[ion/device] Make sure start()'s prologue doesn't use VFP registers

ion/src/device/boot/rt0.cpp

@@ -28,6 +28,19 @@ void abort() {
 #endif
 }
 
+/* By default, the compiler is free to inline any function call he wants. If the
+ * compiler decides to inline some functions that make use of VFP registers, it
+ * will need to push VFP them onto the stack in calling function's prologue.
+ * Problem: in start()'s prologue, we would never had a chance to enable the FPU
+ * since this function is the first thing called after reset.
+ * We can safely assume that neither memcpy, memset, nor any Ion::Device::init*
+ * method will use floating-point numbers, but ion_main very well can.
+ * To make sure ion_main's potential usage of VFP registers doesn't bubble-up to
+ * start(), we isolate it in its very own non-inlined function call. */
+static void __attribute__((noinline)) non_inlined_ion_main() {
+  return ion_main(0, nullptr);
+}
+
 void start() {
   // This is where execution starts after reset.
   // Many things are not initialized yet so the code here has to pay attention.
@@ -75,7 +88,7 @@ void start() {
 
   Ion::Device::init();
 
-  ion_main(0, nullptr);
+  non_inlined_ion_main();
 
   abort();
 }