Move zero-init-ram to just before bss initialisation, so that pre_init occurs before

Author: adamgreig

cortex-m-rt/CHANGELOG.md

@@ -7,14 +7,17 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 ## [Unreleased]
 
+- Add `zero-init-ram` feature to initialize RAM with zeros on startup. This can be necessary on
+  safety-critical hardware to properly initialize memory integrity measures.
+
+## [v0.7.3]
+
 - Fixed a potential miscompilation caused by the initial stack pointer
   not being 8-byte aligned. This issue affected 0.7.1 and 0.7.2; for
   more details please see [the advisory] ([#467]).
 - A linker error is generated if the initial stack pointer is not 8-byte aligned ([#464]).
 - The initial stack pointer is now forced to be 8-byte aligned in the linker script,
   to defend against it being overridden outside of the cortex-m-rt linker script ([#465]).
-- Add `zero-init-ram` feature to initialize RAM with zeros on startup. This can be necessary on
-  safety-critical hardware to properly initialize memory integrity measures.
 
 [the advisory]: https://github.com/rust-embedded/cortex-m/discussions/469
 [#464]: https://github.com/rust-embedded/cortex-m/issues/464

cortex-m-rt/src/lib.rs

@@ -521,19 +521,6 @@ cfg_global_asm! {
     "ldr r0, =_stack_start
      msr msp, r0",
 
-    // If enabled, initialize RAM with zeros. This is not usually required, but might be necessary
-    // to properly initialize checksum-based memory integrity measures on safety-critical hardware.
-    #[cfg(feature = "zero-init-ram")]
-    "ldr r0, =_ram_start
-     ldr r1, =_ram_end
-     movs r2, #0
-     0:
-     cmp r1, r0
-     beq 1f
-     stm r0!, {{r2}}
-     b 0b
-     1:",
-
     // If enabled, initialise VTOR to the start of the vector table. This is normally initialised
     // by a bootloader when the non-reset value is required, but some bootloaders do not set it,
     // leading to frustrating issues where everything seems to work but interrupts are never
@@ -549,6 +536,19 @@ cfg_global_asm! {
     // Example use cases include disabling default watchdogs or enabling RAM.
     "bl __pre_init",
 
+    // If enabled, initialize RAM with zeros. This is not usually required, but might be necessary
+    // to properly initialize checksum-based memory integrity measures on safety-critical hardware.
+    #[cfg(feature = "zero-init-ram")]
+    "ldr r0, =_ram_start
+     ldr r1, =_ram_end
+     movs r2, #0
+     0:
+     cmp r1, r0
+     beq 1f
+     stm r0!, {{r2}}
+     b 0b
+     1:",
+
     // Initialise .bss memory. `__sbss` and `__ebss` come from the linker script.
     #[cfg(not(feature = "zero-init-ram"))]
     "ldr r0, =__sbss