Re: [PATCH] arm64: perf: Simplify the ARMv8 PMUv3 event attributes

From: Will Deacon <will@kernel.org>
To: Mark Rutland <mark.rutland@arm.com>
Cc: Shaokun Zhang <zhangshaokun@hisilicon.com>,
	Robin Murphy <robin.murphy@arm.com>,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH] arm64: perf: Simplify the ARMv8 PMUv3 event attributes
Date: Fri, 1 Nov 2019 16:05:39 +0000	[thread overview]
Message-ID: <20191101160538.GA3603@willie-the-truck> (raw)
In-Reply-To: <20191101143602.GB13020@blommer>

On Fri, Nov 01, 2019 at 02:36:03PM +0000, Mark Rutland wrote:
> On Fri, Nov 01, 2019 at 11:11:49AM +0000, Robin Murphy wrote:
> > On 2019-11-01 10:55 am, Will Deacon wrote:
> > > On Fri, Nov 01, 2019 at 10:54:21AM +0000, Robin Murphy wrote:
> > > > On 2019-11-01 10:36 am, Will Deacon wrote:
> > > > > On Fri, Nov 01, 2019 at 08:53:19AM +0000, Mark Rutland wrote:
> > > > > > You need want &(obj.attr.attr) rather than &(obj).attr.attr, i.e.
> > > > > > 
> > > > > > #define ARMV8_EVENT_ATTR(name, config) \
> > > > > > 	(&((struct perf_pmu_events_attr) { \
> > > > > > 		.attr = __ATTR(name, 0444, armv8pmu_events_sysfs_show, NULL), \
> > > > > > 		.id = config, \
> > > > > > 	}.attr.attr))
> > > > > > ... which compiles for me.
> > > > > 
> > > > > Weird, the following compiles fine for me with both GCC and clang:
> > > > > 
> > > > > #define ARMV8_EVENT_ATTR(name, config)						\
> > > > > 	(&((struct perf_pmu_events_attr) {					\
> > > > > 		.attr = __ATTR(name, 0444, armv8pmu_events_sysfs_show, NULL),	\
> > > > > 		.id = config,							\
> > > > > 	}).attr.attr)
> > > > 
> > > > You know that the expressions are equivalent because unary "&" has lower
> > > > precedence than ".", right? ;)
> > > 
> > > Right, which is why it's weird that Shaokun claims that the version I posted
> > > doesn't compile. I assume it didn't build for Mark either, hence his extra
> > > brackets.
> 
> I must've meessed up locally -- sorry for the noise.
> 
> > Because different compilers have different ideas of whether "obj" is a valid
> > thing to dereference at all, regardless of where you put parentheses. From
> > what I remember, the array trick was the only way to convince older GCCs to
> > treat the floating struct initialiser as an actual object definition. I
> > guess newer versions are a bit more lenient.
> 
> I strongly suspect Will's (much cleaner) version would work with those older
> compilers too, and I just didn't know what I was doing ~8 years ago when I came
> up with the trick.
> 
> I can have a go with my toolchain museum on Monday; if old GCCs are happy we
> can clean up the other instances of the trick to be much more legible.

I've thrown it into -next to see how it gets on.

Will

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