RFC on low level features

active/0000-better-low-level-handling.md

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+- Start Date: 2014-04-29
+- RFC PR #: (leave this empty)
+- Rust Issue #: (leave this empty)
+
+# Summary
+
+Add a few features that would allow to write better system-level code: weak
+symbols, naked functions, pure assembly functions.
+
+# Motivation
+
+Those features are available in C and C++ and are often used in embedded
+development.
+
+# Detailed design
+
+### Weak symbols
+
+`linkage` attribute should be extended to be allowed in function context
+(alternatively a new attribute should be provided):
+
+```rust
+#[no_mangle]
+#[linkage(weak)]
+pub fn isr_handler() {
+  ...
+}
+```
+
+This would allow to provide default implementations that could be overriden by
+specific code.
+
+### Naked functions
+
+Provide an attribute that would allow to generate a naked function, without
+stack guard prologue, common function prologue and epilogue. The only statement
+that could be safely used in naked function is `asm!` that doesn't modify
+operands (alternatively, naked functions are always considered _unsafe_).
+
+```rust
+#[naked]
+pub fn isr_handler {
+  asm!("mrs r0, psp
+        stmdb r0!, {r4-r11}
+        msr psp, r0" :::: "volatile");
+  switch_stack();
+}
+```
+
+would generate the following machine code:
+
+```
+isr_handler:
+    // asm! block
+    mrs r0, psp
+    stmdb r0!, {r4-r11}
+    msr psp, r0
+
+    // function call
+    bl switch_stack
+
+    // the only generated instruction
+    bx lr
+```
+
+Note how in this example the provided code is incorrect, as `bl` will overwrite
+the contents of `lr` register, so that the final `bx lr` instruction will do
+something unexpected. It might be useful to disallow function calls from naked
+function unless those are done explicity from `asm!` (which is quite hard,
+unless the called function has `#[no_mangle]`).
+
+### Pure assembly functions
+
+This the extension of naked funcations above, but no code would be generated by
+compiler at all. Those functions could be written in an external .s file, but
+including them in source code would allow better management, including `#[cfg]`
+usage to pick the correct implementation.
+
+# Alternatives
+
+N/A
+
+# Unresolved questions
+
+N/A