On Wed, Aug 28, 2019 at 06:11:23PM +0200, Peter Zijlstra wrote: > On Wed, Aug 28, 2019 at 05:19:21PM +0200, Peter Zijlstra wrote: > > On Mon, Aug 26, 2019 at 07:47:35AM -0700, kan.li...@linux.intel.com wrote: > > > > + return mul_u64_u32_div(slots, val, 0xff); > > > > But also; x86_64 seems to lack a sane implementation of that function, > > and it currently compiles into utter crap (it can be 2 instructions).
This one actually builds defconfig :-) --- Subject: x86/math64: Provide a sane mul_u64_u32_div() implementation for x86_64 From: Peter Zijlstra <pet...@infradead.org> Date: Wed Aug 28 17:39:46 CEST 2019 On x86_64 we can do a u64 * u64 -> u128 widening multiply followed by a u128 / u64 -> u64 division to implement a sane version of mul_u64_u32_div(). Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org> --- arch/x86/include/asm/div64.h | 13 +++++++++++++ 1 file changed, 13 insertions(+) diff --git a/arch/x86/include/asm/div64.h b/arch/x86/include/asm/div64.h index 20a46150e0a8..9b8cb50768c2 100644 --- a/arch/x86/include/asm/div64.h +++ b/arch/x86/include/asm/div64.h @@ -73,6 +73,19 @@ static inline u64 mul_u32_u32(u32 a, u32 b) #else # include <asm-generic/div64.h> + +static inline u64 mul_u64_u32_div(u64 a, u32 mul, u32 div) +{ + u64 q; + + asm ("mulq %2; divq %3" : "=a" (q) + : "a" (a), "rm" ((u64)mul), "rm" ((u64)div) + : "rdx"); + + return q; +} +#define mul_u64_u32_div mul_u64_u32_div + #endif /* CONFIG_X86_32 */ #endif /* _ASM_X86_DIV64_H */