Re: [PATCH v2] overflow: Match arithmetic in both parts of __builtin_choose_expr

[David Laight <david.laight.linux@gmail.com>]

On Tue, 13 Oct 2026 20:31:46 +0000
Bill Wendling <morbo@google.com> wrote:

> The two parts of '__builtin_choose_expr()' had different arithmetic
> expressions. This patch corrects it so that both parts calculate:
> 
>   COUNT * sizeof(*MEMBER) + __must_be_array(MEMBER)
> 
> In practice, '__must_be_array' is meant to assert if 'MEMBER' isn't an
> array and return '0' otherwise. However, symmetry is less prone to
> subtle errors (and far more satisfying).
> 
> Signed-off-by: Bill Wendling <morbo@google.com>
> ---
> v2: Don't use 'size_add', which just adds more complicated macro cruft to the
>     expression. Instead, just adjust the parentheses.
> ---
>  include/linux/overflow.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/overflow.h b/include/linux/overflow.h
> index a8cb6319b4fb..d0f04436343e 100644
> --- a/include/linux/overflow.h
> +++ b/include/linux/overflow.h
> @@ -418,7 +418,7 @@ static __always_inline size_t __must_check size_sub(size_t minuend, size_t subtr
>  #define flex_array_size(p, member, count)				\
>  	__builtin_choose_expr(__is_constexpr(count),			\
>  		(count) * sizeof(*(p)->member) + __must_be_array((p)->member),	\
> -		size_mul(count, sizeof(*(p)->member) + __must_be_array((p)->member)))
> +		size_mul(count, sizeof(*(p)->member)) + __must_be_array((p)->member))

Why does this use __builtin_choose_expr().
The types of the two expressions are the same so it might as well use ?:

Also doesn't this optimisation apply to all uses of size_mul() (aka saturating_mul()).
Presumably relying on the compiler to report the compilation time numeric overflow.

David

>  
>  /**
>   * struct_size() - Calculate size of structure with trailing flexible array.