[PATCH] x86/math64: Provide a sane mul_u64_u32_div() implementation for x86_64

Peter Zijlstra
2019-08-28T16:11:32Z

On Wed, Aug 28, 2019 at 05:19:21PM +0200, Peter Zijlstra wrote:
> On Mon, Aug 26, 2019 at 07:47:35AM -0700, kan.liang@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).

---
Subject: x86/math64: Provide a sane mul_u64_u32_div() implementation for x86_64
From: Peter Zijlstra <peterz@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) <peterz@infradead.org>
---
arch/x86/include/asm/div64.h | 13 +++++++++++++
1 file changed, 13 insertions(+)

--- 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

#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" (mul), "rm" (div)
+ : "rdx");
+
+ return q;
+}
+#define mul_u64_u32_div mul_u64_u32_div
+
#endif /* CONFIG_X86_32 */

#endif /* _ASM_X86_DIV64_H */