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Date: Tue, 1 Jun 2021 09:02:54 -0500
From: Segher Boessenkool <segher@kernel.crashing.org>
To: Hongtao Liu <crazylht@gmail.com>
Cc: Andrew Pinski <pinskia@gmail.com>, GCC Patches <gcc-patches@gcc.gnu.org>
Subject: Re: [PATCH][i386] Split not+broadcast+pand to broadcast+pandn.
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> 	PR target/100711
> 	* simplify-rtx.c (simplify_unary_operation_1):
> 	Simplify (vec_duplicate (not op)) to (not (vec_duplicate op)).

This is not a simplification.  If we want to do this we need to document
this canonicalisation (in md.texi, "Insn Canonicalizations").

> +    /* Prefer (not (vec_duplicate (nonimmedaite_operand)))
> +       to (vec_duplicate (not (nonimmedaite_operand))).  */

What Andrew said here (also, it's misspelled :-) )

> +    case VEC_DUPLICATE:
> +      if (GET_CODE (op) == NOT)
> +	return gen_rtx_NOT (mode, gen_rtx_VEC_DUPLICATE (mode, XEXP (op, 0)));
> +      break;

If it isn't a canonicalisation you need to simplify the result, and then
only do it if it does in fact simplify.  You risk "simplification" loops
if you don't.


Segher