[riot-notifications] [RIOT-OS/RIOT] sys: Added simple memory barrier API (#11438)

Juan I Carrano notifications at github.com
Wed Apr 24 17:01:12 CEST 2019

jcarrano commented on this pull request.

> + * uint16_t measure_time(void) {
+ *     uint16_t pre = TIMER;
+ *     do_computing();
+ *     uint16_t post = TIMER;
+ *     return pre - post;
+ * }
+ * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ *
+ * In the above example the compiler will now dutifully generate code to access
+ * the timer register two times. The compiler will also not reorder any
+ * `volatile` memory access in regard to any other `volatile` memory access.
+ * But some compilers will reorder non-`volatile` accesses across `volatile`
+ * memory accesses. If the implementation of `do_computing()` is visible to the
+ * compiler, the compiler may conclude that do_computing() could be moved
+ * above the first or below the second access to `TIMER`, which will render the
+ * benchmark meaningless. (**Beware:** Even if `do_something()` is an external

@kaspar030 _library_ function calls. I made the experiment a while ago with a some functions defined in a single C file (no LTO), mixing accesses to globals and calls. GCC is smart enough to realize that some function do not touch global memory. I'm not sure if LTO is also as smart.

Your point still applies to https://github.com/RIOT-OS/RIOT/blob/11da5e8e671c12e5182c1f7aa651489edbe5390a/core/atomic_c11.c#L354

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