1//===----------------------------------------------------------------------===//
2//
3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4// See https://llvm.org/LICENSE.txt for license information.
5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6//
7//
8// Implements Wasm exception handling proposal
9// (https://github.com/WebAssembly/exception-handling) based C++ exceptions
10//
11//===----------------------------------------------------------------------===//
12
13#include <stdbool.h>
14
15#include "config.h"
16
17#ifdef __WASM_EXCEPTIONS__
18
19#include "unwind.h"
20#include <threads.h>
21
22_Unwind_Reason_Code __gxx_personality_wasm0(int version, _Unwind_Action actions,
23 uint64_t exceptionClass,
24 _Unwind_Exception *unwind_exception,
25 _Unwind_Context *context);
26
27struct _Unwind_LandingPadContext {
28 // Input information to personality function
29 uintptr_t lpad_index; // landing pad index
30 uintptr_t lsda; // LSDA address
31
32 // Output information computed by personality function
33 uintptr_t selector; // selector value
34};
35
36// Communication channel between compiler-generated user code and personality
37// function
38thread_local struct _Unwind_LandingPadContext __wasm_lpad_context;
39
40/// Calls to this function are in landing pads in compiler-generated user code.
41/// In other EH schemes, stack unwinding is done by libunwind library, which
42/// calls the personality function for each frame it lands. On the other hand,
43/// WebAssembly stack unwinding process is performed by a VM, and the
44/// personality function cannot be called from there. So the compiler inserts a
45/// call to this function in landing pads in the user code, which in turn calls
46/// the personality function.
47_Unwind_Reason_Code _Unwind_CallPersonality(void *exception_ptr) {
48 struct _Unwind_Exception *exception_object =
49 (struct _Unwind_Exception *)exception_ptr;
50 _LIBUNWIND_TRACE_API("_Unwind_CallPersonality(exception_object=%p)",
51 (void *)exception_object);
52
53 // Reset the selector.
54 __wasm_lpad_context.selector = 0;
55
56 // Call personality function. Wasm does not have two-phase unwinding, so we
57 // only do the cleanup phase.
58 return __gxx_personality_wasm0(
59 1, _UA_SEARCH_PHASE, exception_object->exception_class, exception_object,
60 (struct _Unwind_Context *)&__wasm_lpad_context);
61}
62
63/// Called by __cxa_throw.
64_LIBUNWIND_EXPORT _Unwind_Reason_Code
65_Unwind_RaiseException(_Unwind_Exception *exception_object) {
66 _LIBUNWIND_TRACE_API("_Unwind_RaiseException(exception_object=%p)",
67 (void *)exception_object);
68 // Use Wasm EH's 'throw' instruction.
69 __builtin_wasm_throw(0, exception_object);
70}
71
72// Define the `__cpp_exception` symbol which `__builtin_wasm_throw` above will
73// reference. This is defined here in `libunwind` as the single canonical
74// definition for this API and it's required for users to ensure that there's
75// only one copy of `libunwind` within a wasm module to ensure this is only
76// defined once and exactly once.
77__asm__(".globl __cpp_exception\n"
78#if defined(__wasm32__)
79 ".tagtype __cpp_exception i32\n"
80#elif defined(__wasm64__)
81 ".tagtype __cpp_exception i64\n"
82#else
83#error "Unsupported Wasm architecture"
84#endif
85 "__cpp_exception:\n");
86
87/// Called by __cxa_end_catch.
88_LIBUNWIND_EXPORT void
89_Unwind_DeleteException(_Unwind_Exception *exception_object) {
90 _LIBUNWIND_TRACE_API("_Unwind_DeleteException(ex_obj=%p)",
91 (void *)(exception_object));
92 if (exception_object->exception_cleanup != NULL)
93 (*exception_object->exception_cleanup)(_URC_FOREIGN_EXCEPTION_CAUGHT,
94 exception_object);
95}
96
97/// Called by personality handler to alter register values.
98_LIBUNWIND_EXPORT void _Unwind_SetGR(struct _Unwind_Context *context, int index,
99 uintptr_t value) {
100 _LIBUNWIND_TRACE_API("_Unwind_SetGR(context=%p, index=%d, value=%lu)",
101 (void *)context, index, value);
102 // We only use this function to set __wasm_lpad_context.selector field, which
103 // is index 1 in the personality function.
104 if (index == 1)
105 ((struct _Unwind_LandingPadContext *)context)->selector = value;
106}
107
108/// Called by personality handler to get instruction pointer.
109_LIBUNWIND_EXPORT uintptr_t _Unwind_GetIP(struct _Unwind_Context *context) {
110 // The result will be used as a 1-based index after decrementing 1, so we
111 // increment 2 here
112 uintptr_t result =
113 ((struct _Unwind_LandingPadContext *)context)->lpad_index + 2;
114 _LIBUNWIND_TRACE_API("_Unwind_GetIP(context=%p) => %lu", (void *)context,
115 result);
116 return result;
117}
118
119/// Not used in Wasm.
120_LIBUNWIND_EXPORT void _Unwind_SetIP(struct _Unwind_Context *context,
121 uintptr_t value) {}
122
123/// Called by personality handler to get LSDA for current frame.
124_LIBUNWIND_EXPORT uintptr_t
125_Unwind_GetLanguageSpecificData(struct _Unwind_Context *context) {
126 uintptr_t result = ((struct _Unwind_LandingPadContext *)context)->lsda;
127 _LIBUNWIND_TRACE_API("_Unwind_GetLanguageSpecificData(context=%p) => 0x%lx",
128 (void *)context, result);
129 return result;
130}
131
132/// Not used in Wasm.
133_LIBUNWIND_EXPORT uintptr_t
134_Unwind_GetRegionStart(struct _Unwind_Context *context) {
135 return 0;
136}
137
138#endif // defined(__WASM_EXCEPTIONS__)
139