Re: annoying new gcc 4.6.0 warnings.

[Stratos Psomadakis <psomas@ece.ntua.gr>]

On 21/04/2011 10:21 μμ, Dave Jones wrote:
> gcc 4.6.0 enables a new warning by default, which spews ~3000 lines
> of extra warnings to my build. The warning looks like ..
>
> arch/x86/kernel/cpu/cpufreq/powernow-k8.c: In function ‘pending_bit_stuck’:
> arch/x86/kernel/cpu/cpufreq/powernow-k8.c:108:10: warning: variable ‘hi’ set but not used [-Wunused-but-set-variable]
> arch/x86/kernel/cpu/cpufreq/powernow-k8.c: In function ‘core_voltage_pre_transition’:
> arch/x86/kernel/cpu/cpufreq/powernow-k8.c:340:14: warning: variable ‘lo’ set but not used [-Wunused-but-set-variable]
>
> In those cases, there's no bug there, the code just doesn't use all the arguments
> that get passed to rdmsr...   For eg,
>
> static int pending_bit_stuck(void)
> {
>         u32 lo, hi;
>
>         if (cpu_family == CPU_HW_PSTATE)
>                 return 0;
>
>         rdmsr(MSR_FIDVID_STATUS, lo, hi); 
>         return lo & MSR_S_LO_CHANGE_PENDING ? 1 : 0; 
> }
>
>
> gcc's manpage says to dismiss this warning with attribute(unused), but sprinkling thousands
> of those through the source seems pretty ugly.  As does rewriting functions
> like rdmsr to handle NULL as an argument.
>
> There might be some valid bugs found (I think DaveM found a few already) from this new
> warning, but it seems like everything I've looked at so far is just noise.
>
> Any thoughts ?
>
> 	Dave
>
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This new gcc-4.6.0 behavior can also cause build failures for archs
which use -Werror by default (sparc, ppc) [1][2].

[1] https://lkml.org/lkml/2011/4/19/383
[2] https://lkml.org/lkml/2011/4/18/287

-- 
Stratos Psomadakis
<psomas@ece.ntua.gr>