Use setjmp/longjmp to handle LLVM exceptions.

lib/llvm-backend/cpp/object_loader.cpp

@@ -5,6 +5,41 @@
 extern "C" void __register_frame(uint8_t *);
 extern "C" void __deregister_frame(uint8_t *);
 
+struct UnwindPoint {
+    UnwindPoint *prev;
+    jmp_buf unwind_info;
+    std::unique_ptr<std::exception> exception;
+};
+
+static thread_local UnwindPoint *unwind_state = nullptr;
+
+void catch_unwind(std::function<void()>&& f) {
+    UnwindPoint current;
+    current.prev = unwind_state;
+    unwind_state = &current;
+
+    bool rethrow = false;
+
+    if(setjmp(current.unwind_info)) {
+        rethrow = true;
+    } else {
+        f();
+    }
+
+    unwind_state = current.prev;
+    if(rethrow) throw *current.exception;
+}
+
+void unsafe_unwind(std::exception *exception) {
+    UnwindPoint *state = unwind_state;
+    if(state) {
+        state->exception.reset(exception);
+        longjmp(state->unwind_info, 42);
+    } else {
+        abort();
+    }
+}
+
 struct MemoryManager : llvm::RuntimeDyld::MemoryManager {
 public:
   MemoryManager(callbacks_t callbacks) : callbacks(callbacks) {}

lib/llvm-backend/cpp/object_loader.hh

@@ -3,6 +3,8 @@
 #include <exception>
 #include <iostream>
 #include <sstream>
+#include <setjmp.h>
+#include <functional>
 
 #include <llvm/ExecutionEngine/RuntimeDyld.h>
 
@@ -48,7 +50,10 @@ typedef struct {
   size_t data, vtable;
 } box_any_t;
 
-struct WasmException {
+void catch_unwind(std::function<void()>&& f);
+[[noreturn]] void unsafe_unwind(std::exception *exception);
+
+struct WasmException : std::exception {
 public:
   virtual std::string description() const noexcept = 0;
 };
@@ -174,27 +179,29 @@ result_t module_load(const uint8_t *mem_ptr, size_t mem_size,
   return RESULT_OK;
 }
 
-[[noreturn]] void throw_trap(WasmTrap::Type ty) { throw WasmTrap(ty); }
+[[noreturn]] void throw_trap(WasmTrap::Type ty) { unsafe_unwind(new WasmTrap(ty)); }
 
 void module_delete(WasmModule *module) { delete module; }
 
 // Throw a fat pointer that's assumed to be `*mut dyn Any` on the rust
 // side.
 [[noreturn]] void throw_any(size_t data, size_t vtable) {
-  throw UserException(data, vtable);
+  unsafe_unwind(new UserException(data, vtable));
 }
 
 // Throw a pointer that's assumed to be codegen::BreakpointHandler on the
 // rust side.
 [[noreturn]] void throw_breakpoint(uintptr_t callback) {
-  throw BreakpointException(callback);
+  unsafe_unwind(new BreakpointException(callback));
 }
 
 bool invoke_trampoline(trampoline_t trampoline, void *ctx, void *func,
                        void *params, void *results, WasmTrap::Type *trap_out,
                        box_any_t *user_error, void *invoke_env) noexcept {
   try {
+    catch_unwind([trampoline, ctx, func, params, results]() {
         trampoline(ctx, func, params, results);
+    });
     return true;
   } catch (const WasmTrap &e) {
     *trap_out = e.type;