libunwind/src/Unwind-wasm.c

   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
  27 struct _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
  38 thread_local struct _Unwind_LandingPadContext __wasm_lpad_context;
  39
  40 /// Calls to this function is 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 each frame it lands. On the other
  43 /// hand, WebAssembly stack unwinding process is performed by a VM, and the
  44 /// personality function cannot be called from there. So the compiler inserts
  45 /// a call to this function in landing pads in the user code, which in turn
  46 /// calls 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 /// Called by __cxa_end_catch.
  73 _LIBUNWIND_EXPORT void
  74 _Unwind_DeleteException(_Unwind_Exception *exception_object) {
  75   _LIBUNWIND_TRACE_API("_Unwind_DeleteException(ex_obj=%p)",
  76                        (void *)(exception_object));
  77   if (exception_object->exception_cleanup != NULL)
  78     (*exception_object->exception_cleanup)(_URC_FOREIGN_EXCEPTION_CAUGHT,
  79                                            exception_object);
  80 }
  81
  82 /// Called by personality handler to alter register values.
  83 _LIBUNWIND_EXPORT void _Unwind_SetGR(struct _Unwind_Context *context, int index,
  84                                      uintptr_t value) {
  85   _LIBUNWIND_TRACE_API("_Unwind_SetGR(context=%p, index=%d, value=%lu)",
  86                        (void *)context, index, value);
  87   // We only use this function to set __wasm_lpad_context.selector field, which
  88   // is index 1 in the personality function.
  89   if (index == 1)
  90     ((struct _Unwind_LandingPadContext *)context)->selector = value;
  91 }
  92
  93 /// Called by personality handler to get instruction pointer.
  94 _LIBUNWIND_EXPORT uintptr_t _Unwind_GetIP(struct _Unwind_Context *context) {
  95   // The result will be used as an 1-based index after decrementing 1, so we
  96   // increment 2 here
  97   uintptr_t result =
  98       ((struct _Unwind_LandingPadContext *)context)->lpad_index + 2;
  99   _LIBUNWIND_TRACE_API("_Unwind_GetIP(context=%p) => %lu", (void *)context,
 100                        result);
 101   return result;
 102 }
 103
 104 /// Not used in Wasm.
 105 _LIBUNWIND_EXPORT void _Unwind_SetIP(struct _Unwind_Context *context,
 106                                      uintptr_t value) {}
 107
 108 /// Called by personality handler to get LSDA for current frame.
 109 _LIBUNWIND_EXPORT uintptr_t
 110 _Unwind_GetLanguageSpecificData(struct _Unwind_Context *context) {
 111   uintptr_t result = ((struct _Unwind_LandingPadContext *)context)->lsda;
 112   _LIBUNWIND_TRACE_API("_Unwind_GetLanguageSpecificData(context=%p) => 0x%lx",
 113                        (void *)context, result);
 114   return result;
 115 }
 116
 117 /// Not used in Wasm.
 118 _LIBUNWIND_EXPORT uintptr_t
 119 _Unwind_GetRegionStart(struct _Unwind_Context *context) {
 120   return 0;
 121 }
 122
 123 #endif // defined(__WASM_EXCEPTIONS__)