Re: [PATCH] x86/unwind: remove unneeded initialization

["Lukas Bulwahn" <lukas.bulwahn@gmail.com>]

On Tue, Oct 20, 2020 at 9:20 AM Lukas Bulwahn <lukas.bulwahn@gmail.com> 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 these unneeded assignments 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@gmail.com>
> ---


Please help me with your review.

Also help by compiling and comparing the disassembly before and after
with your compiler.


>  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
>