cortex-m-rt: Remove LR push, to ensure the stack is 8-byte aligned.

This was causing incorrect execution of code optimized with the assumption the stack is 8-byte aligned.

Author: Dirbaio

cortex-m-rt/src/lib.rs

@@ -499,12 +499,10 @@ macro_rules! cfg_global_asm {
 // Finally jumps to the user main function.
 #[cfg(cortex_m)]
 cfg_global_asm! {
-    ".cfi_sections .debug_frame
-     .section .Reset, \"ax\"
+    ".section .Reset, \"ax\"
      .global Reset
      .type Reset,%function
-     .thumb_func",
-    ".cfi_startproc
+     .thumb_func
      Reset:",
 
     // Ensure LR is loaded with 0xFFFF_FFFF at startup to help debuggers find the first call frame.
@@ -572,20 +570,16 @@ cfg_global_asm! {
      dsb
      isb",
 
-    // Push `lr` to the stack for debuggers, to prevent them unwinding past Reset.
-    // See https://sourceware.org/binutils/docs/as/CFI-directives.html.
-    ".cfi_def_cfa sp, 0
-     push {{lr}}
-     .cfi_offset lr, 0",
+    // Put the stack pointer in r7 so the next callee-saved frame pointer (i.e. main's) is correct
+    "mov r7, sp",
 
     // Jump to user main function.
     // `bl` is used for the extended range, but the user main function should not return,
     // so trap on any unexpected return.
     "bl main
      udf #0",
 
-    ".cfi_endproc
-     .size Reset, . - Reset",
+    ".size Reset, . - Reset",
 }
 
 /// Attribute to declare an interrupt (AKA device-specific exception) handler