util/cacheflush: Avoid possible redundant dcache flush on Darwin

<libkern/OSCacheControl.h> describes sys_icache_invalidate() as
"equivalent to sys_cache_control(kCacheFunctionPrepareForExecution)",
having kCacheFunctionPrepareForExecution defined as:

  /* Prepare memory for execution.  This should be called
   * after writing machine instructions to memory, before
   * executing them.  It syncs the dcache and icache. [...]
   */

Since the dcache is also sync'd, we can avoid the sys_dcache_flush()
call when both rx/rw pointers are equal.

Suggested-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Message-Id: <20230605195911.96033-1-philmd@linaro.org>

util/cacheflush.c

@@ -241,7 +241,14 @@ static void __attribute__((constructor)) init_cache_info(void)
 
 void flush_idcache_range(uintptr_t rx, uintptr_t rw, size_t len)
 {
-    sys_dcache_flush((void *)rw, len);
+    if (rx == rw) {
+        /*
+         * sys_icache_invalidate() syncs the dcache and icache,
+         * so no need to call sys_dcache_flush().
+         */
+    } else {
+        sys_dcache_flush((void *)rw, len);
+    }
     sys_icache_invalidate((void *)rx, len);
 }
 #else