RFC: linker placement attribute

active/0000-linker-placement.md

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+- Start Date: 2014-04-14
+- RFC PR #: (leave this empty)
+- Rust Issue #: (leave this empty)
+
+# Summary
+
+Implement `#[link_placement]` attribute to specify where is static expected to be found in address space.
+
+# Motivation
+
+*(copied over from my other RFC)* In most of the cases, embedded hardware is configured with memory-mapped registers. Often a peripheral would have a moderately big number of registers to the point where tracking each one by its address is hard and prone to errors.
+
+Given the unsafe nature of *T, I suggest to use &T instead for IO registers mapping.
+
+# Detailed design
+
+It is currently possible to provide placement information through a linker script:
+
+example.rs
+```rust
+// this struct maps over a number of ioregs
+pub struct UART {
+  REG_A:  u32,
+  REG_B:  u32,
+  _pad_0: u32,
+  REG_C:  u32,
+}
+
+impl UART {
+  // given the nature of regs, we must use volatile access
+  pub fn get_REG_A(&self) {
+    unsafe { volatile_load(&(self.REG_A)) }
+  }
+
+  pub fn set_REG_A(&self, val: u32) {
+    unsafe { volatile_store(&mut (self.REG_A), val) }
+  }
+  ...
+}
+
+// instantiate all peripherals over corresponding ioregs -> hard to read code
+// pub static UART0 : *mut UART = 0xdeadbeef as *mut UART;
+
+// ask the linker to provide correct address
+extern {
+  pub static mut UART0: UART;
+}
+```
+
+script.ld
+```
+UART0 = 0xdeadbeef;
+```
+
+I propose to add `#[link_placement]` attribute that would allow to specify placement address directly in the code:
+
+```rust
+#[link_placement=0xdeadbeef]
+pub static mut UART0: UART;
+```
+
+This way compiler knows symbol location and can provide better optimisations.
+
+## Found in other languages
+
+Proprietary C compilers for embedded systems are known to provide similar feature:
+
+```c
+static int reg @ 0xdeadbeef;
+```
+
+# Alternatives
+
+Alternative implementation, that keeps the exsisting static parser intact is:
+
+```rust
+#[unsafe_override_address]
+static UART0: UART = 0xdeadbeef;
+```
+
+or even
+
+```rust
+#[unsafe_override_address]
+static UART0: &UART = 0xdeadbeef;
+```
+
+The last case is apparently the simplest one to implement.
+
+# Unresolved questions
+
+N/A