linux-kernel - Re: [PATCH] x86/unwind: remove unneeded initialization

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Message-ID: <20201029023630.GA2316@Ryzen-9-3900X.localdomain>
Date:   Wed, 28 Oct 2020 19:36:30 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Lukas Bulwahn <lukas.bulwahn@...il.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Josh Poimboeuf <jpoimboe@...hat.com>, x86@...nel.org,
        "H . Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com,
        kernel-janitors@...r.kernel.org, linux-safety@...ts.elisa.tech
Subject: Re: [PATCH] x86/unwind: remove unneeded initialization

On Wed, Oct 28, 2020 at 01:21:02PM +0100, Lukas Bulwahn wrote:
> make clang-analyzer on x86_64 defconfig caught my attention with:
> 
>   arch/x86/kernel/unwind_orc.c:38:7: warning: Value stored to 'mid' during
>   its initialization is never read [clang-analyzer-deadcode.DeadStores]
>           int *mid = first, *found = first;
>                ^
> 
> Commit ee9f8fce9964 ("x86/unwind: Add the ORC unwinder") introduced
> __orc_find() with this unneeded dead-store initialization.
> 
> Put the variable in local scope and initialize only once the value is
> needed to make clang-analyzer happy.
> 
> As compilers will detect this unneeded assignment and optimize this
> anyway, the resulting object code is effectively identical before and
> after this change.
> 
> No functional change. Effectively, no change to object code.
> 
> Signed-off-by: Lukas Bulwahn <lukas.bulwahn@...il.com>

Seems fine to me.

Reviewed-by: Nathan Chancellor <natechancellor@...il.com>

> ---
> applies cleanly on current master and next-20201028
> 
> Josh, please ack.
> Ingo, Borislav, please pick this minor non-urgent clean-up patch.
> 
>  arch/x86/kernel/unwind_orc.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kernel/unwind_orc.c b/arch/x86/kernel/unwind_orc.c
> index 6a339ce328e0..5c64eed08257 100644
> --- a/arch/x86/kernel/unwind_orc.c
> +++ b/arch/x86/kernel/unwind_orc.c
> @@ -35,7 +35,7 @@ static struct orc_entry *__orc_find(int *ip_table, struct orc_entry *u_table,
>  {
>  	int *first = ip_table;
>  	int *last = ip_table + num_entries - 1;
> -	int *mid = first, *found = first;
> +	int *found = first;
>  
>  	if (!num_entries)
>  		return NULL;
> @@ -47,7 +47,7 @@ static struct orc_entry *__orc_find(int *ip_table, struct orc_entry *u_table,
>  	 * ignored when they conflict with a real entry.
>  	 */
>  	while (first <= last) {
> -		mid = first + ((last - first) / 2);
> +		int *mid = first + ((last - first) / 2);
>  
>  		if (orc_ip(mid) <= ip) {
>  			found = mid;
> -- 
> 2.17.1
>