diff --git a/artiq/compiler/builtins.py b/artiq/compiler/builtins.py
index 73035600d..903900540 100644
--- a/artiq/compiler/builtins.py
+++ b/artiq/compiler/builtins.py
@@ -72,6 +72,26 @@ class TRange(types.TMono):
         ])
 
 class TException(types.TMono):
+    # All exceptions share the same internal layout:
+    #  * Pointer to the unique global with the name of the exception (str)
+    #    (which also serves as the EHABI type_info).
+    #  * File, line and column where it was raised (str, int, int).
+    #  * Message, which can contain substitutions {0}, {1} and {2} (str).
+    #  * Three 64-bit integers, parameterizing the message (int(width=64)).
+
+
+    # Keep this in sync with the function ARTIQIRGenerator.alloc_exn.
+    attributes = OrderedDict([
+        ("__name__",    TStr()),
+        ("__file__",    TStr()),
+        ("__line__",    TInt(types.TValue(32))),
+        ("__col__",     TInt(types.TValue(32))),
+        ("__message__", TStr()),
+        ("__param0__",  TInt(types.TValue(64))),
+        ("__param1__",  TInt(types.TValue(64))),
+        ("__param2__",  TInt(types.TValue(64))),
+    ])
+
     def __init__(self, name="Exception"):
         super().__init__(name)
 
@@ -170,8 +190,12 @@ def is_range(typ, elt=None):
     else:
         return types.is_mono(typ, "range")
 
-def is_exception(typ):
-    return isinstance(typ.find(), TException)
+def is_exception(typ, name=None):
+    if name is None:
+        return isinstance(typ.find(), TException)
+    else:
+        return isinstance(typ.find(), TException) and \
+            typ.name == name
 
 def is_iterable(typ):
     typ = typ.find()
@@ -189,4 +213,5 @@ def is_collection(typ):
 
 def is_allocated(typ):
     return typ.fold(False, lambda accum, typ:
-        is_list(typ) or is_str(typ) or types.is_function(typ))
+        is_list(typ) or is_str(typ) or types.is_function(typ) or
+        is_exception(typ))
diff --git a/artiq/compiler/ir.py b/artiq/compiler/ir.py
index f94df0ed4..8e042e14e 100644
--- a/artiq/compiler/ir.py
+++ b/artiq/compiler/ir.py
@@ -29,6 +29,13 @@ class TOption(types.TMono):
 def is_option(typ):
     return isinstance(typ, TOption)
 
+class TExceptionTypeInfo(types.TMono):
+    def __init__(self):
+        super().__init__("exntypeinfo")
+
+def is_exn_typeinfo(typ):
+    return isinstance(typ, TExceptionTypeInfo)
+
 class Value:
     """
     An SSA value that keeps track of its uses.
@@ -620,7 +627,7 @@ class SetAttr(Instruction):
             assert value.type == obj.type.elts[attr]
         else:
             assert value.type == obj.type.attributes[attr]
-        super().__init__([obj, value], typ, name)
+        super().__init__([obj, value], builtins.TNone(), name)
         self.attr = attr
 
     def opcode(self):
@@ -1004,7 +1011,7 @@ class Invoke(Terminator):
     def opcode(self):
         return "invoke"
 
-    def function(self):
+    def target_function(self):
         return self.operands[0]
 
     def arguments(self):
@@ -1038,11 +1045,15 @@ class LandingPad(Terminator):
         super().__init__([], builtins.TException(), name)
         self.types = []
 
+    def clauses(self):
+        return zip(self.operands, self.types)
+
     def add_clause(self, target, typ):
         assert isinstance(target, BasicBlock)
-        assert builtins.is_exception(typ)
+        assert typ is None or builtins.is_exception(typ)
         self.operands.append(target)
-        self.types.append(typ.find())
+        self.types.append(typ.find() if typ is not None else None)
+        target.uses.add(self)
 
     def opcode(self):
         return "landingpad"
@@ -1050,5 +1061,8 @@ class LandingPad(Terminator):
     def _operands_as_string(self):
         table = []
         for typ, target in zip(self.types, self.operands):
-            table.append("{} => {}".format(types.TypePrinter().name(typ), target.as_operand()))
+            if typ is None:
+                table.append("... => {}".format(target.as_operand()))
+            else:
+                table.append("{} => {}".format(types.TypePrinter().name(typ), target.as_operand()))
         return "[{}]".format(", ".join(table))
diff --git a/artiq/compiler/module.py b/artiq/compiler/module.py
index 38bccda92..9903eaaeb 100644
--- a/artiq/compiler/module.py
+++ b/artiq/compiler/module.py
@@ -33,7 +33,7 @@ class Module:
         escape_validator.visit(self.typedtree)
         self.artiq_ir = artiq_ir_generator.visit(self.typedtree)
         dead_code_eliminator.process(self.artiq_ir)
-        local_access_validator.process(self.artiq_ir)
+        # local_access_validator.process(self.artiq_ir)
         self.llvm_ir = llvm_ir_generator.process(self.artiq_ir)
 
