linux-kernel - Re: ftrace breaks sparc64 build

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Date:	Tue, 6 Jan 2009 13:52:36 -0500 (EST)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	David Miller <davem@...emloft.net>
cc:	sam@...nborg.org, linux-kernel@...r.kernel.org,
	srostedt@...hat.com, mingo@...e.hu, sparclinux@...r.kernel.org
Subject: Re: ftrace breaks sparc64 build


On Tue, 6 Jan 2009, David Miller wrote:

> From: Steven Rostedt <rostedt@...dmis.org>
> Date: Mon, 5 Jan 2009 15:05:11 -0500 (EST)
> 
> > Probably the same issue. The problem is that the first use of a variable 
> > is in the OR section of an if statement that does a return.
> > 
> > 	if (x || !(y = init_me())
> > 		return;
> > 
> > 	use_me(y);
> > 
> > IMHO I find this sloppy code. When reading the code it can cause reviewers 
> > trouble, and wasted time, to see that y is indeed initialized. I'm 
> > impressed that gcc was able to figure it out.
> 
> I'm pretty much in firm disagreement here.

fair enough.

> 
> This code is pretty clear.  The only way to get to use_me() is
> by initializing 'y'.  It's very straightforward.
> 
> There are many conditionals in the kernel where this order
> of evaluation and side effects is depended upon.  Some of them
> just happen to warn now because of the branch tracer.
> 
> > Have you always been compiling with -Werror?
> 
> arch/sparc*/ builds with -Werror for 5+ years.
> 
> > The reason that gcc complains is because you have the
> > "branch_tracer" on that converts 'if ()' into a macro (as you saw in
> > your -E compile). This makes the if statement more complex, and goes
> > beyond gcc's ability to know that the above 'y' is initialized
> > properly. I would work on fixing this in the branch tracer, but
> > honestly, I'm kind of glad that gcc barfs on it.  This will help us
> > point out this kind of sloppy initializations (sorry if I'm
> > offending anybody about calling it sloppy). I just believe that it
> > makes the code a bit more obfuscated to initialize in an if
> > statement, and a second part of a complex if statement at that!
> 
> Keep in mind all this code was fine and built warning free before the
> if() obfuscation done by the branch tracer.  If I wrote the branch
> tracer, I'd probably search for these kinds of excuses too :-)

Yeah, I'm trying different ways to handle the if magic to see if I can 
come up with a way to keep gcc happy. Unfortunately, every solution so far 
still seems to confuse sparc (I don't know why it does not confuse x86 or 
powerpc, although some of my variations do).

I would hate to black list archs just because it gives warnings. I've 
analyzed the code greatly to make sure that it keeps the semantics of the 
original code the same.

It's just that it pushes gcc beyond its threshold of knowing that a 
variable is initialized. I do not blame gcc because it has to have a limit 
to handle logic cases (otherwise it has solved the halting problem).

What bothers me is that some versions of gcc are not affected by the if 
macro and some are. I'm impressed to hear that sparc has been compiling 
fine for years with -Werror since I'm not sure other archs can say the 
same.

I'll have to keep hacking at it. :-/

-- Steve

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