Message-ID: <20131127113321.67f34ee6@gandalf.local.home>
Date:	Wed, 27 Nov 2013 11:33:21 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Juri Lelli <juri.lelli@...il.com>, tglx@...utronix.de,
	mingo@...hat.com, oleg@...hat.com, fweisbec@...il.com,
	darren@...art.com, johan.eker@...csson.com, p.faure@...tech.ch,
	linux-kernel@...r.kernel.org, claudio@...dence.eu.com,
	michael@...rulasolutions.com, fchecconi@...il.com,
	tommaso.cucinotta@...up.it, nicola.manica@...i.unitn.it,
	luca.abeni@...tn.it, dhaval.giani@...il.com, hgu1972@...il.com,
	paulmck@...ux.vnet.ibm.com, raistlin@...ux.it,
	insop.song@...il.com, liming.wang@...driver.com, jkacur@...hat.com,
	harald.gustafsson@...csson.com, vincent.guittot@...aro.org,
	bruce.ashfield@...driver.com
Subject: Re: [PATCH 08/14] sched: add latency tracing for -deadline tasks.

On Wed, 27 Nov 2013 17:13:08 +0100
Ingo Molnar <mingo@...nel.org> wrote:

> 
> * Steven Rostedt <rostedt@...dmis.org> wrote:
> 
> > On Wed, 27 Nov 2013 16:46:00 +0100
> > Ingo Molnar <mingo@...nel.org> wrote:
> > 
> > > 
> > > * Peter Zijlstra <peterz@...radead.org> wrote:
> > > 
> > > > On Wed, Nov 27, 2013 at 04:35:19PM +0100, Ingo Molnar wrote:
> > > > > So why does GCC then behave like this:
> > > > 
> > > > I think because its a much saner behaviour; also it might still be the
> > > > spec actually says this, its a somewhat opaque text.
> > > > 
> > > > Anyway, yes GCC seems to behave as we 'expect' it to; I just can't find
> > > > the language spec actually guaranteeing this.
> > > 
> > > So from C99 standard §6.7.8 (Initialization)/21:
> > > 
> > >     "If there are fewer initializers in a brace-enclosed list than 
> > >   there are elements or members of an aggregate, or fewer characters 
> > >   in a string literal used to initialize an array of known size than 
> > >   there are elements in the array, the remainder of the aggregate 
> > >   shall be initialized implicitly the same as objects that have static 
> > >   storage duration."
> > > 
> > > static initialization == zeroing in this case.
> > > 
> > 
> > The confusion here is that the above looks to be talking about arrays.
> > But it really doesn't specify structures.
> 
> It talks about neither 'arrays' nor 'structures', it talks about 
> 'aggregates' - which is defined as _both_: 'structures and arrays'.

Yeah, I misread it. I was reading the array section for awhile, and got
confused.


> 
> That's what compiler legalese brings you ;-)

Yep.

> 
> > But searching the internet, it looks as though most people believe 
> > it applies to structures, and any compiler that does otherwise will 
> > most likely break applications.
> > 
> > That is, this looks to be one of the gray areas that the compiler 
> > writers just happen to do what's most sane. And they probably assume 
> > it's talking about structures as well, hence the lack of warnings.
> 
> I don't think it's grey, I think it's pretty well specified.
> 
> > It gets confusing, as the doc also shows:
> > 
> > struct { int a[3], b; } w[] = { { 1 }, 2 };
> 
> I don't think this is valid syntax, I think this needs one more set of 
> braces:
> 
>  struct { int a[3], b; } w[] = { { { 1 }, 2 } };
> 
> > Then points out that w.a[0] is 1 and w.b[0] is 2, and all other 
> > elements are zero.
> 
> If by 'w.a[0]' you mean 'w[0].a[0]', and if by 'w.b[0]' you mean 
> 'w[0].b' then yes, this comes from the definition and it's what I'd 
> call 'obvious' initialization behavior.
> 
> What makes it confusing to you?

Well, because it's mixing arrays and structures, and I was on the
misconception that the paragraph was talking about just arrays.

Can I just have my turkey now?

-- Steve
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