linux-kernel - Re: bit fields && data tearing

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Date:	Fri, 05 Sep 2014 12:17:16 -0400
From:	Peter Hurley <peter@...leysoftware.com>
To:	David Laight <David.Laight@...LAB.COM>,
	"'paulmck@...ux.vnet.ibm.com'" <paulmck@...ux.vnet.ibm.com>
CC:	Jakub Jelinek <jakub@...hat.com>,
	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"linux-ia64@...r.kernel.org" <linux-ia64@...r.kernel.org>,
	Mikael Pettersson <mikpelinux@...il.com>,
	Oleg Nesterov <oleg@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	James Bottomley <James.Bottomley@...senPartnership.com>,
	Tony Luck <tony.luck@...el.com>,
	Paul Mackerras <paulus@...ba.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	"linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
	Miroslav Franc <mfranc@...hat.com>,
	Richard Henderson <rth@...ddle.net>,
	"linux-arm@...r.kernel.org" <linux-arm@...r.kernel.org>
Subject: Re: bit fields && data tearing

On 09/05/2014 08:37 AM, David Laight wrote:
> From: Peter Hurley
>> On 09/05/2014 04:30 AM, David Laight wrote:
>>> I've seen gcc generate 32bit accesses for 16bit structure members on arm.
>>> It does this because of the more limited range of the offsets for the 16bit access.
>>> OTOH I don't know if it ever did this for writes - so it may be moot.
>>
>> Can you recall the particulars, like what ARM config or what code?
>>
>> I tried an overly-simple test to see if gcc would bump up to the word load for
>> the 12-bit offset mode, but it stuck with register offset rather than immediate
>> offset. [I used the compiler options for allmodconfig and a 4.8 cross-compiler.]
>>
>> Maybe the test doesn't generate enough register pressure on the compiler?
> 
> Dunno, I would have been using a much older version of the compiler.
> It is possible that it doesn't do it any more.
> It might only have done it for loads.
> 
> The compiler used to use misaligned 32bit loads for structure
> members on large 4n+2 byte boundaries as well.
> I'm pretty sure it doesn't do that either.
> 
> There have been a lot of compiler versions since I was compiling
> anything for arm.

Yeah, it seems gcc for ARM no longer uses the larger operand size as a
substitute for 12-bit immediate offset addressing mode, even for reads.

While this test:

struct x {
	short b[12];
};

short load_b(struct x *p) {
	return p->b[8];
}

generates the 8-bit immediate offset form,

short load_b(struct x *p) {
   0:	e1d001f0 	ldrsh	r0, [r0, #16]
   4:	e12fff1e 	bx	lr


pushing the offset out past 256:

struct x {
	long unused[64];
	short b[12];
};

short load_b(struct x *p) {
	return p->b[8];
}

generates the register offset addressing mode instead of 12-bit immediate:

short load_b(struct x *p) {
   0:	e3a03e11 	mov	r3, #272	; 0x110
   4:	e19000f3 	ldrsh	r0, [r0, r3]
   8:	e12fff1e 	bx	lr

Regards,
Peter Hurley

[Note: I compiled without the frame pointer to simplify the code generation]
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