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From: Richard Sandiford <richard.sandiford@arm.com>
To: Prathamesh Kulkarni <prathamesh.kulkarni@linaro.org>
Mail-Followup-To: Prathamesh Kulkarni <prathamesh.kulkarni@linaro.org>,gcc Patches <gcc-patches@gcc.gnu.org>,  Richard Biener <richard.guenther@gmail.com>, richard.sandiford@arm.com
Cc: gcc Patches <gcc-patches@gcc.gnu.org>,  Richard Biener <richard.guenther@gmail.com>
Subject: Re: Extend fold_vec_perm to fold VEC_PERM_EXPR in VLA manner
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Date: Fri, 30 Sep 2022 17:08:10 +0100
In-Reply-To: <mptr0zsn98c.fsf@arm.com> (Richard Sandiford via Gcc-patches's
	message of "Fri, 30 Sep 2022 17:00:03 +0100")
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Richard Sandiford via Gcc-patches <gcc-patches@gcc.gnu.org> writes:
> Prathamesh Kulkarni <prathamesh.kulkarni@linaro.org> writes:
>> Sorry to ask a silly question but in which case shall we select 2nd vector ?
>> For num_poly_int_coeffs == 2,
>> a1 /trunc n1 == (a1 + 0x) / (n1.coeffs[0] + n1.coeffs[1]*x)
>> If a1/trunc n1 succeeds,
>> 0 / n1.coeffs[1] == a1/n1.coeffs[0] == 0.
>> So, a1 has to be < n1.coeffs[0] ?
>
> Remember that a1 is itself a poly_int.  It's not necessarily a constant.
>
> E.g. the TRN1 .D instruction maps to a VEC_PERM_EXPR with the selector:
>
>   { 0, 2 + 2x, 1, 4 + 2x, 2, 6 + 2x, ... }

Sorry, should have been:

  { 0, 2 + 2x, 2, 4 + 2x, 4, 6 + 2x, ... }

> which is an interleaving of the two patterns:
>
>   { 0, 2, 4, ... }                  a0 = 0, a1 = 2, S = 2
>   { 2 + 2x, 4 + 2x, 6 + 2x }        a0 = 2 + 2x, a1 = 4 + 2x, S = 2