Merge #423

423: Swap to just-stabilised asm!() and global_asm!() macros r=thejpster a=adamgreig

Once Rust 1.59 is released in a couple of days, the `asm!()` and `global_asm!()` macros will become stable, and we will no longer require the various precompiled binaries we've used until now in cortex-m and cortex-m-rt. cc #420.

This PR uses `asm!()` in cortex-m, and removes the inline-asm feature, since I anticipate this going into cortex-m 0.8 and so we don't need to leave it behind for compatibility. In various places the previous version would call an extern C method when built for the native target, which would only fail at link time; to preserve the ability to build on x86 I've either made the whole method require the `cortex_m` configuration, or where appropriate/convenient simply skipped the `asm!()` call.

This PR replaces the old gcc-preprocessed `asm.S` in cortex-m-rt with use of `global_asm!()`, although since you can't normally use `#[cfg(...)]` attributes with `global_asm!()`, there's also a slightly scary macro modified from one invented by `@Dirbaio` for a similar purpose. I considered putting the initialisation of LR behind an armv6m flag, but since we want to restore it after calling `__pre_init` it seemed better to just leave it the same on both targets. I added Cargo features to optionally set SP and VTOR at startup, which has been variously requested but would previously have required multiplicatively more pre-built binaries. Now: no problem. Relevant issues:

* rust-embedded/cortex-m-rt#283
* rust-embedded/cortex-m-rt#55
* rust-embedded/cortex-m-rt#254
* rust-embedded/cortex-m-rt#102
* rust-embedded/cortex-m-rt#338

I've tested these on a couple of targets (and updated the CI): on the whole there's a small improvement in code size due to everyone getting inlined asm, especially in `cortex_m::interrupt::free()`.

The major downside is we bump our MSRV from 1.42 (March 2020) to 1.59 (Feb 2022). For cortex-m, I propose putting these changes in the upcoming 0.8 release (which is technically what the master branch is already on) and not backporting. For cortex-m-rt I'm not sure: we don't have any other pending breaking changes, so we could consider a patch release. Anyway, this PR doesn't commit to any particular releases, so we can decide that later.

For cortex-m-semihosting/panic-semihosting I think a patch release would be ideal, especially since we had to yank the last c-m-sh release due to conflicting prebuilt binaries (a problem that should now vanish).

Also tagging these issues that I think might also benefit from new inline asm:
* #265
* #215
* #406

Co-authored-by: Adam Greig <adam@adamgreig.com>

Author: bors[bot]

.github/bors.toml

@@ -3,9 +3,9 @@ delete_merged_branches = true
 required_approvals = 1
 status = [
   "ci-linux (stable)",
-  "ci-linux (1.42.0)",
+  "ci-linux (1.59.0)",
   "rt-ci-linux (stable)",
-  "rt-ci-linux (1.42.0)",
+  "rt-ci-linux (1.59.0)",
   "rt-ci-other-os (macOS-latest)",
   "rt-ci-other-os (windows-latest)",
   "rustfmt",

.github/workflows/ci.yml

@@ -16,7 +16,7 @@ jobs:
 
         include:
           # Test MSRV
-          - rust: 1.42.0
+          - rust: 1.59.0
 
           # Test nightly but don't fail
           - rust: nightly

.github/workflows/rt-ci.yml

@@ -11,8 +11,7 @@ jobs:
     continue-on-error: ${{ matrix.experimental || false }}
     strategy:
       matrix:
-        # All generated code should be running on stable now
-        rust: [nightly, stable, 1.42.0]
+        rust: [nightly, stable, 1.59.0]
 
         include:
           # Nightly is only for reference and allowed to fail

CHANGELOG.md

@@ -21,8 +21,12 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 ### Fixed
 - Fixed `singleton!()` statics sometimes ending up in `.data` instead of `.bss` (#364, #380).
 
+### Changed
+- Inline assembly is now always used, requiring Rust 1.59.
+
 ### Removed
 - removed all peripherals `ptr()` functions in favor of the associated constant `PTR` (#385).
+- removed `inline-asm` feature which is now always enabled
 
 ## [v0.7.4] - 2021-12-31
 

Cargo.toml

@@ -12,7 +12,8 @@ name = "cortex-m"
 readme = "README.md"
 repository = "https://github.com/rust-embedded/cortex-m"
 version = "0.7.4"
-edition = "2018"
+edition = "2021"
+rust-version = "1.59"
 links = "cortex-m"  # prevent multiple versions of this crate to be linked together
 
 [dependencies]
@@ -29,7 +30,6 @@ optional = true
 [features]
 cm7 = []
 cm7-r0p1 = ["cm7"]
-inline-asm = []
 linker-plugin-lto = []
 std = []
 

README.md

@@ -11,7 +11,7 @@ This project is developed and maintained by the [Cortex-M team][team].
 
 ## Minimum Supported Rust Version (MSRV)
 
-This crate is guaranteed to compile on stable Rust 1.42 and up. It might compile with older versions but that may change in any new patch release.
+This crate is guaranteed to compile on stable Rust 1.59 and up. It might compile with older versions but that may change in any new patch release.
 
 ## License