On Mon, Mar 4, 2024, at 14:01, Ard Biesheuvel wrote:
> On Mon, 4 Mar 2024 at 13:35, Arnd Bergmann <arnd@xxxxxxxx> wrote:
>> On Mon, Mar 4, 2024, at 12:45, Andre Przywara wrote:
>> It's not critical if this is called infrequently, and as Maxime
>> just replied, the 64-bit division is in fact required here.
>> Since we are dividing by a constant value (200), there is a good
>> chance that this will be get turned into fairly efficient
>> multiply/shift code.
>>
>
> Clang does not implement that optimization for 64-bit division. That
> is how we ended up with this error in the first place.
I meant it will use the optimization after the patch to convert
the plain '/' to div_u64().
> Perhaps it is worthwhile to make div_u64() check its divisor, e.g.,
>
> --- a/include/linux/math64.h
> +++ b/include/linux/math64.h
> @@ -127,6 +127,9 @@
> static inline u64 div_u64(u64 dividend, u32 divisor)
> {
> u32 remainder;
> +
> + if (IS_ENABLED(CONFIG_CC_IS_GCC) && __builtin_constant_p(divisor))
> + return dividend / divisor;
> return div_u64_rem(dividend, divisor, &remainder);
> }
I think the div_u64()->do_div()->__div64_const32()->__arch_xprod_64()
optimization in asm-generic/div64.h already produces what we want
on both compilers. Is there something missing there?
Arnd
