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

[Saravana Kannan]

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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