MDEV-34297 get_rnd_value() of ib_counter_t is unnecessarily complex

The shared counter template ib_counter_t uses the function
my_timer_cycles() as a source of pseudo-random numbers to pick a shard.
On some platforms, my_timer_cycles() could return the constant value 0.

get_rnd_value(): Remove.

my_pseudo_random(): Implement as an alias of my_timer_cycles() or
a wrapper for pthread_self().

Reviewed by: Vladislav Vaintroub

include/my_rdtsc.h

@@ -177,10 +177,23 @@ static inline ulonglong my_timer_cycles(void)
   /* gethrtime may appear as either cycle or nanosecond counter */
   return (ulonglong) gethrtime();
 #else
+# define MY_TIMER_CYCLES_IS_ZERO
   return 0;
 #endif
 }
 
+#ifdef MY_TIMER_CYCLES_IS_ZERO
+static inline size_t my_pseudo_random(void)
+{
+  /* In some platforms, pthread_self() might return a structure
+  that cannot be converted to a number like this. Possible alternatives
+  could include gettid() or sched_getcpu(). */
+  return ((size_t) pthread_self()) / 16;
+}
+#else
+# define my_pseudo_random my_timer_cycles
+#endif
+
 /**
   A nanosecond timer.
   @return the current timer value, in nanoseconds.

storage/innobase/include/sync0arr.inl

@@ -44,7 +44,7 @@ sync_array_get()
 		return(sync_wait_array[0]);
 	}
 
-	return(sync_wait_array[get_rnd_value() % sync_array_size]);
+	return(sync_wait_array[my_pseudo_random() % sync_array_size]);
 }
 
 /******************************************************************//**

storage/innobase/include/ut0counter.h

@@ -41,30 +41,6 @@ Created 2012/04/12 by Sunny Bains
 /** Default number of slots to use in ib_counter_t */
 #define IB_N_SLOTS		64
 
-/** Use the result of my_timer_cycles(), which mainly uses RDTSC for cycles
-as a random value. See the comments for my_timer_cycles() */
-/** @return result from RDTSC or similar functions. */
-static inline size_t
-get_rnd_value()
-{
-	size_t c = static_cast<size_t>(my_timer_cycles());
-
-	if (c != 0) {
-		return c;
-	}
-
-	/* We may go here if my_timer_cycles() returns 0,
-	so we have to have the plan B for the counter. */
-#if !defined(_WIN32)
-	return (size_t)os_thread_get_curr_id();
-#else
-	LARGE_INTEGER cnt;
-	QueryPerformanceCounter(&cnt);
-
-	return static_cast<size_t>(cnt.QuadPart);
-#endif /* !_WIN32 */
-}
-
 /** Class for using fuzzy counters. The counter is multi-instance relaxed atomic
 so the results are not guaranteed to be 100% accurate but close
 enough. Creates an array of counters and separates each element by the
@@ -80,7 +56,7 @@ struct ib_counter_t {
 
 	/** Add to the counter.
 	@param[in]	n	amount to be added */
-	void add(Type n) { add(get_rnd_value(), n); }
+	void add(Type n) { add(my_pseudo_random(), n); }
 
 	/** Add to the counter.
 	@param[in]	index	a reasonably thread-unique identifier