Start implementing exception handling by adding frame descriptor entry processing.

- Soon, we should be able basic exceptions.

lib/llvm-backend/src/platform/unix.rs

@@ -0,0 +1,35 @@
+/// `__register_frame` and `__deregister_frame` on macos take a single fde as an
+/// argument, so we need to parse the fde table here.
+///
+/// This is a pretty direct port of llvm's fde handling code:
+///     https://llvm.org/doxygen/RTDyldMemoryManager_8cpp_source.html.
+#[cfg(target_os = "macos")]
+pub unsafe fn visit_fde(addr: *mut u8, size: usize, visitor: extern "C" fn(*mut u8)) {
+    unsafe fn process_fde(entry: *mut u8, visitor: extern "C" fn(*mut u8)) -> *mut u8 {
+        let mut p = entry;
+        let length = (p as *const u32).read_unaligned();
+        p = p.add(4);
+        let offset = (p as *const u32).read_unaligned();
+
+        if offset != 0 {
+            visitor(entry);
+        }
+        p.add(length as usize)
+    }
+
+    let mut p = addr;
+    let end = p.add(size);
+
+    loop {
+        if p >= end {
+            break;
+        }
+
+        p = process_fde(p, visitor);
+    }
+}
+
+#[cfg(not(target_os = "macos"))]
+pub unsafe fn visit_fde(addr: *mut u8, size: usize, visitor: extern "C" fn(*mut u8)) {
+    visitor(addr);
+}