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kernel: optimize ms-to-ticks for certain tick frequencies

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Some tick frequencies lend themselves to optimized conversions from ms
to ticks and vice-versa.

- 1000Hz which does not need any conversion
- 500Hz, 250Hz, 125Hz where the division/multiplication are a straight
  shift since they are power-of-two factors of 1000.

In addition, some more generally used values are made to use optimized
conversion equations rather than the generic one that uses 64-bit math,
and often results in calling compiler intrinsics.

These values are: 100Hz, 50Hz, 25Hz, 20Hz, 10Hz, 1Hz (the last one used
in some testing).

Avoiding the 64-bit math intrisics has the additional benefit, in
addition to increased performance, of using a significant lower amount
of stack space: 52 bytes on ARM Cortex-M and 80 bytes on x86.

Change-Id: I080eb338a2637d6b1c6838c119af1a9fa37fe869
Signed-off-by: Benjamin Walsh <benjamin.walsh@windriver.com>
This commit is contained in:
Benjamin Walsh 2016-12-20 14:39:08 -05:00 • committed by Anas Nashif
commit 6209218f40
6 changed files with 73 additions and 10 deletions
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2
kernel/include/ksched.h
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@ -31,7 +31,9 @@ extern void _pend_thread(struct k_thread *thread,
extern void _pend_current_thread(_wait_q_t *wait_q, int32_t timeout);
extern void _move_thread_to_end_of_prio_q(struct k_thread *thread);
extern int __must_switch_threads(void);
#ifdef _NON_OPTIMIZED_TICKS_PER_SEC
extern int32_t _ms_to_ticks(int32_t ms);
#endif
extern void idle(void *, void *, void *);
/* find which one is the next thread to run */