Adds a paint-stack feature.

Paints the stack with a fixed 0xcccccccc value, which makes it easier to see the stack high-water mark.

Also update the qemu.rs example to verify a .data value and a .bss value.

cortex-m-rt/Cargo.toml

@@ -46,6 +46,7 @@ device = []
 set-sp = []
 set-vtor = []
 zero-init-ram = []
+paint-stack = []
 
 [package.metadata.docs.rs]
 features = ["device"]

cortex-m-rt/examples/qemu.rs

@@ -1,15 +1,25 @@
 #![no_main]
 #![no_std]
 
-use core::fmt::Write;
+use core::{
+    fmt::Write,
+    sync::atomic::{AtomicU32, Ordering},
+};
+
+static DATA_VAL: AtomicU32 = AtomicU32::new(1234);
+
+static BSS_VAL: AtomicU32 = AtomicU32::new(0);
 
 #[cortex_m_rt::entry]
 fn main() -> ! {
     let x = 42;
 
     loop {
         let mut hstdout = cortex_m_semihosting::hio::hstdout().unwrap();
-        write!(hstdout, "x = {}\n", x).unwrap();
+        // check that .data and .bss were initialised OK
+        if DATA_VAL.load(Ordering::Relaxed) == 1234 && BSS_VAL.load(Ordering::Relaxed) == 0 {
+            _ = writeln!(hstdout, "x = {}", x);
+        }
         cortex_m_semihosting::debug::exit(cortex_m_semihosting::debug::EXIT_SUCCESS);
     }
 }

cortex-m-rt/src/lib.rs

@@ -185,6 +185,13 @@
 //! value to the `_ram_end` value from the linker script. This is not usually required, but might be
 //! necessary to properly initialize memory integrity measures on some hardware.
 //!
+//! ## `paint-stack`
+//!
+//! Everywhere between `__sheap` and `___stack_start` is painted with the fixed value `STACK_PAINT_VALUE`.
+//! You can then inspect memory during debugging to determine how much of the stack has been used
+//! - where the stack has been used the 'paint' will have been 'scrubbed off' and the memory will
+//! have a value other than `STACK_PAINT_VALUE`.
+//!
 //! # Inspection
 //!
 //! This section covers how to inspect a binary that builds on top of `cortex-m-rt`.
@@ -467,6 +474,13 @@
 
 extern crate cortex_m_rt_macros as macros;
 
+/// The 32-bit value the stack is painted with before the program runs.
+///
+/// Note: keep this value in-sync with the start-up assembly code, as we can't
+/// use const values in `global_asm!` yet.
+#[cfg(feature = "paint-stack")]
+pub const STACK_PAINT_VALUE: u32 = 0xcccc_cccc;
+
 #[cfg(cortex_m)]
 use core::arch::global_asm;
 use core::fmt;
@@ -545,24 +559,37 @@ cfg_global_asm! {
     "ldr r0, =__sbss
      ldr r1, =__ebss
      movs r2, #0
-     2:
+     0:
      cmp r1, r0
-     beq 3f
+     beq 1f
      stm r0!, {{r2}}
-     b 2b
-     3:",
+     b 0b
+     1:",
+
+    // If enabled, paint stack/heap RAM with 0xcccccccc.
+    // `__sheap` and `_stack_start` come from the linker script.
+    #[cfg(feature = "paint-stack")]
+    "ldr r0, =__sheap
+     ldr r1, =_stack_start
+     ldr r2, =0xcccccccc // This must match STACK_PAINT_VALUE
+     0:
+     cmp r1, r0
+     beq 1f
+     stm r0!, {{r2}}
+     b 0b
+     1:",
 
     // Initialise .data memory. `__sdata`, `__sidata`, and `__edata` come from the linker script.
     "ldr r0, =__sdata
      ldr r1, =__edata
      ldr r2, =__sidata
-     4:
+     0:
      cmp r1, r0
-     beq 5f
+     beq 1f
      ldm r2!, {{r3}}
      stm r0!, {{r3}}
-     b 4b
-     5:",
+     b 0b
+     1:",
 
     // Potentially enable an FPU.
     // SCB.CPACR is 0xE000_ED88.