[EmscriptenEH] Always use custom JS class for C/C++ exceptions

This has a slight codesize cost but it means we no longer have to worry
about the ambiguity in when we catch a Number.

Author: sbc100

src/lib/libcore.js

@@ -484,11 +484,7 @@ addToLibrary({
   // a proxy and declare the dependency here.
   _emscripten_throw_longjmp__deps: ['setThrew'],
   _emscripten_throw_longjmp: () => {
-#if EXCEPTION_STACK_TRACES
     throw new EmscriptenSjLj;
-#else
-    throw Infinity;
-#endif
   },
 #elif !SUPPORT_LONGJMP
 #if !INCLUDE_FULL_LIBRARY

src/lib/libdylink.js

@@ -93,16 +93,9 @@ var LibraryDylink = {
     } catch(e) {
       stackRestore(sp);
       // Create a try-catch guard that rethrows the Emscripten EH exception.
-#if EXCEPTION_STACK_TRACES
       // Exceptions thrown from C++ and longjmps will be an instance of
       // EmscriptenEH.
       if (!(e instanceof EmscriptenEH)) throw e;
-#else
-      // Exceptions thrown from C++ will be a pointer (number) and longjmp
-      // will throw the number Infinity. Use the compact and fast "e !== e+0"
-      // test to check if e was not a Number.
-      if (e !== e+0) throw e;
-#endif
       _setThrew(1, 0);
 #if WASM_BIGINT
       // In theory this if statement could be done on

src/lib/libemval.js

@@ -393,35 +393,18 @@ ${functionBody}
   },
 
   _emval_throw__deps: ['$Emval',
-#if !WASM_EXCEPTIONS
+#if !WASM_EXCEPTIONS && !DISABLE_EXCEPTION_CATCHING
     '$exceptionLast',
 #endif
   ],
   _emval_throw: (object) => {
     object = Emval.toValue(object);
-#if !WASM_EXCEPTIONS
+#if !WASM_EXCEPTIONS && !DISABLE_EXCEPTION_CATCHING
     // If we are throwing Emcripten C++ exception, set exceptionLast, as we do
-    // in __cxa_throw. When EXCEPTION_STACK_TRACES is set, a C++ exception will
-    // be an instance of EmscriptenEH, and when EXCEPTION_STACK_TRACES is not
-    // set, it will be a pointer (number).
-    //
-    // This is different from __cxa_throw() in libexception.js because
-    // __cxa_throw() is called from the user C++ code when the 'throw' keyword
-    // is used, and the value thrown is a C++ pointer. When
-    // EXCEPTION_STACK_TRACES is true, we wrap it with CppException. But this
-    // _emval_throw is called when we throw whatever is contained in 'object',
-    // which can be anything including a CppException object, or a number, or
-    // other JS object. So we don't use storeException() wrapper here and we
-    // throw it as is.
-#if EXCEPTION_STACK_TRACES
+    // in __cxa_throw. C++ exception will be an instance of CppEmscripten.
     if (object instanceof CppException) {
       exceptionLast = object;
     }
-#else
-    if (object === object+0) { // Check if it is a number
-      exceptionLast = object;
-    }
-#endif
 #endif
     throw object;
   },

src/lib/libexceptions.js

@@ -7,7 +7,9 @@
 var LibraryExceptions = {
 #if !WASM_EXCEPTIONS
   $uncaughtExceptionCount: '0',
-  $exceptionLast: '0',
+#if !DISABLE_EXCEPTION_CATCHING
+  $exceptionLast: null,
+#endif
   $exceptionCaught: ' []',
 
   // This class is the exception metadata which is prepended to each thrown object (in WASM memory).
@@ -82,14 +84,15 @@ var LibraryExceptions = {
 
   // Here, we throw an exception after recording a couple of values that we need to remember
   // We also remember that it was the last exception thrown as we need to know that later.
-  __cxa_throw__deps: ['$ExceptionInfo', '$exceptionLast', '$uncaughtExceptionCount',
+  __cxa_throw__deps: ['$ExceptionInfo', '$uncaughtExceptionCount',
 #if !DISABLE_EXCEPTION_CATCHING
+    '$exceptionLast',
     '__cxa_increment_exception_refcount',
 #endif
 #if EXCEPTION_STACK_TRACES
-    // When EXCEPTION_STACK_TRACES is enabled, storeException contains a call to
-    // 'new CppException', whose constructor calls getExceptionMessage. We can't
-    // track the dependency there, so we track it here.
+    // When EXCEPTION_STACK_TRACES is enabled, the 'CppException' constructor
+    // calls getExceptionMessage. We can't track the dependency there, so we
+    // track it here.
     '$getExceptionMessage',
     // These functions can be necessary to prevent memory leaks from the JS
     // side. Even though they are not used it here directly, we export them when
@@ -106,17 +109,18 @@ var LibraryExceptions = {
     info.init(type, destructor);
 #if !DISABLE_EXCEPTION_CATCHING
     ___cxa_increment_exception_refcount(ptr);
+    exceptionLast = new CppException(ptr);
 #endif
-    {{{ storeException('exceptionLast', 'ptr') }}}
     uncaughtExceptionCount++;
-    {{{ makeThrow('exceptionLast') }}}
+    {{{ makeThrow() }}}
   },
 