     @classmethod
diff --git a/artiq/compiler/transforms/artiq_ir_generator.py b/artiq/compiler/transforms/artiq_ir_generator.py
index b4d803134..62f7f995f 100644
--- a/artiq/compiler/transforms/artiq_ir_generator.py
+++ b/artiq/compiler/transforms/artiq_ir_generator.py
@@ -465,8 +465,17 @@ class ARTIQIRGenerator(algorithm.Visitor):
     def visit_Continue(self, node):
         self.append(ir.Branch(self.continue_target))
 
+    def raise_exn(self, exn):
+        loc_file = ir.Constant(self.current_loc.source_buffer.name, builtins.TStr())
+        loc_line = ir.Constant(self.current_loc.line(), builtins.TInt(types.TValue(32)))
+        loc_column = ir.Constant(self.current_loc.column(), builtins.TInt(types.TValue(32)))
+        self.append(ir.SetAttr(exn, "__file__", loc_file))
+        self.append(ir.SetAttr(exn, "__line__", loc_line))
+        self.append(ir.SetAttr(exn, "__col__", loc_column))
+        self.append(ir.Raise(exn))
+
     def visit_Raise(self, node):
-        self.append(ir.Raise(self.visit(node.exc)))
+        self.raise_exn(self.visit(node.exc))
 
     def visit_Try(self, node):
         dispatcher = self.add_block("try.dispatch")
@@ -521,15 +530,25 @@ class ARTIQIRGenerator(algorithm.Visitor):
             self.return_target = old_return
 
         handlers = []
+        has_catchall = False
         for handler_node in node.handlers:
-            handler = self.add_block("handler." + handler_node.name_type.find().name)
+            exn_type = handler_node.name_type.find()
+            if handler_node.filter is not None and \
+                    not builtins.is_exception(exn_type, 'Exception'):
+                handler = self.add_block("handler." + exn_type.name)
+                landingpad.add_clause(handler, exn_type)
+            else:
+                handler = self.add_block("handler.catchall")
+                landingpad.add_clause(handler, None)
+                has_catchall = True
+
             self.current_block = handler
             handlers.append(handler)
-            landingpad.add_clause(handler, handler_node.name_type)
 
             if handler_node.name is not None:
                 exn = self.append(ir.Builtin("exncast", [landingpad], handler_node.name_type))
                 self._set_local(handler_node.name, exn)
+
             self.visit(handler_node.body)
 
         if any(node.finalbody):
@@ -537,36 +556,44 @@ class ARTIQIRGenerator(algorithm.Visitor):
             self.current_block = finalizer
 
             self.visit(node.finalbody)
+            post_finalizer = self.current_block
 
-            if not self.current_block.is_terminated():
-                dest = self.append(ir.GetLocal(final_state, ".k"))
-                self.append(ir.IndirectBranch(dest, final_targets))
-
-        tail = self.add_block("try.tail")
+        self.current_block = tail = self.add_block("try.tail")
         if any(node.finalbody):
+            final_targets.append(tail)
+
             if self.break_target:
                 break_proxy.append(ir.Branch(finalizer))
             if self.continue_target:
                 continue_proxy.append(ir.Branch(finalizer))
             return_proxy.append(ir.Branch(finalizer))
-        if not body.is_terminated():
-            if any(node.finalbody):
+
+            if not body.is_terminated():
                 body.append(ir.SetLocal(final_state, ".k", tail))
                 body.append(ir.Branch(finalizer))
-                for handler in handlers:
-                    if not handler.is_terminated():
-                        handler.append(ir.SetLocal(final_state, ".k", tail))
-                        handler.append(ir.Branch(tail))
-            else:
-                body.append(ir.Branch(tail))
-                for handler in handlers:
-                    if not handler.is_terminated():
-                        handler.append(ir.Branch(tail))
 
-        if any(tail.predecessors()):
-            self.current_block = tail
+            if not has_catchall:
+                # Add a catch-all handler so that finally would have a chance
+                # to execute.
+                handler = self.add_block("handler.catchall")
+                landingpad.add_clause(handler, None)
+                handlers.append(handler)
+
+            for handler in handlers:
+                if not handler.is_terminated():
+                    handler.append(ir.SetLocal(final_state, ".k", tail))
+                    handler.append(ir.Branch(tail))
+
+            if not post_finalizer.is_terminated():
+                dest = post_finalizer.append(ir.GetLocal(final_state, ".k"))
+                post_finalizer.append(ir.IndirectBranch(dest, final_targets))
         else:
-            self.current_function.remove(tail)
+            if not body.is_terminated():
+                body.append(ir.Branch(tail))
+
+            for handler in handlers:
+                if not handler.is_terminated():
+                    handler.append(ir.Branch(tail))
 
