On 28/10/12 21:20, Ingo Molnar wrote:
>
> * Andrew Morton <a...@linux-foundation.org> wrote:
>
>> On Sat, 27 Oct 2012 15:12:03 +0200 Ingo Molnar <mi...@kernel.org> wrote:
>>
>>> There's 3 types of conversions done:
>>>
>>> uninitialized_var(x) => x = 0 /* for scalar types */
>>> uninitialized_var(x) => x = NULL /* for pointers */
>>> uninitialized_var(x) => x = { } /* for structures, unions */
>>
>> It's regrettable that we lose information.
>> uninitialized_var() says "this isn't needed - it's just there
>> for gcc". The reader can of course work out the reason with
>> careful code inspection, but that's a lot more time consuming.
>>
>> We could go add "/* keep gcc quiet */" to every site, or add
>> self-documenting macros for the above.
>
> Ok, agreed - I've added /* GCC */ to every site to make people
> think about it.
>
> I left it a bit mystic because in some cases this macro was
> mis-used not to suppress GCC being wrong, but to hide GCC being
> *right*: for example unused variable warnings in cases like:
>
> int uninitialized_var(var);
>
> #ifdef XYZ
> var = ...;
> ...
> #endif
>
> which (ab-)use was no doubt actively dangerous beyond being
> ugly. One such example is in arch/x86/mm/numa.c. (These cases
> now turn into clear (and always harmless) compiler warnings, as
> they should.)
Shouldn't those cases be using:
int var __maybe_unused;
To clarify that the variable is not used in some configurations. Or
moving the variable declaration inside the #ifdef block if possible. The
alternative:
#ifdef XYZ
int var;
#endif
...
#ifdef XYZ
var = ...;
#endif
Does get pretty clunky.
>
> This further strengthens the argument that we should remove this
> whole thing.
>
> Thanks,
>
> Ingo
>
> ---
> Signed-off-by: Ingo Molnar <mi...@kernel.org>
<snip>
> 185 files changed, 281 insertions(+), 291 deletions(-)
It might be interesting to know how many instances of uninitialized_var
are no longer required because of code change around them. Possibly
redefining uninitialized_var as an empty macro and then checking how
many don't cause errors would identify if any can just be removed
outright rather than converting them to an assignment.
> diff --git a/arch/arm/mach-sa1100/assabet.c b/arch/arm/mach-sa1100/assabet.c
> index 6a7ad3c..5ec34d6 100644
> --- a/arch/arm/mach-sa1100/assabet.c
> +++ b/arch/arm/mach-sa1100/assabet.c
> @@ -388,7 +388,7 @@ static void __init map_sa1100_gpio_regs( void )
> */
> static void __init get_assabet_scr(void)
> {
> - unsigned long uninitialized_var(scr), i;
> + unsigned long scr = 0 /* GCC */, i;
>
> GPDR |= 0x3fc; /* Configure GPIO 9:2 as outputs */
> GPSR = 0x3fc; /* Write 0xFF to GPIO 9:2 */
> diff --git a/arch/ia64/kernel/process.c b/arch/ia64/kernel/process.c
> index 35e106f..2c2da64 100644
> --- a/arch/ia64/kernel/process.c
> +++ b/arch/ia64/kernel/process.c
> @@ -493,7 +493,7 @@ static void
> do_copy_task_regs (struct task_struct *task, struct unw_frame_info *info,
> void *arg)
> {
> unsigned long mask, sp, nat_bits = 0, ar_rnat, urbs_end, cfm;
> - unsigned long uninitialized_var(ip); /* GCC be quiet */
> + unsigned long ip = 0 /* GCC */; /* GCC be quiet */
Doubled up comment. Same in a few other places.
> elf_greg_t *dst = arg;
> struct pt_regs *pt;
> char nat;
> diff --git a/arch/ia64/mm/discontig.c b/arch/ia64/mm/discontig.c
> index c641333..75cc8da 100644
> --- a/arch/ia64/mm/discontig.c
> +++ b/arch/ia64/mm/discontig.c
> @@ -185,7 +185,7 @@ static void *per_cpu_node_setup(void *cpu_data, int node)
> void __init setup_per_cpu_areas(void)
> {
> struct pcpu_alloc_info *ai;
> - struct pcpu_group_info *uninitialized_var(gi);
> + struct pcpu_group_info *gi = NULL /* GCC */;
The placement of the comment here is a bit ugly, can you change it so it
appears after the semicolon when it is the last initialiser?
~Ryan
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