[ARMv6-M] initialize the LR register

asm.s

@@ -40,3 +40,17 @@ FpuTrampoline:
   # Hand execution over to `main`.
   bl main
   # Note: `main` must not return. `bl` is used only because it has a wider range than `b`.
+
+  # ARMv6-M leaves LR in an unknown state on Reset
+  # this trampoline sets LR before it's pushed onto the stack by Reset
+  .section .PreResetTrampoline, "ax"
+  .global PreResetTrampoline
+  # .type and .thumb_func are both required; otherwise its Thumb bit does not
+  # get set and an invalid vector table is generated
+  .type PreResetTrampoline,%function
+  .thumb_func
+PreResetTrampoline:
+  # set LR to the initial value used by the ARMv7-M (0xFFFF_FFFF)
+  ldr r0,=0xffffffff
+  mov lr,r0
+  b Reset

link.x.in

@@ -85,6 +85,10 @@ SECTIONS
   /* ### .text */
   .text _stext :
   {
+    /* place these 2 close to each other or the `b` instruction will fail to link */
+    *(.PreResetTrampoline);
+    *(.Reset);
+
     *(.text .text.*);
     *(.HardFaultTrampoline);
     *(.HardFault.*);

src/lib.rs

@@ -923,9 +923,17 @@ pub fn heap_start() -> *mut u32 {
 #[doc(hidden)]
 #[link_section = ".vector_table.reset_vector"]
 #[no_mangle]
+#[cfg(not(armv6m))]
 pub static __RESET_VECTOR: unsafe extern "C" fn() -> ! = Reset;
 
 #[doc(hidden)]
+#[link_section = ".vector_table.reset_vector"]
+#[no_mangle]
+#[cfg(armv6m)]
+pub static __RESET_VECTOR: unsafe extern "C" fn() -> ! = PreResetTrampoline;
+
+#[doc(hidden)]
+#[link_section = ".Reset"]
 #[no_mangle]
 pub unsafe extern "C" fn Reset() -> ! {
     extern "C" {
@@ -1030,6 +1038,9 @@ pub enum Exception {
 pub use self::Exception as exception;
 
 extern "C" {
+    #[cfg(armv6m)]
+    fn PreResetTrampoline() -> !;
+
     fn NonMaskableInt();
 
     fn HardFaultTrampoline();