     # TODO: With
 
@@ -655,15 +682,16 @@ class ARTIQIRGenerator(algorithm.Visitor):
         mapped_lt_len = self.append(ir.Compare(end_cmpop, mapped_index, length))
         in_bounds     = self.append(ir.Select(mapped_ge_0, mapped_lt_len,
                                               ir.Constant(False, builtins.TBool())))
+        head = self.current_block
 
-        out_of_bounds_block = self.add_block()
-        exn = out_of_bounds_block.append(ir.Alloc([], builtins.TIndexError()))
-        out_of_bounds_block.append(ir.Raise(exn))
+        self.current_block = out_of_bounds_block = self.add_block()
+        exn = self.alloc_exn(builtins.TIndexError(),
+            ir.Constant("index {0} out of bounds 0:{1}", builtins.TStr()),
+            index, length)
+        self.raise_exn(exn)
 
-        in_bounds_block = self.add_block()
-
-        self.append(ir.BranchIf(in_bounds, in_bounds_block, out_of_bounds_block))
-        self.current_block = in_bounds_block
+        self.current_block = in_bounds_block = self.add_block()
+        head.append(ir.BranchIf(in_bounds, in_bounds_block, out_of_bounds_block))
 
         return mapped_index
 
@@ -673,7 +701,7 @@ class ARTIQIRGenerator(algorithm.Visitor):
         cond_block = self.current_block
 
         self.current_block = body_block = self.add_block()
-        self.append(ir.Raise(exn_gen()))
+        self.raise_exn(exn_gen())
 
         self.current_block = tail_block = self.add_block()
         cond_block.append(ir.BranchIf(cond, tail_block, body_block))
@@ -736,9 +764,10 @@ class ARTIQIRGenerator(algorithm.Visitor):
 
             if node.slice.step is not None:
                 step = self.visit(node.slice.step)
-                self._make_check(self.append(ir.Compare(ast.NotEq(loc=None), step,
-                                                        ir.Constant(0, step.type))),
-                                 lambda: self.append(ir.Alloc([], builtins.TValueError())))
+                self._make_check(
+                    self.append(ir.Compare(ast.NotEq(loc=None), step, ir.Constant(0, step.type))),
+                    lambda: self.alloc_exn(builtins.TValueError(),
+                        ir.Constant("step cannot be zero", builtins.TStr())))
             else:
                 step = ir.Constant(1, node.slice.type)
             counting_up = self.append(ir.Compare(ast.Gt(loc=None), step,
@@ -755,8 +784,12 @@ class ARTIQIRGenerator(algorithm.Visitor):
             slice_size = self.append(ir.Select(rem_not_empty,
                                                slice_size_c, slice_size_a,
                                                name="slice.size"))
-            self._make_check(self.append(ir.Compare(ast.LtE(loc=None), slice_size, length)),
-                             lambda: self.append(ir.Alloc([], builtins.TValueError())))
+            self._make_check(
+                self.append(ir.Compare(ast.LtE(loc=None), slice_size, length)),
+                lambda: self.alloc_exn(builtins.TValueError(),
+                    ir.Constant("slice size {0} is larger than iterable length {1}",
+                                builtins.TStr()),
+                    slice_size, iterable_len))
 
             if self.current_assign is None:
                 is_neg_size = self.append(ir.Compare(ast.Lt(loc=None),
@@ -832,9 +865,12 @@ class ARTIQIRGenerator(algorithm.Visitor):
             return lst
         else:
             length = self.iterable_len(self.current_assign)
-            self._make_check(self.append(ir.Compare(ast.Eq(loc=None), length,
-                                                    ir.Constant(len(node.elts), self._size_type))),
-                             lambda: self.append(ir.Alloc([], builtins.TValueError())))
+            self._make_check(
+                self.append(ir.Compare(ast.Eq(loc=None), length,
+                                       ir.Constant(len(node.elts), self._size_type))),
+                lambda: self.alloc_exn(builtins.TValueError(),
+                    ir.Constant("list must be {0} elements long to decompose", builtins.TStr()),
+                    length))
 
