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

From: Mark Rutland <mark.rutland@arm.com>
To: Andrew Murray <andrew.murray@arm.com>
Cc: peterz@infradead.org, catalin.marinas@arm.com,
	ndesaulniers@google.com, robin.murphy@arm.com,
	Ard.Biesheuvel@arm.com, natechancellor@gmail.com,
	Will Deacon <will@kernel.org>,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v5 10/10] arm64: atomics: Use K constraint when toolchain appears to support it
Date: Fri, 30 Aug 2019 11:40:53 +0100	[thread overview]
Message-ID: <20190830104053.GA47586@lakrids.cambridge.arm.com> (raw)
In-Reply-To: <20190830091155.GS14582@e119886-lin.cambridge.arm.com>

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?

> 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.

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.

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