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

[Peter Zijlstra]

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@xxxxxxxxxxxxxxx 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@xxxxxxxxxxxxx>
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@xxxxxxxxxxxxx>
---
 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 */