             for index, elt_node in enumerate(node.elts):
                 elt = self.append(ir.GetElem(self.current_assign,
@@ -937,13 +973,15 @@ class ARTIQIRGenerator(algorithm.Visitor):
             rhs = self.visit(node.right)
             if isinstance(node.op, (ast.LShift, ast.RShift)):
                 # Check for negative shift amount.
-                self._make_check(self.append(ir.Compare(ast.GtE(loc=None), rhs,
-                                                        ir.Constant(0, rhs.type))),
-                                 lambda: self.append(ir.Alloc([], builtins.TValueError())))
+                self._make_check(
+                    self.append(ir.Compare(ast.GtE(loc=None), rhs, ir.Constant(0, rhs.type))),
+                    lambda: self.alloc_exn(builtins.TValueError(),
+                        ir.Constant("shift amount must be nonnegative", builtins.TStr())))
             elif isinstance(node.op, (ast.Div, ast.FloorDiv)):
-                self._make_check(self.append(ir.Compare(ast.NotEq(loc=None), rhs,
-                                                        ir.Constant(0, rhs.type))),
-                                 lambda: self.append(ir.Alloc([], builtins.TZeroDivisionError())))
+                self._make_check(
+                    self.append(ir.Compare(ast.NotEq(loc=None), rhs, ir.Constant(0, rhs.type))),
+                    lambda: self.alloc_exn(builtins.TZeroDivisionError(),
+                        ir.Constant("cannot divide by zero", builtins.TStr())))
 
             return self.append(ir.Arith(node.op, self.visit(node.left), rhs))
         elif isinstance(node.op, ast.Add): # list + list, tuple + tuple
@@ -1179,6 +1217,31 @@ class ARTIQIRGenerator(algorithm.Visitor):
                 result_tail.append(ir.Branch(tail))
         return phi
 
+    # Keep this function with builtins.TException.attributes.
+    def alloc_exn(self, typ, message=None, param0=None, param1=None, param2=None):
+        attributes = [
+            ir.Constant(typ.find().name, ir.TExceptionTypeInfo()),  # typeinfo
+            ir.Constant("<not thrown>", builtins.TStr()),           # file
+            ir.Constant(0, builtins.TInt(types.TValue(32))),        # line
+            ir.Constant(0, builtins.TInt(types.TValue(32))),        # column
+        ]
+
+        if message is None:
+            attributes.append(ir.Constant(typ.find().name, builtins.TStr()))
+        else:
+            attributes.append(message)                              # message
+
+        param_type = builtins.TInt(types.TValue(64))
+        for param in [param0, param1, param2]:
+            if param is None:
+                attributes.append(ir.Constant(0, builtins.TInt(types.TValue(64))))
+            else:
+                if param.type != param_type:
+                    param = self.append(ir.Coerce(param, param_type))
+                attributes.append(param)                            # paramN, N=0:2
+
+        return self.append(ir.Alloc(attributes, typ))
+
     def visit_builtin_call(self, node):
         # A builtin by any other name... Ignore node.func, just use the type.
         typ = node.func.type
@@ -1275,7 +1338,7 @@ class ARTIQIRGenerator(algorithm.Visitor):
                                    separator=" ", suffix="\n")
             return ir.Constant(None, builtins.TNone())
         elif types.is_exn_constructor(typ):
-            return self.append(ir.Alloc([self.visit(arg) for args in node.args], node.type))
+            return self.alloc_exn(node.type, *[self.visit(arg_node) for arg_node in node.args])
         else:
             assert False
 
@@ -1485,8 +1548,14 @@ class ARTIQIRGenerator(algorithm.Visitor):
 
                 format_string += ")"
             elif builtins.is_exception(value.type):
-                # TODO: print exceptions
-                assert False
+                name    = self.append(ir.GetAttr(value, "__name__"))
+                message = self.append(ir.GetAttr(value, "__message__"))
+                param1  = self.append(ir.GetAttr(value, "__param0__"))
+                param2  = self.append(ir.GetAttr(value, "__param1__"))
+                param3  = self.append(ir.GetAttr(value, "__param2__"))
+
+                format_string += "%s(%s, %lld, %lld, %lld)"
+                args += [name, message, param1, param2, param3]
             else:
                 assert False
 
diff --git a/artiq/compiler/transforms/inferencer.py b/artiq/compiler/transforms/inferencer.py
index 3422abf85..15604e5b8 100644
--- a/artiq/compiler/transforms/inferencer.py
+++ b/artiq/compiler/transforms/inferencer.py
@@ -440,23 +440,32 @@ class Inferencer(algorithm.Visitor):
             self.engine.process(diag)
 
         if types.is_exn_constructor(typ):
-            exns = {
-                "IndexError": builtins.TIndexError,
-                "ValueError": builtins.TValueError,
-            }
-            for exn in exns:
-                if types.is_exn_constructor(typ, exn):
-                    valid_forms = lambda: [
-                        valid_form("{exn}() -> {exn}".format(exn=exn))
-                    ]
+            valid_forms = lambda: [
+                valid_form("{exn}() -> {exn}".format(exn=typ.name)),
+                valid_form("{exn}(message:str) -> {exn}".format(exn=typ.name)),
+                valid_form("{exn}(message:str, param1:int(width=64)) -> {exn}".format(exn=typ.name)),
+                valid_form("{exn}(message:str, param1:int(width=64), "
+                           "param2:int(width=64)) -> {exn}".format(exn=typ.name)),
+                valid_form("{exn}(message:str, param1:int(width=64), "
+                           "param2:int(width=64), param3:int(width=64)) "
+                           "-> {exn}".format(exn=typ.name)),
+            ]
 
