Re: [RFC PATCH] xen: Remove -Wdeclaration-after-statement

[Frediano Ziglio <frediano.ziglio@xxxxxxxxx>]

On Fri, Aug 9, 2024 at 2:04 PM Alejandro Vallejo
<alejandro.vallejo@xxxxxxxxx> wrote:
>
> This warning only makes sense when developing using a compiler with C99
> support on a codebase meant to be built with C89 compilers too, and
> that's no longer the case (nor should it be, as it's been 25 years since
> C99 came out already).
>
> Signed-off-by: Alejandro Vallejo <alejandro.vallejo@xxxxxxxxx>
> ---
> Yes, I'm opening this can of worms. I'd like to hear others people's
> thoughts on this and whether this is something MISRA has views on. If
> there's an ulterior non-obvious reason besides stylistic preference I
> think it should be documented somewhere, but I haven't seen such an
> explanation.
>
> IMO, the presence of this warning causes several undesirable effects:
>
>   1. Small functions are hampered by the preclusion of check+declare
>      patterns that improve readability via concision. e.g: Consider a
>      silly example like:
>
>      /* with warning */                     /* without warning */
>      void foo(uint8_t *p)                   void foo(uint8_t *p)
>      {                                      {
>          uint8_t  tmp1;                         if ( !p )
>          uint16_t tmp2;                             return;
>          uint32_t tmp3;
>                                                 uint8_t  tmp1 = OFFSET1 + *p;
>          if ( !p )                              uint16_t tmp2 = OFFSET2 + *p;
>              return;                            uint32_t tmp3 = OFFSET3 + *p;
>
>          tmp1 = OFFSET1 + *p;                   /* Lots of uses of `tmpX` */
>          tmp2 = OFFSET2 + *p;               }
>          tmp2 = OFFSET2 + *p;
>
>          /* Lots of uses of tmpX */
>      }
>
>   2. Promotes scope-creep. On small functions it doesn't matter much,
>      but on bigger ones to prevent declaring halfway through the body
>      needlessly increases variable scope to the full scope in which they
>      are defined rather than the subscope of point-of-declaration to
>      end-of-current-scope. In cases in which they can be trivially
>      defined at that point, it also means they can be trivially misused
>      before they are meant to. i.e: On the example in (1) assume the
>      conditional in "with warning" is actually a large switch statement.
>
>   3. It facilitates a disconnect between time-of-declaration and
>      time-of-use that lead very easily to "use-before-init" bugs.
>      While a modern compiler can alleviate the most egregious cases of
>      this, there's cases it simply cannot cover. A conditional
>      initialization on anything with external linkage combined with a
>      conditional use on something else with external linkage will squash
>      the warning of using an uninitialised variable. Things are worse
>      where the variable in question is preinitialised to something
>      credible (e.g: a pointer to NULL), as then that can be misused
>      between its declaration and its original point of intended use.
>
> So... thoughts? yay or nay?

Hi,
   I personally agree with this change. Even compiler is stating that
this is just C89 compatibility.
Yes, it's question of style and surely having all declaration at the
beginning reduce the options however there are technical reason why
having mixed declarations can help
- you can use "const" to tell compiler (and code editor) that
something should not be changed;
- in many cases reduces commit changes;

> ---
>  xen/Makefile | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/xen/Makefile b/xen/Makefile
> index 2e1a925c8417..288b7ac8bb2d 100644
> --- a/xen/Makefile
> +++ b/xen/Makefile
> @@ -394,7 +394,7 @@ CFLAGS-$(CONFIG_CC_SPLIT_SECTIONS) += -ffunction-sections 
> -fdata-sections
>
>  CFLAGS += -nostdinc -fno-builtin -fno-common
>  CFLAGS += -Werror -Wredundant-decls -Wwrite-strings -Wno-pointer-arith
> -CFLAGS += -Wdeclaration-after-statement -Wuninitialized
> +CFLAGS += -Wuninitialized
>  $(call cc-option-add,CFLAGS,CC,-Wvla)
>  $(call cc-option-add,CFLAGS,CC,-Wflex-array-member-not-at-end)
>  $(call cc-option-add,CFLAGS,CC,-Winit-self)

Frediano