linux-kernel - Re: [PATCH v1 1/2] powerpc/bitops: Use immediate operand when possible

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Message-ID: <20210412215428.GM26583@gate.crashing.org>
Date:   Mon, 12 Apr 2021 16:54:28 -0500
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Christophe Leroy <christophe.leroy@...roup.eu>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 1/2] powerpc/bitops: Use immediate operand when possible

Hi!

On Thu, Apr 08, 2021 at 03:33:44PM +0000, Christophe Leroy wrote:
> For clear bits, on 32 bits 'rlwinm' can be used instead or 'andc' for
> when all bits to be cleared are consecutive.

Also on 64-bits, as long as both the top and bottom bits are in the low
32-bit half (for 32 bit mode, it can wrap as well).

> For the time being only
> handle the single bit case, which we detect by checking whether the
> mask is a power of two.

You could look at rs6000_is_valid_mask in GCC:
  <https://gcc.gnu.org/git/?p=gcc.git;a=blob;f=gcc/config/rs6000/rs6000.c;h=48b8efd732b251c059628096314848305deb0c0b;hb=HEAD#l11148>
used by rs6000_is_valid_and_mask immediately after it.  You probably
want to allow only rlwinm in your case, and please note this checks if
something is a valid mask, not the inverse of a valid mask (as you
want here).

So yes this is pretty involved :-)

Your patch looks good btw.  But please use "n", not "i", as constraint?

Segher