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From: Richard Sandiford <richard.sandiford@arm.com>
To: =?utf-8?B?6ZKf5bGF5ZOy?= <juzhe.zhong@rivai.ai>
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Cc: gcc-patches <gcc-patches@gcc.gnu.org>,  rguenther <rguenther@suse.de>
Subject: Re: [PATCH V14] VECT: Add decrement IV iteration loop control by variable amount support
References: <20230524144801.73537-1-juzhe.zhong@rivai.ai>
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Date: Wed, 24 May 2023 16:47:06 +0100
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=E9=92=9F=E5=B1=85=E5=93=B2 <juzhe.zhong@rivai.ai> writes:
> Hi, Richard. I still don't understand it. Sorry about that.
>
>>>  loop_len_48 =3D MIN_EXPR <loop_len_34 * 2, 4>;
>   >>   _74 =3D 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 =3D 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 al=
ready have
> execution check and passed. I am not sure whether this patch has the issu=
e that I didn't notice.

Think about the maximum values of each SSA name:

   loop_len_34 =3D MIN_EXPR <ivtmp_72, 8>;       // MAX 8
   loop_len_48 =3D MIN_EXPR <loop_len_34, 4>;    // MAX 4
   _74 =3D loop_len_34 - loop_len_48;            // MAX 4
   loop_len_49 =3D MIN_EXPR <_74, 4>;            // MAX 4 (always =3D=3D _7=
4)
   _75 =3D _74 - loop_len_49;                    // 0
   loop_len_50 =3D MIN_EXPR <_75, 4>;            // 0
   loop_len_51 =3D _75 - loop_len_50;            // 0

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

Thanks,
Richard