   // This exception will be caught twice, but while begin_catch runs twice,
   // we early-exit from end_catch when the exception has been rethrown, so
   // pop that here from the caught exceptions.
-  __cxa_rethrow__deps: ['$exceptionCaught', '$exceptionLast', '$uncaughtExceptionCount',
+  __cxa_rethrow__deps: ['$exceptionCaught', '$uncaughtExceptionCount',
 #if !DISABLE_EXCEPTION_CATCHING
+    '$exceptionLast',
     '__cxa_increment_exception_refcount',
 #endif
   ],
@@ -140,8 +144,10 @@ var LibraryExceptions = {
     dbg('__cxa_rethrow, popped ' +
       [ptrToString(ptr), exceptionLast, 'stack', exceptionCaught]);
 #endif
-    {{{ storeException('exceptionLast', 'ptr') }}}
-    {{{ makeThrow('exceptionLast') }}}
+#if !DISABLE_EXCEPTION_CATCHING
+    exceptionLast = new CppException(ptr);
+#endif
+    {{{ makeThrow() }}}
   },
 
   llvm_eh_typeid_for: (type) => type,
@@ -166,7 +172,11 @@ var LibraryExceptions = {
   // and free the exception. Note that if the dynCall on the destructor fails
   // due to calling apply on undefined, that means that the destructor is
   // an invalid index into the FUNCTION_TABLE, so something has gone wrong.
-  __cxa_end_catch__deps: ['$exceptionCaught', '$exceptionLast', '__cxa_decrement_exception_refcount', 'setThrew'],
+  __cxa_end_catch__deps: ['$exceptionCaught', '__cxa_decrement_exception_refcount', 'setThrew',
+#if !DISABLE_EXCEPTION_CATCHING
+    '$exceptionLast',
+#endif
+  ],
   __cxa_end_catch: () => {
     // Clear state flag.
     _setThrew(0, 0);
@@ -177,10 +187,12 @@ var LibraryExceptions = {
     var info = exceptionCaught.pop();
 
 #if EXCEPTION_DEBUG
-    dbg('__cxa_end_catch popped ' + [info, exceptionLast, 'stack', exceptionCaught]);
+    dbg('__cxa_end_catch popped ' + [info, 'stack', exceptionCaught]);
 #endif
     ___cxa_decrement_exception_refcount(info.excPtr);
-    exceptionLast = 0; // XXX in decRef?
+#if !DISABLE_EXCEPTION_CATCHING
+    exceptionLast = null; // XXX in decRef?
+#endif
   },
 
   __cxa_uncaught_exceptions__deps: ['$uncaughtExceptionCount'],
@@ -224,14 +236,15 @@ var LibraryExceptions = {
   // unwinding using 'if' blocks around each function, so the remaining
   // functionality boils down to picking a suitable 'catch' block.
   // We'll do that here, instead, to keep things simpler.
+#if !DISABLE_EXCEPTION_CATCHING
   $findMatchingCatch__deps: ['$exceptionLast', '$ExceptionInfo', '__cxa_can_catch', '$setTempRet0'],
+#endif
   $findMatchingCatch: (args) => {
-    var thrown =
-#if EXCEPTION_STACK_TRACES
-      exceptionLast?.excPtr;
+#if DISABLE_EXCEPTION_CATCHING
+    setTempRet0(0);
+    return 0;
 #else
-      exceptionLast;
-#endif
+    var thrown = exceptionLast?.excPtr;
     if (!thrown) {
       // just pass through the null ptr
       setTempRet0(0);
@@ -270,17 +283,22 @@ var LibraryExceptions = {
     }
     setTempRet0(thrownType);
     return thrown;
+#endif
   },
 