-                    if len(node.args) == 0 and len(node.keywords) == 0:
-                        pass # False
-                    else:
-                        diagnose(valid_forms())
+            if len(node.args) == 0 and len(node.keywords) == 0:
+                pass # Default message, zeroes as parameters
+            elif len(node.args) >= 1 and len(node.args) <= 4 and len(node.keywords) == 0:
+                message, *params = node.args
 
-                    self._unify(node.type, exns[exn](),
-                                node.loc, None)
+                self._unify(message.type, builtins.TStr(),
+                            message.loc, None)
+                for param in params:
+                    self._unify(param.type, builtins.TInt(types.TValue(64)),
+                                param.loc, None)
+            else:
+                diagnose(valid_forms())
+
+            self._unify(node.type, getattr(builtins, "T" + typ.name)(),
+                        node.loc, None)
         elif types.is_builtin(typ, "bool"):
             valid_forms = lambda: [
                 valid_form("bool() -> bool"),
@@ -829,26 +838,27 @@ class Inferencer(algorithm.Visitor):
     def visit_ExceptHandlerT(self, node):
         self.generic_visit(node)
 
-        if not types.is_exn_constructor(node.filter.type):
-            diag = diagnostic.Diagnostic("error",
-                "this expression must refer to an exception constructor",
-                {"type": types.TypePrinter().name(node.filter.type)},
-                node.filter.loc)
-            self.engine.process(diag)
-        else:
-            def makenotes(printer, typea, typeb, loca, locb):
-                return [
-                    diagnostic.Diagnostic("note",
-                        "expression of type {typea}",
-                        {"typea": printer.name(typea)},
-                        loca),
-                    diagnostic.Diagnostic("note",
-                        "constructor of an exception of type {typeb}",
-                        {"typeb": printer.name(typeb)},
-                        locb)
-                ]
-            self._unify(node.name_type, builtins.TException(node.filter.type.name),
-                        node.name_loc, node.filter.loc, makenotes)
+        if node.filter is not None:
+            if not types.is_exn_constructor(node.filter.type):
+                diag = diagnostic.Diagnostic("error",
+                    "this expression must refer to an exception constructor",
+                    {"type": types.TypePrinter().name(node.filter.type)},
+                    node.filter.loc)
+                self.engine.process(diag)
+            else:
+                def makenotes(printer, typea, typeb, loca, locb):
+                    return [
+                        diagnostic.Diagnostic("note",
+                            "expression of type {typea}",
+                            {"typea": printer.name(typea)},
+                            loca),
+                        diagnostic.Diagnostic("note",
+                            "constructor of an exception of type {typeb}",
+                            {"typeb": printer.name(typeb)},
+                            locb)
+                    ]
+                self._unify(node.name_type, builtins.TException(node.filter.type.name),
+                            node.name_loc, node.filter.loc, makenotes)
 
     def _type_from_arguments(self, node, ret):
         self.generic_visit(node)
@@ -943,6 +953,17 @@ class Inferencer(algorithm.Visitor):
             self._unify(self.function.return_type, node.value.type,
                         self.function.name_loc, node.value.loc, makenotes)
 
+    def visit_Raise(self, node):
+        self.generic_visit(node)
+
+        exc_type = node.exc.type
+        if not types.is_var(exc_type) and not builtins.is_exception(exc_type):
+            diag = diagnostic.Diagnostic("error",
+                "cannot raise a value of type {type}, which is not an exception",
+                {"type": types.TypePrinter().name(exc_type)},
+                node.exc.loc)
+            self.engine.process(diag)
+
     def visit_Assert(self, node):
         self.generic_visit(node)
         self._unify(node.test.type, builtins.TBool(),
diff --git a/artiq/compiler/transforms/llvm_ir_generator.py b/artiq/compiler/transforms/llvm_ir_generator.py
index de7a75307..79f214097 100644
--- a/artiq/compiler/transforms/llvm_ir_generator.py
+++ b/artiq/compiler/transforms/llvm_ir_generator.py
@@ -43,7 +43,7 @@ class LLVMIRGenerator:
             return ll.IntType(builtins.get_int_width(typ))
         elif builtins.is_float(typ):
             return ll.DoubleType()
-        elif builtins.is_str(typ):
+        elif builtins.is_str(typ) or ir.is_exn_typeinfo(typ):
             return ll.IntType(8).as_pointer()
         elif builtins.is_list(typ):
             lleltty = self.llty_of_type(builtins.get_iterable_elt(typ))
@@ -51,11 +51,8 @@ class LLVMIRGenerator:
         elif builtins.is_range(typ):
             lleltty = self.llty_of_type(builtins.get_iterable_elt(typ))
             return ll.LiteralStructType([lleltty, lleltty, lleltty])
-        elif builtins.is_exception(typ):
-            # TODO: hack before EH is working
-            return ll.LiteralStructType([])
         elif ir.is_basic_block(typ):
-            return ll.LabelType()
+            return ll.IntType(8).as_pointer()
         elif ir.is_option(typ):
             return ll.LiteralStructType([ll.IntType(1), self.llty_of_type(typ.params["inner"])])
         elif ir.is_environment(typ):
@@ -65,8 +62,21 @@ class LLVMIRGenerator:
                 return llty
             else:
                 return llty.as_pointer()
-        else:
-            assert False
+        else: # Catch-all for exceptions and custom classes
+            if builtins.is_exception(typ):
+                name = 'Exception' # they all share layout
+            else:
+                name = typ.name
+
+            llty = self.llcontext.get_identified_type(name)
+            if llty.elements is None:
+                llty.elements = [self.llty_of_type(attrtyp)
+                                 for attrtyp in typ.attributes.values()]
+
+            if bare or not builtins.is_allocated(typ):
+                return llty
+            else:
+                return llty.as_pointer()
 
