linux-kernel - Re: [PATCH v2 4/5] math64: Silence a clang -Wshorten-64-to-32 warning

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Message-ID: <20250501210729.60558b33@pumpkin>
Date: Thu, 1 May 2025 21:07:29 +0100
From: David Laight <david.laight.linux@...il.com>
To: Ian Rogers <irogers@...gle.com>
Cc: Yury Norov <yury.norov@...il.com>, Rasmus Villemoes
 <linux@...musvillemoes.dk>, Arnd Bergmann <arnd@...db.de>, Nathan
 Chancellor <nathan@...nel.org>, Nick Desaulniers
 <nick.desaulniers+lkml@...il.com>, Bill Wendling <morbo@...gle.com>, Justin
 Stitt <justinstitt@...gle.com>, Adrian Hunter <adrian.hunter@...el.com>,
 Thomas Gleixner <tglx@...utronix.de>, Jakub Kicinski <kuba@...nel.org>,
 Jacob Keller <jacob.e.keller@...el.com>, linux-arch@...r.kernel.org,
 linux-kernel@...r.kernel.org, llvm@...ts.linux.dev, Leo Yan
 <leo.yan@....com>
Subject: Re: [PATCH v2 4/5] math64: Silence a clang -Wshorten-64-to-32
 warning

On Wed, 30 Apr 2025 10:15:33 -0700
Ian Rogers <irogers@...gle.com> wrote:

> The clang warning -Wshorten-64-to-32 can be useful to catch
> inadvertent truncation. In some instances this truncation can lead to
> changing the sign of a result, for example, truncation to return an
> int to fit a sort routine. Silence the warning by making the implicit
> truncation explicit. This isn't to say the code is currently incorrect
> but without silencing the warning it is hard to spot the erroneous
> cases.

Except that the extra casts make the reader think something 'extra'
is going on.
For readability you want as few casts as possible.

	David


> 
> Signed-off-by: Ian Rogers <irogers@...gle.com>
> ---
>  include/linux/math64.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/math64.h b/include/linux/math64.h
> index 6aaccc1626ab..f32fcb2a2331 100644
> --- a/include/linux/math64.h
> +++ b/include/linux/math64.h
> @@ -179,7 +179,7 @@ static __always_inline u64 mul_u64_u64_shr(u64 a, u64 mul, unsigned int shift)
>  #ifndef mul_u64_u32_shr
>  static __always_inline u64 mul_u64_u32_shr(u64 a, u32 mul, unsigned int shift)
>  {
> -	u32 ah = a >> 32, al = a;
> +	u32 ah = a >> 32, al = (u32)a;
>  	u64 ret;
>  
>  	ret = mul_u32_u32(al, mul) >> shift;