linux-kernel - Re: [PATCH] perf/x86: Fix 'may be used uninitialized' build warnings in core.c

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Date:   Thu, 12 Jan 2017 09:30:00 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Augusto Mecking Caringi <augustocaringi@...il.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] perf/x86: Fix 'may be used uninitialized' build warnings
 in core.c


* Augusto Mecking Caringi <augustocaringi@...il.com> wrote:

> This patch fixes the following build warnings in core.c:
> 
> linux/arch/x86/events/core.c: In function ‘init_hw_perf_events’:
> linux/include/linux/printk.h:292:2: warning: ‘reg_fail’ may be used
> uninitialized in this function [-Wmaybe-uninitialized]
> printk(KERN_ERR pr_fmt(fmt), ##__VA_ARGS__)
> ^
> linux/arch/x86/events/core.c:194:14: note: ‘reg_fail’ was declared here
> int i, reg, reg_fail, ret = 0;
> 
> linux/include/linux/printk.h:292:2: warning: ‘val_fail’ may be used
> uninitialized in this function [-Wmaybe-uninitialized]
> printk(KERN_ERR pr_fmt(fmt), ##__VA_ARGS__)
> ^
> linux/arch/x86/events/core.c:193:11: note: ‘val_fail’ was declared here
> u64 val, val_fail, val_new= ~0;
> 
> Signed-off-by: Augusto Mecking Caringi <augustocaringi@...il.com>
> ---
>  arch/x86/events/core.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
> index 019c588..f6e41b4 100644
> --- a/arch/x86/events/core.c
> +++ b/arch/x86/events/core.c
> @@ -190,8 +190,8 @@ static void release_pmc_hardware(void) {}
>  
>  static bool check_hw_exists(void)
>  {
> -	u64 val, val_fail, val_new= ~0;
> -	int i, reg, reg_fail, ret = 0;
> +	u64 val, val_fail = 0, val_new= ~0;
> +	int i, reg, reg_fail = 0, ret = 0;
>  	int bios_fail = 0;
>  	int reg_safe = -1;

What's not mentioned in the changelog is whether the warning was right or wrong - 
i.e. whether this patch changes behavior or silences a false positive warning.

Whether the compiler changed object code as result of this change would be good to 
know as well.

Thanks,

	Ingo