linux-kernel - RE: [RFC 12/12] asm-generic: simplify asm/unaligned.h

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Message-ID: <0b599cc80612436bb8d688fa2ad1dc34@AcuMS.aculab.com>
Date:   Sat, 8 May 2021 11:03:20 +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>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [RFC 12/12] asm-generic: simplify asm/unaligned.h

From: Arnd Bergmann
> Sent: 07 May 2021 23:08
> 
> The get_unaligned()/put_unaligned() implementations are much more complex
> than necessary, now that all architectures use the same code.
> 
...
> +#define __get_unaligned_t(type, ptr) ({						\
> +	const struct { type x; } __packed *__pptr = (typeof(__pptr))(ptr);	\
> +	__pptr->x; 								\
> +})

I thought gcc was likely to track through the alignment of the
variable holding the source pointer (through any (void *) casts
implied by inlined function calls) through to the pointer used
for the actual access - so it would tend to issue a single
instruction that assumed an aligned address.

I know that has caused grief trying to copy unaligned data
to an aligned structure.

Possibly adding:
	asm ("" :: "+r" (__pptr)) );
in the middle stops that assumption without generating any code.

	David

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