[RFC 1/1] Optimizing the rand function on arm64 platforms

[Tian Tao]

Armv8.5 adds the FEAT_RNG feature so that true random numbers can be
obtained from the cpu. This optimization is therefore proposed for the
arm64 platform.

Signed-off-by: Tian Tao <tiantao6@hisilicon.com>
---
 stdlib/rand.c | 32 ++++++++++++++++++++++++++++++++
 1 file changed, 32 insertions(+)

diff --git a/stdlib/rand.c b/stdlib/rand.c
index c250c065e4..f199dbd65f 100644
--- a/stdlib/rand.c
+++ b/stdlib/rand.c
@@ -19,10 +19,42 @@
 
 #undef	rand
 
+#ifdef __aarch64__
+// A global variable to indicate whether rndr instruction is supported
+static int rndr_supported = 0;
+// A function prototype to check rndr support
+static void check_rndr_support (void) __attribute__ ((constructor));
+#endif
 
 /* Return a random integer between 0 and RAND_MAX.  */
 int
 rand (void)
 {
+  // Use armv8.5 rndr instruction if supported
+#ifdef __aarch64__
+  if (rndr_supported) {
+    unsigned int r;
+    asm volatile("mrs %0, s3_3_c2_c4_0" : "=r" (r));
+    // Discard the least random bit
+    r >>=1;
+    return (int) r;
+  }
+#endif
   return (int) __random ();
 }
+
+#ifdef __aarch64__
+// A function to check rndr support and set the global variable accordingly
+static void
+check_rndr_support (void)
+{
+  unsigned long isar0;
+  // Read the ID_AA64ISAR0_EL1 register to check the rndr feature
+  asm volatile("mrs %0, id_aa64isar0_el1" : "=r" (isar0));
+  // Check the bits [63:60] for the rndr feature
+  if ((isar0 >> 60) & 0xf) {
+    // Set the global variable to indicate rndr support
+    rndr_supported = 1;
+  }
+}
+#endif
-- 
2.33.0