On Mon, Mar 4, 2024, at 14:01, Ard Biesheuvel wrote:
On Mon, 4 Mar 2024 at 13:35, Arnd Bergmann arnd@arndb.de 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