linux-kernel - Re: [PATCH] perf/x86/p4: Fix "Wunused-but-set-variable" warning

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Date:   Mon, 9 Oct 2023 09:29:50 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Lucy Mielke <lucymielke@...oud.com>, mingo@...hat.com,
        acme@...nel.org, mark.rutland@....com,
        alexander.shishkin@...ux.intel.com, jolsa@...nel.org,
        namhyung@...nel.org, irogers@...gle.com, adrian.hunter@...el.com,
        tglx@...utronix.de, bp@...en8.de, dave.hansen@...ux.intel.com,
        x86@...nel.org, hpa@...or.com, linux-perf-users@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] perf/x86/p4: Fix "Wunused-but-set-variable" warning

On Sat, Oct 07, 2023, Peter Zijlstra wrote:
> On Sat, Oct 07, 2023 at 08:55:19PM +0200, Lucy Mielke wrote:
> > This fixes a compiler warning when compiling an allyesconfig with W=1:
> > warning: variable ´high´ set but not used [-Wunused-but-set-variable]
> 
> What compiler and what .config?
> 
> > Signed-off-by: Lucy Mielke <lucymielke@...oud.com>
> > ---
> >  arch/x86/events/intel/p4.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/arch/x86/events/intel/p4.c b/arch/x86/events/intel/p4.c
> > index 35936188db01..69aaf7c0f340 100644
> > --- a/arch/x86/events/intel/p4.c
> > +++ b/arch/x86/events/intel/p4.c
> > @@ -1366,7 +1366,7 @@ static __initconst const struct x86_pmu p4_pmu = {
> >  
> >  __init int p4_pmu_init(void)
> >  {
> > -	unsigned int low, high;
> > +	unsigned int low, __maybe_unused high;
> >  	int i, reg;
> >  
> >  	/* If we get stripped -- indexing fails */
> 
> Right after this we have:
> 
> 	rdmsr(MSR_IA32_MISC_ENABLE, low, high);
> 
> which should get high unconditionally used. If there's a problem then
> it's probably inside that rdmsr macro.

rdmsr() writes to "high", but nothing ever reads from high.  FWIW, I would _love_
for rdmsrl() to have return semantics, e.g. to be able to do:

	low  = (u32)rdmsrl(MSR_IA32_MISC_ENABLE);

or even

	if (!(rdmsrl(MSR_IA32_MISC_ENABLE) & BIT(7)))