Messages in this thread | | | Date | Thu, 14 Mar 2019 12:13:15 +0000 | From | Patrick Bellasi <> | Subject | Re: [PATCH v7 01/15] sched/core: uclamp: Add CPU's clamp buckets refcounting |
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On 13-Mar 20:48, Peter Zijlstra wrote: > On Wed, Mar 13, 2019 at 04:12:29PM +0000, Patrick Bellasi wrote: > > On 13-Mar 14:40, Peter Zijlstra wrote: > > > On Fri, Feb 08, 2019 at 10:05:40AM +0000, Patrick Bellasi wrote: > > > > +static inline unsigned int uclamp_bucket_id(unsigned int clamp_value) > > > > +{ > > > > + return clamp_value / UCLAMP_BUCKET_DELTA; > > > > +} > > > > + > > > > +static inline unsigned int uclamp_bucket_value(unsigned int clamp_value) > > > > +{ > > > > + return UCLAMP_BUCKET_DELTA * uclamp_bucket_id(clamp_value); > > > > > > return clamp_value - (clamp_value % UCLAMP_BUCKET_DELTA); > > > > > > might generate better code; just a single division, instead of a div and > > > mult. > > > > Wondering if compilers cannot do these optimizations... but yes, looks > > cool and will do it in v8, thanks. > > I'd be most impressed if they pull this off. Check the generated code > and see I suppose :-)
On x86 the code generated looks exactly the same:
https://godbolt.org/z/PjmA7k
While on on arm64 it seems the difference boils down to: - one single "mul" instruction vs - two instructions: "sub" _plus_ one "multiply subtract"
https://godbolt.org/z/0shU0S
So, if I din't get something wrong... perhaps the original version is even better, isn't it?
Test code:
---8<--- #define UCLAMP_BUCKET_DELTA 52
static inline unsigned int uclamp_bucket_id(unsigned int clamp_value) { return clamp_value / UCLAMP_BUCKET_DELTA; }
static inline unsigned int uclamp_bucket_value1(unsigned int clamp_value) { return UCLAMP_BUCKET_DELTA * uclamp_bucket_id(clamp_value); }
static inline unsigned int uclamp_bucket_value2(unsigned int clamp_value) { return clamp_value - (clamp_value % UCLAMP_BUCKET_DELTA); }
int test1(int argc, char *argv[]) { return uclamp_bucket_value1(argc); }
int test2(int argc, char *argv[]) { return uclamp_bucket_value2(argc); }
int test3(int argc, char *argv[]) { return uclamp_bucket_value1(argc) - uclamp_bucket_value2(argc); } ---8<---
which gives on arm64:
---8<--- test1: mov w1, 60495 movk w1, 0x4ec4, lsl 16 umull x0, w0, w1 lsr x0, x0, 36 mov w1, 52 mul w0, w0, w1 ret test2: mov w1, 60495 movk w1, 0x4ec4, lsl 16 umull x1, w0, w1 lsr x1, x1, 36 mov w2, 52 msub w1, w1, w2, w0 sub w0, w0, w1 ret test3: mov w0, 0 ret ---8<---
-- #include <best/regards.h>
Patrick Bellasi
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