     def llconst_of_const(self, const):
         llty = self.llty_of_type(const.type)
@@ -79,14 +89,27 @@ class LLVMIRGenerator:
         elif isinstance(const.value, (int, float)):
             return ll.Constant(llty, const.value)
         elif isinstance(const.value, str):
-            as_bytes = (const.value + '\0').encode('utf-8')
+            assert "\0" not in const.value
+
+            as_bytes = (const.value + "\0").encode("utf-8")
+            if ir.is_exn_typeinfo(const.type):
+                # Exception typeinfo; should be merged with identical others
+                name = "__artiq_exn_" + const.value
+                linkage = "linkonce"
+                unnamed_addr = False
+            else:
+                # Just a string
+                name = self.llmodule.get_unique_name("str")
+                linkage = "private"
+                unnamed_addr = True
+
             llstrty = ll.ArrayType(ll.IntType(8), len(as_bytes))
-            llconst = ll.GlobalVariable(self.llmodule, llstrty,
-                                        name=self.llmodule.get_unique_name("str"))
+            llconst = ll.GlobalVariable(self.llmodule, llstrty, name)
             llconst.global_constant = True
-            llconst.unnamed_addr = True
-            llconst.linkage = 'internal'
             llconst.initializer = ll.Constant(llstrty, bytearray(as_bytes))
+            llconst.linkage = linkage
+            llconst.unnamed_addr = unnamed_addr
+
             return llconst.bitcast(ll.IntType(8).as_pointer())
         else:
             assert False
@@ -112,6 +135,10 @@ class LLVMIRGenerator:
             llty = ll.FunctionType(ll.DoubleType(), [ll.DoubleType(), ll.DoubleType()])
         elif name == "printf":
             llty = ll.FunctionType(ll.VoidType(), [ll.IntType(8).as_pointer()], var_arg=True)
+        elif name == "__artiq_raise":
+            llty = ll.FunctionType(ll.VoidType(), [self.llty_of_type(builtins.TException())])
+        elif name == "__artiq_personality":
+            llty = ll.FunctionType(ll.IntType(32), [], var_arg=True)
         else:
             assert False
         return ll.Function(self.llmodule, llty, name)
@@ -124,8 +151,10 @@ class LLVMIRGenerator:
         elif isinstance(value, ir.Function):
             llfun = self.llmodule.get_global(value.name)
             if llfun is None:
-                return ll.Function(self.llmodule, self.llty_of_type(value.type, bare=True),
-                                   value.name)
+                llfun = ll.Function(self.llmodule, self.llty_of_type(value.type, bare=True),
+                                    value.name)
+                llfun.linkage = 'internal'
+                return llfun
             else:
                 return llfun
         else:
@@ -186,6 +215,9 @@ class LLVMIRGenerator:
         self.fixups.append(fixup)
         return llinsn
 
+    def llindex(self, index):
+        return ll.Constant(ll.IntType(32), index)
+
     def process_Alloc(self, insn):
         if ir.is_environment(insn.type):
             return self.llbuilder.alloca(self.llty_of_type(insn.type, bare=True),
@@ -210,6 +242,13 @@ class LLVMIRGenerator:
                                             size=llsize)
             llvalue = self.llbuilder.insert_value(llvalue, llalloc, 1, name=insn.name)
             return llvalue
+        elif builtins.is_exception(insn.type):
+            llalloc = self.llbuilder.alloca(self.llty_of_type(insn.type, bare=True))
+            for index, operand in enumerate(insn.operands):
+                lloperand = self.map(operand)
+                llfieldptr = self.llbuilder.gep(llalloc, [self.llindex(0), self.llindex(index)])
+                self.llbuilder.store(lloperand, llfieldptr)
+            return llalloc
         elif builtins.is_allocated(insn.type):
             assert False
         else: # immutable
@@ -219,9 +258,6 @@ class LLVMIRGenerator:
             llvalue.name = insn.name
             return llvalue
 
