linux-kernel - Re: [PATCH 2/4] msm: scm: Fix improper register assignment

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Date:	Tue, 01 Mar 2011 13:29:14 -0800
From:	Saravana Kannan <skannan@...eaurora.org>
To:	Will Deacon <will.deacon@....com>
CC:	Nicolas Pitre <nico@...xnic.net>,
	David Brown <davidb@...eaurora.org>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	linux-arm-msm@...r.kernel.org, Stephen Boyd <sboyd@...eaurora.org>,
	lkml <linux-kernel@...r.kernel.org>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 2/4] msm: scm: Fix improper register assignment

On 03/01/2011 02:37 AM, Will Deacon wrote:
> Hi Nicolas,
>
> On Sat, 2011-02-26 at 20:04 +0000, Nicolas Pitre wrote:
>> Right.  A minimal test case may look like this if someone feels like
>> filling a gcc bug report:
>>
>> extern int foo(int x);
>>
>> int bar(int x)
>> {
>>          register int a asm("r0") = 1;
>>          x = foo(x);
>>          asm ("add %0, %1, %2" : "=r" (x) : "r" (a), "r" (x));
>>          return x;
>> }
>>
>> And the produced code is:
>>
>> bar:
>>          stmfd   sp!, {r3, lr}
>>          bl      foo
>> #APP
>>          add r0, r0, r0
>>          ldmfd   sp!, {r3, pc}
>>
>> So this is clearly bogus.
>>
>
> I agree that this is wrong, but the compiler people may try and argue
> the other way. I'll ask some of the compiler guys at ARM and see what
> they think.

Nicolas and Will,

Thanks for the sample bug code and thanks for checking with the compiler 
guys and validating (in another thread) that this is indeed a bug in 
GCC. Glad to know we weren't doing something stupid.

>>> In any case, fortunately it works with the fix.
>>
>> Please add a comment in your patch to explain the issue.
>>
>
> Perhaps a more robust fix would be to remove the register int
> declarations and handle the parameter marshalling in the same asm block
> that contains the smc?

I was thinking the same, but the opposing idea I heard was that not 
doing it inside the asm block would allow GCC to be make better use of 
the registers. Didn't have a strong opinion either way, so we went with 
the implementation that was sent out.

Thanks,
Saravana
-- 
Sent by an employee of the Qualcomm Innovation Center, Inc.
The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum.
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