Re: [PATCH v1 2/5] bitmap: Silence a clang -Wshorten-64-to-32 warning

Arnd Bergmann
2025-04-04T05:49:58Z

On Thu, Apr 3, 2025, at 18:56, Ian Rogers 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.

> unsigned int bitmap_weight(const unsigned long *src, unsigned int nbits)
> {
> if (small_const_nbits(nbits))
> - return hweight_long(*src & BITMAP_LAST_WORD_MASK(nbits));
> + return (int)hweight_long(*src & BITMAP_LAST_WORD_MASK(nbits));
> return __bitmap_weight(src, nbits);
> }

I don't understand this one. hweight_long() and bitmap_weight()
both return unsigned value, so why do you need to cast this to
a signed value to avoid a signedness problem?

hweight_long() should never return anything larger than 64ul
anyway, which is way outside of the range where it would get
sign-extended.

A more logical change to me would be to make hweight_long()
and bitmap_weight() have the same return type, either
'unsigned long' or 'unsigned int'.

Arnd