-    def llindex(self, index):
-        return ll.Constant(ll.IntType(32), index)
-
     def llptr_to_var(self, llenv, env_ty, var_name):
         if var_name in env_ty.params:
             var_index = list(env_ty.params.keys()).index(var_name)
@@ -241,6 +277,8 @@ class LLVMIRGenerator:
         env = insn.environment()
         llptr = self.llptr_to_var(self.map(env), env.type, insn.var_name)
         llvalue = self.map(insn.value())
+        if isinstance(llvalue, ll.Block):
+            llvalue = ll.BlockAddress(self.llfunction, llvalue)
         if llptr.type.pointee != llvalue.type:
             # The environment argument is an i8*, so that all closures can
             # unify with each other regardless of environment type or size.
@@ -266,11 +304,11 @@ class LLVMIRGenerator:
             return self.llbuilder.load(llptr)
 
     def process_SetAttr(self, insn):
-        assert builtins.is_allocated(insns.object().type)
+        assert builtins.is_allocated(insn.object().type)
         llptr = self.llbuilder.gep(self.map(insn.object()),
                                    [self.llindex(0), self.llindex(self.attr_index(insn))],
                                    name=insn.name)
-        return self.llbuilder.store(llptr, self.map(insn.value()))
+        return self.llbuilder.store(self.map(insn.value()), llptr)
 
     def process_GetElem(self, insn):
         llelts = self.llbuilder.extract_value(self.map(insn.list()), 1)
@@ -483,7 +521,9 @@ class LLVMIRGenerator:
             llargs = map(self.map, insn.operands)
             return self.llbuilder.call(self.llbuiltin("printf"), llargs,
                                        name=insn.name)
-        # elif insn.op == "exncast":
+        elif insn.op == "exncast":
+            # This is an identity cast at LLVM IR level.
+            return self.map(insn.operands[0])
         else:
             assert False
 
@@ -495,13 +535,24 @@ class LLVMIRGenerator:
                                               name=insn.name)
         return llvalue
 
-    def process_Call(self, insn):
+    def prepare_call(self, insn):
         llclosure, llargs = self.map(insn.target_function()), map(self.map, insn.arguments())
         llenv = self.llbuilder.extract_value(llclosure, 0)
         llfun = self.llbuilder.extract_value(llclosure, 1)
-        return self.llbuilder.call(llfun, [llenv] + list(llargs),
+        return llfun, [llenv] + list(llargs)
+
+    def process_Call(self, insn):
+        llfun, llargs = self.prepare_call(insn)
+        return self.llbuilder.call(llfun, llargs,
                                    name=insn.name)
 
+    def process_Invoke(self, insn):
+        llfun, llargs = self.prepare_call(insn)
+        llnormalblock = self.map(insn.normal_target())
+        llunwindblock = self.map(insn.exception_target())
+        return self.llbuilder.invoke(llfun, llargs, llnormalblock, llunwindblock,
+                                     name=insn.name)
+
     def process_Select(self, insn):
         return self.llbuilder.select(self.map(insn.condition()),
                                      self.map(insn.if_true()), self.map(insn.if_false()))
@@ -513,8 +564,11 @@ class LLVMIRGenerator:
         return self.llbuilder.cbranch(self.map(insn.condition()),
                                       self.map(insn.if_true()), self.map(insn.if_false()))
 
-    # def process_IndirectBranch(self, insn):
-    #     pass
+    def process_IndirectBranch(self, insn):
+        llinsn = self.llbuilder.branch_indirect(self.map(insn.target()))
+        for dest in insn.destinations():
+            llinsn.add_destination(self.map(dest))
+        return llinsn
 
     def process_Return(self, insn):
         if builtins.is_none(insn.value().type):
@@ -526,15 +580,33 @@ class LLVMIRGenerator:
         return self.llbuilder.unreachable()
 
     def process_Raise(self, insn):
-        # TODO: hack before EH is working
-        llinsn = self.llbuilder.call(self.llbuiltin("llvm.trap"), [],
+        arg = self.map(insn.operands[0])
+        llinsn = self.llbuilder.call(self.llbuiltin("__artiq_raise"), [arg],
                                      name=insn.name)
+        llinsn.attributes.add('noreturn')
         self.llbuilder.unreachable()
         return llinsn
 
