Optimise mbed_ticker_api.c

The generic code in mbed_ticker_api.c uses run-time polymorphism to
handle different tickers, and has generic run-time calculations for
different ticker widths and frequencies, with a single special-case for
1MHz.

Extend the run-time special casing to handle any conversion cases where
either the multiply or divide can be done as a shift. This is a speed
optimisation for certain platforms.

Add a new option `target.custom-tickers`. If turned off, it promises
that only USTICKER and LPTICKER devices will be used. This then permits
elimination and/or simplification of runtime calculations, saving size
and speed. If either both USTICKER and LPTICKER have the same width, or
same period numerator or denominator, or only one of them exists, then
operations can be hard-coded. This is a significant ROM space saving,
and a minor speed and RAM saving.

We get to optimise all the calculations, but the run-time polymorphism
is retained even if there is only one ticker, as it doesn't
significantly affect code size versus direct calls, and the existence of
lp_ticker_wrapper and various us_ticker optimisations requires it, even
if only LPTICKER is available.

Author: kjbracey

hal/include/hal/ticker_api.h

@@ -70,22 +70,73 @@ typedef struct {
     bool runs_in_deep_sleep;                      /**< Whether ticker operates in deep sleep */
 } ticker_interface_t;
 
+/* Optimizations to avoid run-time computation if custom ticker support is disabled and
+ * there is exactly one of USTICKER or LPTICKER available, or if they have the same
+ * parameter value(s).
+ */
+#define MBED_TICKER_JUST_US      (!MBED_CONF_TARGET_CUSTOM_TICKERS && DEVICE_USTICKER && !DEVICE_LPTICKER)
+#define MBED_TICKER_JUST_LP      (!MBED_CONF_TARGET_CUSTOM_TICKERS && DEVICE_LPTICKER && !DEVICE_USTICKER)
+
+#if (MBED_TICKER_JUST_US && defined US_TICKER_PERIOD_NUM) || \
+    (!MBED_CONF_TARGET_CUSTOM_TICKERS && defined US_TICKER_PERIOD_NUM && defined LP_TICKER_PERIOD_NUM && \
+    US_TICKER_PERIOD_NUM == LP_TICKER_PERIOD_NUM)
+#define MBED_TICKER_CONSTANT_PERIOD_NUM US_TICKER_PERIOD_NUM
+#elif MBED_TICKER_JUST_LP && defined LP_TICKER_PERIOD_NUM
+#define MBED_TICKER_CONSTANT_PERIOD_NUM LP_TICKER_PERIOD_NUM
+#endif
+
+#if (MBED_TICKER_JUST_US && defined US_TICKER_PERIOD_DEN) || \
+    (!MBED_CONF_TARGET_CUSTOM_TICKERS && defined US_TICKER_PERIOD_DEN && defined LP_TICKER_PERIOD_DEN && \
+    US_TICKER_PERIOD_DEN == LP_TICKER_PERIOD_DEN)
+#define MBED_TICKER_CONSTANT_PERIOD_DEN US_TICKER_PERIOD_DEN
+#elif MBED_TICKER_JUST_LP && defined LP_TICKER_PERIOD_DEN
+#define MBED_TICKER_CONSTANT_PERIOD_DEN LP_TICKER_PERIOD_DEN
+#endif
+
+#if defined MBED_TICKER_CONSTANT_PERIOD_NUM && defined MBED_TICKER_CONSTANT_PERIOD_DEN
+#define MBED_TICKER_CONSTANT_PERIOD
+#endif
+
+#if (MBED_TICKER_JUST_US && defined US_TICKER_MASK) || \
+    (!MBED_CONF_TARGET_CUSTOM_TICKERS && defined US_TICKER_MASK && defined LP_TICKER_MASK && \
+    US_TICKER_MASK == LP_TICKER_MASK)
+#define MBED_TICKER_CONSTANT_MASK US_TICKER_MASK
+#elif MBED_TICKER_JUST_LP && defined LP_TICKER_MASK
+#define MBED_TICKER_CONSTANT_MASK LP_TICKER_MASK
+#endif
+
 /** Ticker's event queue structure
  */
 typedef struct {
     ticker_event_handler event_handler; /**< Event handler */
     ticker_event_t *head;               /**< A pointer to head */
-    uint32_t frequency;                 /**< Frequency of the timer in Hz */
+#ifndef MBED_TICKER_CONSTANT_PERIOD_NUM
+    uint32_t period_num;                /**< Ratio of period to 1us, numerator */
+#endif
+#ifndef MBED_TICKER_CONSTANT_PERIOD_DEN
+    uint32_t period_den;                /**< Ratio of period to 1us, denominator */
+#endif
+#ifndef MBED_TICKER_CONSTANT_MASK
     uint32_t bitmask;                   /**< Mask to be applied to time values read */
     uint32_t max_delta;                 /**< Largest delta in ticks that can be used when scheduling */
+#endif
+#if !(defined MBED_TICKER_CONSTANT_PERIOD && defined MBED_TICKER_CONSTANT_MASK)
     uint64_t max_delta_us;              /**< Largest delta in us that can be used when scheduling */
+#endif
     uint32_t tick_last_read;            /**< Last tick read */
-    uint64_t tick_remainder;            /**< Ticks that have not been added to base_time */
+#if MBED_TICKER_CONSTANT_PERIOD_DEN != 1
+    uint32_t tick_remainder;            /**< Ticks that have not been added to base_time */
+#endif
     us_timestamp_t present_time;        /**< Store the timestamp used for present time */
     bool initialized;                   /**< Indicate if the instance is initialized */
     bool dispatching;                   /**< The function ticker_irq_handler is dispatching */
     bool suspended;                     /**< Indicate if the instance is suspended */
-    uint8_t frequency_shifts;           /**< If frequency is a value of 2^n, this is n, otherwise 0 */
+#ifndef MBED_TICKER_CONSTANT_PERIOD_NUM
+    int8_t period_num_shifts;           /**< If numerator is a value of 2^n, this is n, otherwise -1 */
+#endif
+#ifndef MBED_TICKER_CONSTANT_PERIOD_DEN
+    int8_t period_den_shifts;           /**< If denominator is a value of 2^n, this is n, otherwise -1 */
+#endif
 } ticker_event_queue_t;
 
 /** Ticker's data structure