Re: [PATCH v5 10/10] arm64: atomics: Use K constraint when toolchain appears to support it

[Andrew Murray <andrew.murray@arm.com>]

On Fri, Aug 30, 2019 at 11:40:53AM +0100, Mark Rutland wrote:
> On Fri, Aug 30, 2019 at 10:11:55AM +0100, Andrew Murray wrote:
> > On Fri, Aug 30, 2019 at 08:52:20AM +0100, Will Deacon wrote:
> > > On Fri, Aug 30, 2019 at 01:08:03AM +0100, Andrew Murray wrote:
> > > > On Thu, Aug 29, 2019 at 05:54:58PM +0100, Will Deacon wrote:
> > > > > On Thu, Aug 29, 2019 at 04:48:34PM +0100, Will Deacon wrote:
> > > > > > diff --git a/arch/arm64/include/asm/atomic_ll_sc.h b/arch/arm64/include/asm/atomic_ll_sc.h
> > > > > > index 95091f72228b..7fa042f5444e 100644
> > > > > > --- a/arch/arm64/include/asm/atomic_ll_sc.h
> > > > > > +++ b/arch/arm64/include/asm/atomic_ll_sc.h
> > > > > > @@ -23,6 +23,10 @@ asm_ops "\n"								\
> > > > > >  #define __LL_SC_FALLBACK(asm_ops) asm_ops
> > > > > >  #endif
> > 
> > I downloaded your original patches and tried them, and also got the
> > build error. After playing with this I think something isn't quite right...
> 
> Can you post the error you see?

Doh, it looks like I didn't apply the __stringify patches - this is why it
didn't work for me.

> 
> > This is your current test:
> > 
> >  echo 'int main(void) {asm volatile("and w0, w0, %w0" :: "K" (4294967295)); return 0; }' |  aarch64-linux-gnu-gcc -S -x c  - ; echo $?
> > 
> > But on my machine this returns 0, i.e. no error. 
> 
> IIUC that's expected, as this is testing if the compiler erroneously
> accepts the invalid immediate.
> 
> Note that try-run takes (option,option-ok,otherwise), so:
> 
> | cc_has_k_constraint := $(call try-run,echo                             \
> |        'int main(void) {                                               \
> |                asm volatile("and w0, w0, %w0" :: "K" (4294967295));    \
> |                return 0;                                               \
> |        }' | $(CC) -S -x c -o "$$TMP" -,,-DCONFIG_CC_HAS_K_CONSTRAINT=1)
> 
> ... means we do nothing when the compile is successful (i.e. when the compiler
> is broken), and we set -DCONFIG_CC_HAS_K_CONSTRAINT=1 when the compiler
> correctly rejects the invalid immediate.

Yes I see this now. I hadn't realised that the -S allows us to see what the
compiler does prior to assembling. Indeed this test verifies that the compiler
accepts an invalid value - and if so we don't permit use of the 'K' flag.

(I guess I was wrongly expecting the command to fail when we pass an invalid
value and thus expected the option-ok to be where we set the define.)

Thanks for the explanation!

Andrew Murray

> 
> If we drop the -S, we'll get an error in all cases, as either:
> 
> * GCC silently accepts the immediate, GAS aborts
> * GCC aborts as it can't satisfy the constraint
> 
> > > > > > +#ifndef CONFIG_CC_HAS_K_CONSTRAINT
> > > > > > +#define K
> > > > > > +#endif
> 
> Here we define K to nothing if the compiler accepts the broken immediate.
> 
> If the compiler rejects invalid immediates we don't define K to anything, so
> it's treated as a literal later on, and gets added as a constaint.
> 
> Thanks,
> Mark.
> 
> _______________________________________________
> linux-arm-kernel mailing list
> linux-arm-kernel@lists.infradead.org
> http://lists.infradead.org/mailman/listinfo/linux-arm-kernel

_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel