Re: [PATCH] parisc: Prevent using same register as soure and target in extru/shr

From: Sven Schnelle <svens@stackframe.org>
To: Helge Deller <deller@gmx.de>
Cc: linux-parisc@vger.kernel.org,
	James Bottomley <James.Bottomley@hansenpartnership.com>,
	John David Anglin <dave.anglin@bell.net>
Subject: Re: [PATCH] parisc: Prevent using same register as soure and target in extru/shr
Date: Wed, 18 May 2022 08:57:36 +0200	[thread overview]
Message-ID: <87zgjfl44f.fsf@x1.stackframe.org> (raw)
In-Reply-To: <8b5c308a-f3ea-e07f-053a-ad086ce75c43@gmx.de> (Helge Deller's message of "Tue, 17 May 2022 14:54:44 +0200")

Helge Deller <deller@gmx.de> writes:

> On 5/17/22 14:49, Helge Deller wrote:
>> In 2004 Randolph added the shr() assembly macro and noted that the
>> source and target register could not be the same.
>>
>> I did not find any confindence in the docs for this restriction. Maybe
>> it's related that on PA2.0 the upper bits may be clobbered?
>
> Looking at the generated kernel code from C-files, I'll find all over usages of
> 	extru source, x,y,  target
> where source and target are the same register.
> So, at least for 32-bit this restriction can't be true.

I did a quick objdump on the 64 bit HP-UX kernel and that one also uses
extrd/extrw where target and source are the same register. So i don't
think that restriction is true.