Merge branch 'master' into pr-eraseconfig-reset

Author: mhightower83

cores/esp8266/core_esp8266_features.cpp

@@ -38,13 +38,13 @@ void precache(void *f, uint32_t bytes) {
   // page (ie 1 word in 8) for this to work.
   #define CACHE_PAGE_SIZE 32
 
-  uint32_t a0;
-  __asm__("mov.n %0, a0" : "=r"(a0));
-  uint32_t lines = (bytes/CACHE_PAGE_SIZE)+2;
-  volatile uint32_t *p = (uint32_t*)((f ? (uint32_t)f : a0) & ~0x03);
-  uint32_t x;
-  for (uint32_t i=0; i<lines; i++, p+=CACHE_PAGE_SIZE/sizeof(uint32_t)) x=*p;
-  (void)x;
+  uint32_t lines = (bytes / CACHE_PAGE_SIZE) + 2;
+  uint32_t *p = (uint32_t*)((uint32_t)(f ? f : __builtin_return_address(0)) & ~0x03);
+  do {
+    __asm__ volatile ("" : : "r"(*p));  // guarantee that the value of *p will be in some register (forced load)
+    p += CACHE_PAGE_SIZE / sizeof(uint32_t);
+  } while (--lines);
+  __sync_synchronize();  // full memory barrier, mapped to MEMW in Xtensa
 }
 
 /** based on efuse data, we could determine what type of chip this is