-    # def process_Invoke(self, insn):
-    #     pass
+    def process_LandingPad(self, insn):
+        lllandingpad = self.llbuilder.landingpad(self.llty_of_type(insn.type),
+                                                 self.llbuiltin("__artiq_personality"))
+        llexnnameptr = self.llbuilder.gep(lllandingpad, [self.llindex(0), self.llindex(0)])
+        llexnname = self.llbuilder.load(llexnnameptr)
 
-    # def process_LandingPad(self, insn):
-    #     pass
+        for target, typ in insn.clauses():
+            if typ is None:
+                llclauseexnname = ll.Constant(
+                    self.llty_of_type(ir.TExceptionTypeInfo()), None)
+            else:
+                llclauseexnname = self.llconst_of_const(
+                    ir.Constant(typ.name, ir.TExceptionTypeInfo()))
+            lllandingpad.add_clause(ll.CatchClause(llclauseexnname))
+
+            llmatchingclause = self.llbuilder.icmp_unsigned('==', llexnname, llclauseexnname)
+            with self.llbuilder.if_then(llmatchingclause):
+                self.llbuilder.branch(self.map(target))
+
+        self.llbuilder.resume(lllandingpad)
+
+        return lllandingpad
 
diff --git a/artiq/compiler/types.py b/artiq/compiler/types.py
index a84c1777e..a614ce637 100644
--- a/artiq/compiler/types.py
+++ b/artiq/compiler/types.py
@@ -85,7 +85,9 @@ class TMono(Type):
     A monomorphic type, possibly parametric.
 
     :class:`TMono` is supposed to be subclassed by builtin types,
-    unlike all other :class:`Type` descendants.
+    unlike all other :class:`Type` descendants. Similarly,
+    instances of :class:`TMono` should never be allocated directly,
+    as that will break the type-sniffing code in :mod:`builtins`.
     """
 
     attributes = OrderedDict()
diff --git a/artiq/compiler/validators/escape.py b/artiq/compiler/validators/escape.py
index 9f73b936e..df87c6afc 100644
--- a/artiq/compiler/validators/escape.py
+++ b/artiq/compiler/validators/escape.py
@@ -239,13 +239,17 @@ class EscapeValidator(algorithm.Visitor):
     # Only three ways for a pointer to escape:
     #   * Assigning or op-assigning it (we ensure an outlives relationship)
     #   * Returning it (we only allow returning values that live forever)
-    #   * Raising it (we forbid raising mutable data)
+    #   * Raising it (we forbid allocating exceptions that refer to mutable data)¹
     #
     # Literals doesn't count: a constructed object is always
     # outlived by all its constituents.
     # Closures don't count: see above.
     # Calling functions doesn't count: arguments never outlive
     # the function body.
+    #
+    # ¹Strings are currently never allocated with a limited lifetime,
+    # and exceptions can only refer to strings, so we don't actually check
+    # this property. But we will need to, if string operations are ever added.
 
     def visit_assignment(self, target, value, is_aug_assign=False):
         target_region = self._region_of(target)
@@ -298,14 +302,3 @@ class EscapeValidator(algorithm.Visitor):
                 "cannot return a mutable value that does not live forever", {},
                 node.value.loc, notes=self._diagnostics_for(region, node.value.loc) + [note])
             self.engine.process(diag)
-
-    def visit_Raise(self, node):
-        if builtins.is_allocated(node.exc.type):
-            note = diagnostic.Diagnostic("note",
-                "this expression has type {type}",
-                {"type": types.TypePrinter().name(node.exc.type)},
-                node.exc.loc)
-            diag = diagnostic.Diagnostic("error",
-                "cannot raise a mutable value", {},
-                node.exc.loc, notes=[note])
-            self.engine.process(diag)
diff --git a/lit-test/compiler/inferencer/unify.py b/lit-test/compiler/inferencer/unify.py
index 48681fdcb..c6f41b495 100644
--- a/lit-test/compiler/inferencer/unify.py
+++ b/lit-test/compiler/inferencer/unify.py
@@ -56,3 +56,18 @@ lambda x, y=1: x
 
 k = "x"
 # CHECK-L: k:str
+
+IndexError()
+# CHECK-L: IndexError:<constructor IndexError>():IndexError
+
+IndexError("x")
+# CHECK-L: IndexError:<constructor IndexError>("x":str):IndexError
+
+IndexError("x", 1)
+# CHECK-L: IndexError:<constructor IndexError>("x":str, 1:int(width=64)):IndexError
+
+IndexError("x", 1, 1)
+# CHECK-L: IndexError:<constructor IndexError>("x":str, 1:int(width=64), 1:int(width=64)):IndexError
+
+IndexError("x", 1, 1, 1)
+# CHECK-L: IndexError:<constructor IndexError>("x":str, 1:int(width=64), 1:int(width=64), 1:int(width=64)):IndexError