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From: David Laight <David.Laight@ACULAB.COM>
To: 'Arnd Bergmann' <arnd@kernel.org>,
	"linux-arch@vger.kernel.org" <linux-arch@vger.kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	Vineet Gupta <vgupta@synopsys.com>, Arnd Bergmann <arnd@arndb.de>,
	Jonas Bonn <jonas@southpole.se>,
	Stefan Kristiansson <stefan.kristiansson@saunalahti.fi>,
	Stafford Horne <shorne@gmail.com>,
	"openrisc@lists.librecores.org" <openrisc@lists.librecores.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: RE: [RFC 02/12] openrisc: always use unaligned-struct header
Date: Sat, 8 May 2021 11:42:13 +0000	[thread overview]
Message-ID: <7f2ca7e366a444108932c4c3bb95c2f9@AcuMS.aculab.com> (raw)
In-Reply-To: <20210507220813.365382-3-arnd@kernel.org>

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

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)


WARNING: multiple messages have this Message-ID (diff)
From: David Laight <David.Laight@ACULAB.COM>
To: openrisc@lists.librecores.org
Subject: [OpenRISC] [RFC 02/12] openrisc: always use unaligned-struct header
Date: Sat, 8 May 2021 11:42:13 +0000	[thread overview]
Message-ID: <7f2ca7e366a444108932c4c3bb95c2f9@AcuMS.aculab.com> (raw)
In-Reply-To: <20210507220813.365382-3-arnd@kernel.org>

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

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)


  parent reply	other threads:[~2021-05-08 11:42 UTC|newest]

Thread overview: 50+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-05-07 22:07 [RFC 0/12] Unify asm/unaligned.h around struct helper Arnd Bergmann
2021-05-07 22:07 ` [OpenRISC] " Arnd Bergmann
2021-05-07 22:07 ` Arnd Bergmann
2021-05-07 22:07 ` Arnd Bergmann
2021-05-07 22:07 ` [RFC 01/12] asm-generic: use asm-generic/unaligned.h for most architectures Arnd Bergmann
2021-05-07 22:07   ` Arnd Bergmann
2021-05-07 22:07   ` Arnd Bergmann
2021-05-07 23:02   ` Thomas Gleixner
2021-05-07 23:02     ` Thomas Gleixner
2021-05-07 23:02     ` Thomas Gleixner
2021-05-10 10:16   ` Geert Uytterhoeven
2021-05-10 10:16     ` Geert Uytterhoeven
2021-05-10 10:16     ` Geert Uytterhoeven
2021-05-10 13:12     ` Arnd Bergmann
2021-05-10 13:12       ` Arnd Bergmann
2021-05-10 13:12       ` Arnd Bergmann
2021-05-07 22:07 ` [RFC 02/12] openrisc: always use unaligned-struct header Arnd Bergmann
2021-05-07 22:07   ` [OpenRISC] " Arnd Bergmann
2021-05-07 23:02   ` Stafford Horne
2021-05-07 23:02     ` [OpenRISC] " Stafford Horne
2021-05-08 11:42   ` David Laight [this message]
2021-05-08 11:42     ` David Laight
2021-05-07 22:07 ` [RFC 03/12] sh: remove unaligned access for sh4a Arnd Bergmann
2021-05-10 21:11   ` Rob Landley
2021-05-07 22:07 ` [RFC 04/12] m68k: select CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS Arnd Bergmann
2021-05-10 10:18   ` Geert Uytterhoeven
2021-05-07 22:07 ` [RFC 05/12] powerpc: use linux/unaligned/le_struct.h on LE power7 Arnd Bergmann
2021-05-07 22:07   ` Arnd Bergmann
2021-05-07 22:07 ` [RFC 06/12] asm-generic: unaligned: remove byteshift helpers Arnd Bergmann
2021-05-07 22:07   ` Arnd Bergmann
2021-05-08 11:38   ` Arnd Bergmann
2021-05-08 11:38     ` Arnd Bergmann
2021-05-07 22:07 ` [RFC 07/12] asm-generic: unaligned always use struct helpers Arnd Bergmann
2021-05-07 22:07   ` Arnd Bergmann
2021-05-07 22:07 ` [RFC 08/12] partitions: msdos: fix one-byte get_unaligned() Arnd Bergmann
2021-05-07 22:07 ` [RFC 09/12] apparmor: use get_unaligned() only for multi-byte words Arnd Bergmann
2021-05-10  8:17   ` John Johansen
2021-05-07 22:07 ` [RFC 10/12] mwifiex: re-fix for unaligned accesses Arnd Bergmann
2021-05-07 22:07 ` [RFC 11/12] netpoll: avoid put_unaligned() on single character Arnd Bergmann
2021-05-07 22:07 ` [RFC 12/12] asm-generic: simplify asm/unaligned.h Arnd Bergmann
2021-05-07 23:54   ` Linus Torvalds
2021-05-08  9:28     ` Arnd Bergmann
2021-05-08 15:23       ` Linus Torvalds
2021-05-08 11:03   ` David Laight
2021-05-08 14:18     ` David Laight
2021-05-10  6:39   ` Geert Uytterhoeven
2021-05-07 22:07 ` [RFC 0/12] Unify asm/unaligned.h around struct helper Arnd Bergmann
2021-05-07 22:07   ` [OpenRISC] " Arnd Bergmann
2021-05-07 22:07   ` Arnd Bergmann
2021-05-07 22:07   ` Arnd Bergmann

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