[EH] Use _UA_SEARCH_PHASE in personality function (NFC) (#17991)

In two-phase unwinding, the first phase is the search phase
(`_UA_SEARCH_PHASE`) and the second one is the cleanup phase
(`_UA_CLEANUP_PHASE`). The search phase searches up the stack to see if
there is a matching catch handler, and if it finds one, it caches the
result. And in the second cleanup phase, it retrieves the cached result
(to avoid doing same work twice) and unwinds the stack.

Wasm does not do the two-phase unwinding; it only has a single phase. We
used `_UA_CLEANUP_PHASE` for this single phase, so in Wasm the cleanup
phase is supposed to the search. So we several many custom `#ifdef`s to
use the code guarded by `_UA_SEARCH_PHASE`, for example:
https://github.com/aheejin/emscripten/blob/d57db5bea1719319a680699c50b91fa3d88fa0ec/system/lib/libcxxabi/src/cxa_personality.cpp#L771-L776
https://github.com/aheejin/emscripten/blob/d57db5bea1719319a680699c50b91fa3d88fa0ec/system/lib/libcxxabi/src/cxa_personality.cpp#L850-L855

These are apparently gone in #14288, which replaced many `if`s with
`assert`s. This in effect removed our special handling for
`_UA_CLEANUP_PHASE`; there are several `assert`s that asserts the
current phase is `_UA_SEARCH_PHASE`, while Wasm is in
`_UA_CLEANUP_PHASE`. But this has not caused problems so far because we
have built libc++abi with `-NDEBUG`, so all assertions were no-op.
https://github.com/emscripten-core/emscripten/blob/40fb7d2071e439f1de614898b88518df582faa94/tools/system_libs.py#L1366
But this is now a problem because #17979 adds a debug build of
libc++abi, which enables assertions.

Come to think of it, I'm not sure why I decided to use
`_UA_CLEANUP_PHASE` for our single phase in the first place. If we use
`_UA_SEARCH_PHASE`, we can remove more our custom code and reduce the
difference between our port and the upstream library.

Author: aheejin

system/lib/libcxxabi/src/cxa_personality.cpp

@@ -798,6 +798,8 @@ static void scan_eh_tab(scan_results &results, _Unwind_Action actions,
                         {
                             // Native exception caught by exception
                             // specification.
+                            assert(actions & _UA_SEARCH_PHASE);
+                            results.ttypeIndex = ttypeIndex;
                             results.actionRecord = actionRecord;
                             results.adjustedPtr = adjustedPtr;
                             results.reason = _URC_HANDLER_FOUND;
@@ -964,6 +966,11 @@ __gxx_personality_v0
             exc->languageSpecificData = results.languageSpecificData;
             exc->catchTemp = reinterpret_cast<void*>(results.landingPad);
             exc->adjustedPtr = results.adjustedPtr;
+#ifdef __USING_WASM_EXCEPTIONS__
+            // Wasm only uses a single phase (_UA_SEARCH_PHASE), so save the
+            // results here.
+            set_registers(unwind_exception, context, results);
+#endif
         }
         return _URC_HANDLER_FOUND;
     }
@@ -981,16 +988,6 @@ __gxx_personality_v0
       exception_header->catchTemp = 0;
 #endif
     }
-#ifdef __USING_WASM_EXCEPTIONS__
-    // Wasm uses only one phase in _UA_CLEANUP_PHASE, so we should set
-    // these here.
-    __cxa_exception* exception_header = (__cxa_exception*)(unwind_exception+1) - 1;
-    exception_header->handlerSwitchValue = static_cast<int>(results.ttypeIndex);
-    exception_header->actionRecord = results.actionRecord;
-    exception_header->languageSpecificData = results.languageSpecificData;
-    exception_header->catchTemp = reinterpret_cast<void*>(results.landingPad);
-    exception_header->adjustedPtr = results.adjustedPtr;
-#endif
     return _URC_INSTALL_CONTEXT;
 }
 

system/lib/libunwind/src/Unwind-wasm.c

@@ -42,7 +42,7 @@ _Unwind_Reason_Code _Unwind_CallPersonality(void *exception_ptr) {
   // Call personality function. Wasm does not have two-phase unwinding, so we
   // only do the cleanup phase.
   _Unwind_Reason_Code ret = __gxx_personality_wasm0(
-      1, _UA_CLEANUP_PHASE, exception_object->exception_class, exception_object,
+      1, _UA_SEARCH_PHASE, exception_object->exception_class, exception_object,
       (struct _Unwind_Context *)&__wasm_lpad_context);
   return ret;
 }