linux-kernel - RE: [RFC 02/12] openrisc: always use unaligned-struct header

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Date:   Sat, 8 May 2021 11:42:13 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Arnd Bergmann' <arnd@...nel.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>
CC:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Vineet Gupta <vgupta@...opsys.com>,
        Arnd Bergmann <arnd@...db.de>, Jonas Bonn <jonas@...thpole.se>,
        Stefan Kristiansson <stefan.kristiansson@...nalahti.fi>,
        Stafford Horne <shorne@...il.com>,
        "openrisc@...ts.librecores.org" <openrisc@...ts.librecores.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [RFC 02/12] openrisc: always use unaligned-struct header

From: Arnd Bergmann
> Sent: 07 May 2021 23:08
...
> I don't know how the loads/store perform compared to the shift version
> on a particular microarchitecture, but my guess is that the shifts
> are better.

What does the nios use?
Shifts generate reasonable code for put_unaligned() but
they get horrid for get_unaligned().

On the nios writing the 4 bytes to memory and reading back
a 32bit value should generate shorter faster code.
You do need to generate 4 byte loads, 4 bytes stores, 32bit load.
(The load will cause a stall if the data is needed for one
of the next two instructions, and there is a (undocumented)
stall between a write and read to the same memory area.
The shift version requires 3 shifts and 3 ors - but I think
gcc makes a bigger pig's breakfast of it.)

OTOH I'm not sure anyone in their right mind would run Linux on nios.
It is a soft cpu for the altera (now intel) fpgas.
We use them with 4k code and sub 64k data for real time processing.

	David

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