rt: automatically align .text by default

cortex-m-rt/CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 ## [Unreleased]
 
+- Align the default `_stext` to the alignment required by the `.text` section. Fixes
+  the lld warning `address (...) of section .text is not a multiple of alignment (...)`.
+    - Note: `_stext` is not auto-aligned if it's explicitly overridden by the user. It's their responsibility to align it, else the warning will show.
+
 ## [v0.7.6]
 
 - Mark `pre_init` as deprecated

cortex-m-rt/link.x.in

@@ -89,7 +89,11 @@ SECTIONS
     KEEP(*(.vector_table.interrupts)); /* this is the `__INTERRUPTS` symbol */
   } > FLASH
 
-  PROVIDE(_stext = ADDR(.vector_table) + SIZEOF(.vector_table));
+  /* The alignment of .text is the maximum alignment required by its input sections, which
+     can be more than 4 (e.g. `.p2align` directives inside inline assembly). Since the
+     start address of .text is set explicitly, the linker won't align it automatically, so do it
+     here. */
+  PROVIDE(_stext = ALIGN(ADDR(.vector_table) + SIZEOF(.vector_table), ALIGNOF(.text)));
 
   /* ### .text */
   .text _stext :

cortex-m-rt/src/lib.rs

@@ -113,6 +113,10 @@
 //! _stext = ORIGIN(FLASH) + 0x40C;
 //! ```
 //!
+//! It is your responsibility to ensure `_stext` is aligned to what the `.text` section requires.
+//! This is typically 4 but can ocasionally be higher, such as when using `.p2align` directives inside ASM.
+//! If you fail to do so you will see a linker warning like `address (...) of section .text is not a multiple of alignment (...)`.
+//!
 //! # An example
 //!
 //! This section presents a minimal application built on top of `cortex-m-rt`. Apart from the