzhe.zhong@rivai.ai> writes:
> Hi, Richard. I still don't understand it. Sorry about that.
>
>>>  loop_len_48 = MIN_EXPR <loop_len_34 * 2, 4>;
>   >>   _74 = loop_len_34 * 2 - loop_len_48;
>
> I have the tests already tested.
> We have a MIN_EXPR to calculate the total elements:
> loop_len_34 = MIN_EXPR <ivtmp_72, 8>;
> I think "8" is already multiplied by 2?
>
> Why do we need loop_len_34 * 2 ?
> Could you give me more informations, The similiar tests you present we already have
> execution check and passed. I am not sure whether this patch has the issue that I didn't notice.

Think about the maximum values of each SSA name:

   loop_len_34 = MIN_EXPR <ivtmp_72, 8>;       // MAX 8
   loop_len_48 = MIN_EXPR <loop_len_34, 4>;    // MAX 4
   _74 = loop_len_34 - loop_len_48;            // MAX 4
   loop_len_49 = MIN_EXPR <_74, 4>;            // MAX 4 (always == _74)
   _75 = _74 - loop_len_49;                    // 0
   loop_len_50 = MIN_EXPR <_75, 4>;            // 0
   loop_len_51 = _75 - loop_len_50;            // 0

So the final two y vectors will always have 0 controls.

Thanks,
Richard