Change pre-init to use an empty function that is overwridden instead of a sentinel

Using a sentinel creates a conditional branch that cannot be optimized out
whereas an empty default function keeps the codepath linear and can be optimized
out in the best case.

Signed-off-by: Gabriel Smith <[email protected]>

Author: yodaldevoid

cortex-m-rt/link.x.in

@@ -54,7 +54,7 @@ EXTERN(__INTERRUPTS); /* `static` variable similar to `__EXCEPTIONS` */
 /* # Pre-initialization function */
 /* If the user overrides this using the `pre_init!` macro or by creating a `__pre_init` function,
    then the function this points to will be called before the RAM is initialized. */
-PROVIDE(__pre_init = 1);
+PROVIDE(__pre_init = DefaultPreInit);
 
 /* # Sections */
 SECTIONS

cortex-m-rt/src/lib.rs

@@ -237,11 +237,10 @@
 //! have a size of 32 vectors (on ARMv6-M) or 240 vectors (on ARMv7-M). This array is located after
 //! `__EXCEPTIONS` in the `.vector_table` section.
 //!
-//! - `__pre_init`. This is a function to be run before RAM is initialized. It defaults pointing at
-//! `1` and if not changed to point to another address, usually by calling the `pre_init!` macro,
-//! the `_pre_init` function is skipped. The function cannot default to `0` as the compiler
-//! optimizes out the check for `0` under the assumption that a function pointer cannot point to
-//! `0`.
+//! - `__pre_init`. This is a function to be run before RAM is initialized. It defaults to an empty
+//! function. The function called can be changed by calling the `pre_init!` macro. The empty
+//! function is not optimized out by default, but if an empty function is passed to `pre_init!` the
+//! function call will be optimized out.
 //!
 //! If you override any exception handler you'll find it as an unmangled symbol, e.g. `SysTick` or
 //! `SVCall`, in the output of `objdump`,
@@ -493,9 +492,7 @@ pub unsafe extern "C" fn Reset() -> ! {
     }
 
     let pre_init: unsafe extern "C" fn() = __pre_init;
-    if pre_init as usize != 1 {
-        pre_init();
-    }
+    pre_init();
 
     // Initialize RAM
     r0::zero_bss(&mut __sbss, &mut __ebss);
@@ -552,6 +549,10 @@ pub unsafe extern "C" fn DefaultUserHardFault() {
     }
 }
 
+#[doc(hidden)]
+#[no_mangle]
+pub unsafe extern "C" fn DefaultPreInit() {}
+
 /// Macro to define the entry point of the program
 ///
 /// **NOTE** This macro must be invoked once and must be invoked from an accessible module, ideally