linux-kernel - Re: linux-next: kvm tree build warning

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Date:	Wed, 20 Jan 2010 11:42:28 +0100
From:	Alexander Graf <agraf@...e.de>
To:	Stephen Rothwell <sfr@...b.auug.org.au>
Cc:	Avi Kivity <avi@...ranet.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	linux-next@...r.kernel.org, linux-kernel@...r.kernel.org,
	ppc-dev <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: linux-next: kvm tree build warning


On 20.01.2010, at 07:48, Stephen Rothwell wrote:

> [Not sure what happened to the previous versions To/CC lists, sorry]
> 
> Hi all,
> 
> Today's linux-next build (powerpc allyesconfig) produced this warning:
> 
> arch/powerpc/kvm/book3s.c: In function '__kvmppc_vcpu_run':
> arch/powerpc/kvm/book3s.c:1102: warning: 'ext_bkp.vrsave' may be used uninitialized in this function
> 
> Introduced by commit 2609c1d06b6a7952273fb33b33ad32aff0628146 ("KVM: PPC:
> Add support for FPU/Altivec/VSX").  This *may* be a false positive but
> will be a problem for normal builds where warnings are treated as errors
> in arch/powerpc.


This is odd. The same function saves and restores ext_bkp.vrsave under a common condition:

        if (save_vec) {
                if (current->thread.regs->msr & MSR_VEC)
                        giveup_altivec(current);
                memcpy(ext_bkp.vr, current->thread.vr, sizeof(ext_bkp.vr));
                ext_bkp.vscr = current->thread.vscr;
                ext_bkp.vrsave = current->thread.vrsave;
        }


[...]

        if (save_vec && current->thread.used_vr) {
                memcpy(current->thread.vr, ext_bkp.vr, sizeof(ext_bkp.vr));
                current->thread.vscr = ext_bkp.vscr;
                current->thread.vrsave = ext_bkp.vrsave;
        }


So there can't be a case where ext_bkp.vrsave is read, but not written to before, which is what this warning implies, right?
Also, why doesn't gcc complain about vscr too then? They're both used in the very same if statement.

Additionally, while compiling locally I looked for warnings and didn't spot any. So apparently our compiler versions / options don't match. Which version of gcc are you using? I assume a gcc bug.

$ gcc --version
gcc (SUSE Linux) 4.3.2 [gcc-4_3-branch revision 141291]


Alex--
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