Don't unregister unwind tables after the JIT is done

Author: bjorn3

src/debuginfo/unwind.rs

@@ -72,32 +72,32 @@ impl UnwindContext {
     }
 
     #[cfg(feature = "jit")]
-    pub(crate) unsafe fn register_jit(
-        self,
-        jit_module: &cranelift_jit::JITModule,
-    ) -> Option<UnwindRegistry> {
+    pub(crate) unsafe fn register_jit(self, jit_module: &cranelift_jit::JITModule) {
         let mut eh_frame = EhFrame::from(super::emit::WriterRelocate::new(self.endian));
         self.frame_table.write_eh_frame(&mut eh_frame).unwrap();
 
         if eh_frame.0.writer.slice().is_empty() {
-            return None;
+            return;
         }
 
         let mut eh_frame = eh_frame.0.relocate_for_jit(jit_module);
 
         // GCC expects a terminating "empty" length, so write a 0 length at the end of the table.
         eh_frame.extend(&[0, 0, 0, 0]);
 
-        let mut registrations = Vec::new();
+        // FIXME support unregistering unwind tables once cranelift-jit supports deallocating
+        // individual functions
+        #[allow(unused_variables)]
+        let (eh_frame, eh_frame_len, _) = Vec::into_raw_parts(eh_frame);
 
         // =======================================================================
         // Everything after this line up to the end of the file is loosly based on
         // https://github.com/bytecodealliance/wasmtime/blob/4471a82b0c540ff48960eca6757ccce5b1b5c3e4/crates/jit/src/unwind/systemv.rs
         #[cfg(target_os = "macos")]
         {
             // On macOS, `__register_frame` takes a pointer to a single FDE
-            let start = eh_frame.as_ptr();
-            let end = start.add(eh_frame.len());
+            let start = eh_frame;
+            let end = start.add(eh_frame_len);
             let mut current = start;
 
             // Walk all of the entries in the frame table and register them
@@ -107,7 +107,6 @@ impl UnwindContext {
                 // Skip over the CIE
                 if current != start {
                     __register_frame(current);
-                    registrations.push(current as usize);
                 }
 
                 // Move to the next table entry (+4 because the length itself is not inclusive)
@@ -117,41 +116,12 @@ impl UnwindContext {
         #[cfg(not(target_os = "macos"))]
         {
             // On other platforms, `__register_frame` will walk the FDEs until an entry of length 0
-            let ptr = eh_frame.as_ptr();
-            __register_frame(ptr);
-            registrations.push(ptr as usize);
+            __register_frame(eh_frame);
         }
-
-        Some(UnwindRegistry { _frame_table: eh_frame, registrations })
     }
 }
 
-/// Represents a registry of function unwind information for System V ABI.
-pub(crate) struct UnwindRegistry {
-    _frame_table: Vec<u8>,
-    registrations: Vec<usize>,
-}
-
 extern "C" {
     // libunwind import
     fn __register_frame(fde: *const u8);
-    fn __deregister_frame(fde: *const u8);
-}
-
-impl Drop for UnwindRegistry {
-    fn drop(&mut self) {
-        unsafe {
-            // libgcc stores the frame entries as a linked list in decreasing sort order
-            // based on the PC value of the registered entry.
-            //
-            // As we store the registrations in increasing order, it would be O(N^2) to
-            // deregister in that order.
-            //
-            // To ensure that we just pop off the first element in the list upon every
-            // deregistration, walk our list of registrations backwards.
-            for fde in self.registrations.iter().rev() {
-                __deregister_frame(*fde as *const _);
-            }
-        }
-    }
 }

src/driver/jit.rs

@@ -80,12 +80,17 @@ pub(crate) fn run_jit(tcx: TyCtxt<'_>, backend_config: BackendConfig) -> ! {
     }
 
     crate::allocator::codegen(tcx, &mut jit_module, &mut cx.unwind_context);
-    crate::main_shim::maybe_create_entry_wrapper(tcx, &mut jit_module, &mut cx.unwind_context, true);
+    crate::main_shim::maybe_create_entry_wrapper(
+        tcx,
+        &mut jit_module,
+        &mut cx.unwind_context,
+        true,
+    );
 
     tcx.sess.abort_if_errors();
 
     jit_module.finalize_definitions();
-    let _unwind_register_guard = unsafe { cx.unwind_context.register_jit(&jit_module) };
+    unsafe { cx.unwind_context.register_jit(&jit_module) };
 
     println!(
         "Rustc codegen cranelift will JIT run the executable, because -Cllvm-args=mode=jit was passed"
@@ -147,7 +152,7 @@ extern "C" fn __clif_jit_fn(instance_ptr: *const Instance<'static>) -> *const u8
 
             assert!(cx.global_asm.is_empty());
             jit_module.finalize_definitions();
-            std::mem::forget(unsafe { cx.unwind_context.register_jit(&jit_module) });
+            unsafe { cx.unwind_context.register_jit(&jit_module) };
             jit_module.get_finalized_function(func_id)
         })
     })

src/lib.rs

@@ -1,4 +1,4 @@
-#![feature(rustc_private, decl_macro, never_type, hash_drain_filter)]
+#![feature(rustc_private, decl_macro, never_type, hash_drain_filter, vec_into_raw_parts)]
 #![warn(rust_2018_idioms)]
 #![warn(unused_lifetimes)]
 #![warn(unreachable_pub)]