Start implementing exception handling by adding frame descriptor entry processing.

- Soon, we should be able basic exceptions.

lib/llvm-backend/src/backend.rs

@@ -59,6 +59,7 @@ struct Callbacks {
     dealloc_memory: extern "C" fn(*mut u8, usize) -> LLVMResult,
 
     lookup_vm_symbol: extern "C" fn(*const c_char, usize) -> *const vm::Func,
+    visit_fde: extern "C" fn(*mut u8, usize, extern "C" fn(*mut u8)),
 }
 
 extern "C" {
@@ -165,11 +166,18 @@ fn get_callbacks() -> Callbacks {
         }
     }
 
+    extern "C" fn visit_fde(fde: *mut u8, size: usize, visitor: extern "C" fn(*mut u8)) {
+        unsafe {
+            crate::platform::visit_fde(fde, size, visitor);
+        }
+    }
+
     Callbacks {
         alloc_memory,
         protect_memory,
         dealloc_memory,
         lookup_vm_symbol,
+        visit_fde,
     }
 }
 

lib/llvm-backend/src/lib.rs

@@ -14,6 +14,7 @@ use wasmparser::{self, WasmDecoder};
 mod backend;
 mod code;
 mod intrinsics;
+mod platform;
 mod read_info;
 mod state;
 mod trampolines;

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

@@ -1,8 +1,7 @@
-
-#[cfg(target_family = "unix")]
+#[cfg(unix)]
 mod unix;
-#[cfg(target_family = "unix")]
+#[cfg(unix)]
 pub use self::unix::*;
 
 #[cfg(target_family = "windows")]
-compile_error!("windows not yet supported for the llvm-based compiler backend");
+compile_error!("windows not yet supported for the llvm-based compiler backend");

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);
+}

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

@@ -1,40 +0,0 @@
-
-extern "C" {
-    fn __register_frame(frame: *const u8);
-    fn __deregister_frame(frame: *const u8);
-}
-
-pub unsafe fn register_eh_frames(eh_frames: *const u8, num_bytes: usize) {
-    visit_frame_desc_entries(eh_frames, num_bytes, |frame| __register_frame(frame));
-}
-
-unsafe fn visit_frame_desc_entries<F>(eh_frames: *const u8, num_bytes: usize, visitor: F)
-where
-    F: Fn(*const u8),
-{
-    let mut next = eh_frames;
-    let mut end = eh_frames.add(num_bytes);
-
-    loop {
-        if next >= end {
-            break;
-        }
-
-        let cfi = next;
-        let mut cfi_num_bytes = (next as *const u32).read_unaligned() as u64;
-        assert!(cfi_num_bytes != 0);
-
-        next = next.add(4);
-        if num_bytes == 0xffffffff {
-            let cfi_num_bytes64 = (next as *const u64).read_unaligned();
-            cfi_num_bytes = cfi_num_bytes64;
-            next = next.add(8);
-        }
-
-        let cie_offset = (next as *const u32).read_unaligned();
-        if cie_offset != 0 {
-            visitor(cfi);
-        }
-        next = next.add(cfi_num_bytes as usize);
-    }
-}