Re: [PATCH] x86, vsyscall: Fix build warning in vsyscall_64.c

[Ingo Molnar <mingo@elte.hu>]

* Andrew Lutomirski <luto@mit.edu> wrote:

> On Mon, Jun 13, 2011 at 5:29 AM, Ingo Molnar <mingo@elte.hu> wrote:
> >
> > * Andrew Lutomirski <luto@mit.edu> wrote:
> >
> >> On Sat, Jun 11, 2011 at 3:31 AM, Rakib Mullick <rakib.mullick@gmail.com> wrote:
> >> > Due to commit 5cec93c216db77 (x86-64: Emulate legacy vsyscalls), we get the following warning:
> >> >
> >> >   arch/x86/kernel/vsyscall_64.c: In function ‘do_emulate_vsyscall’:
> >> >   arch/x86/kernel/vsyscall_64.c:111:7: warning: ‘ret’ may be used uninitialized in this function
> >>
> >> What's the code path that uses ret without initializing it?
> >
> > If the code is correct but GCC got confused then please use the
> > simplest possible patch to help GCC find its way around the code.
> 
> The simplest patch is to mark ret as uninitialized_var.

No - that primitive really sucks as it might hide *future* debug 
warnings and silently break code.

The problem with uninitialized_var() is that such code:

	int test(void)
	{
		int uninitialized_var(ret);

		return ret;
	}

Builds without a single warning but it is very broken code.

So if we use uninitialized_var() and the code is changed in the 
future to have the above broken sequence, we'll have a silent runtime 
failure ...

So we try to avoid using uninitialized_var() in arch/x86/ and use 
explicit initialization instead.

That way GCC that can see through the flow will optimize away the 
superfluous initialization - GCC versions that are older will 
generate one more instruction but that's OK.

Thanks,

	Ingo