Created Mixed language link time optimization (markdown)

Mixed-language-link-time-optimization.md

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+main.c
+
+~~~
+#include <stdint.h>
+
+uint32_t *foo();
+
+int main() {
+    free(foo());
+}
+~~~
+
+foo.rs
+
+~~~
+#[no_std];
+#[allow(ctypes, cstack)];
+
+extern {
+    fn malloc(size: uint) -> *mut u8;
+    fn free(ptr: *mut u8);
+    fn abort() -> !;
+}
+
+#[lang = "exchange_malloc"]
+#[inline(always)]
+unsafe fn exchange_malloc(size: uint) -> *mut u8 {
+  let ptr = malloc(size);
+  if ptr == 0 as *mut u8 {
+    abort()
+  }
+
+  ptr
+}
+
+#[lang = "exchange_free"]
+#[inline(always)]
+unsafe fn exchange_free(ptr: *mut u8) {
+    free(ptr)
+}
+
+#[no_mangle]
+extern "C" fn foo() -> ~u32 {
+    ~5
+}
+~~~
+
+Build LLVM bytecode from the Rust code:
+
+    $ rustc foo.rs --emit-llvm --lib -O
+
+The `--lib` flag is necessary even if Rust defines `main`, to avoid treating
+the Rust part as the entire application.
+
+Compile together the C and Rust code with `clang`:
+
+    $ clang foo.bc main.c -flto -O3 -o main
+
+Verify that LLVM eliminated the call to `foo` via inlining and removed the
+allocation as a dead store:
+
+    $ gdb main
+    (gdb) disassemble main
+    Dump of assembler code for function main:
+       0x00000000004005b0 <+0>:	xor    %eax,%eax
+       0x00000000004005b2 <+2>:	retq   
+    End of assembler dump.