Force some compilers to respect volatile reads
Inspection of the generated assembly showed that before this commit, armcc 5
was optimizing away the successive reads to the volatile local variable that's
used for double-checks. Inspection also reveals that inserting a call to an
external function is enough to prevent it from doing that.
The tested versions of ARM-GCC, Clang and Armcc 6 (aka armclang) all keep the
double read, with our without a call to an external function in the middle.
The inserted function can also be changed to insert a random delay if
desired in the future, as it is appropriately places between the reads.
diff --git a/include/mbedtls/platform_util.h b/include/mbedtls/platform_util.h
index 586f0d9..e20f1c3 100644
--- a/include/mbedtls/platform_util.h
+++ b/include/mbedtls/platform_util.h
@@ -238,6 +238,13 @@
*/
uint32_t mbedtls_platform_random_in_range( size_t num );
+/**
+ * \brief This function does nothing, but can be inserted between
+ * successive reads to a volatile local variable to prevent
+ * compilers from optimizing them away.
+ */
+void mbedtls_platform_enforce_volatile_reads( void );
+
#if defined(MBEDTLS_HAVE_TIME_DATE)
/**
* \brief Platform-specific implementation of gmtime_r()