+#if !DISABLE_EXCEPTION_CATCHING
   __resumeException__deps: ['$exceptionLast'],
+#endif
   __resumeException: (ptr) => {
+#if !DISABLE_EXCEPTION_CATCHING
 #if EXCEPTION_DEBUG
     dbg("__resumeException " + [ptrToString(ptr), exceptionLast]);
 #endif
     if (!exceptionLast) {
-      {{{ storeException('exceptionLast', 'ptr') }}}
+      exceptionLast = new CppException(ptr);
     }
-    {{{ makeThrow('exceptionLast') }}}
+#endif
+    {{{ makeThrow() }}}
   },
 
 #endif

src/parseTools.mjs

@@ -648,22 +648,21 @@ export function makeReturn64(value) {
   return `(setTempRet0(${pair[1]}), ${pair[0]})`;
 }
 
-function makeThrow(excPtr) {
-  if (ASSERTIONS && DISABLE_EXCEPTION_CATCHING) {
-    var assertInfo =
-      'Exception thrown, but exception catching is not enabled. Compile with -sNO_DISABLE_EXCEPTION_CATCHING or -sEXCEPTION_CATCHING_ALLOWED=[..] to catch.';
-    if (MAIN_MODULE) {
-      assertInfo +=
-        ' (note: in dynamic linking, if a side module wants exceptions, the main module must be built with that support)';
+function makeThrow() {
+  if (DISABLE_EXCEPTION_CATCHING) {
+    if (ASSERTIONS) {
+      var assertInfo =
+        'Exception thrown, but exception catching is not enabled. Compile with -sNO_DISABLE_EXCEPTION_CATCHING or -sEXCEPTION_CATCHING_ALLOWED=[..] to catch.';
+      if (MAIN_MODULE) {
+        assertInfo +=
+          ' (note: in dynamic linking, if a side module wants exceptions, the main module must be built with that support)';
+      }
+      return `assert(false, '${assertInfo}');`;
+    } else {
+      return 'abort()';
     }
-    return `assert(false, '${assertInfo}');`;
   }
-  return `throw ${excPtr};`;
-}
-
-function storeException(varName, excPtr) {
-  var exceptionToStore = EXCEPTION_STACK_TRACES ? `new CppException(${excPtr})` : `${excPtr}`;
-  return `${varName} = ${exceptionToStore};`;
+  return 'throw exceptionLast;';
 }
 
 function charCode(char) {
@@ -1255,7 +1254,6 @@ addToCompileTimeContext({
   runtimeKeepalivePop,
   runtimeKeepalivePush,
   splitI64,
-  storeException,
   to64,
   toIndexType,
   nodePthreadDetection,

src/preamble.js

@@ -360,11 +360,7 @@ function makeAbortWrapper(original) {
         ABORT // rethrow exception if abort() was called in the original function call above
         || abortWrapperDepth > 1 // rethrow exceptions not caught at the top level if exception catching is enabled; rethrow from exceptions from within callMain
 #if SUPPORT_LONGJMP == 'emscripten' // Rethrow longjmp if enabled
-#if EXCEPTION_STACK_TRACES
-        || e instanceof EmscriptenSjLj // EXCEPTION_STACK_TRACES=1 will throw an instance of EmscriptenSjLj
-#else
-        || e === Infinity // EXCEPTION_STACK_TRACES=0 will throw Infinity
-#endif // EXCEPTION_STACK_TRACES
+        || e instanceof EmscriptenSjLj
 #endif
         || e === 'unwind'
       ) {

src/runtime_exceptions.js

@@ -4,23 +4,35 @@
  * SPDX-License-Identifier: MIT
  */
 
-#if EXCEPTION_STACK_TRACES && !WASM_EXCEPTIONS
+#if !WASM_EXCEPTIONS
+
 // Base Emscripten EH error class
+#if EXCEPTION_STACK_TRACES
 class EmscriptenEH extends Error {}
+#else
+class EmscriptenEH {}
+#endif
 
 #if SUPPORT_LONGJMP == 'emscripten'
 class EmscriptenSjLj extends EmscriptenEH {}
 #endif
 
+#if !DISABLE_EXCEPTION_CATCHING
 class CppException extends EmscriptenEH {
   constructor(excPtr) {
+#if EXCEPTION_STACK_TRACES
     super(excPtr);
+#else
+    super();
+#endif
     this.excPtr = excPtr;
-#if !DISABLE_EXCEPTION_CATCHING
+#if !DISABLE_EXCEPTION_CATCHING && EXCEPTION_STACK_TRACES
     const excInfo = getExceptionMessage(excPtr);
     this.name = excInfo[0];
     this.message = excInfo[1];
 #endif
   }
 }
-#endif // EXCEPTION_STACK_TRACES && !WASM_EXCEPTIONS
+#endif
+
+#endif // !WASM_EXCEPTIONS

test/codesize/test_codesize_cxx_ctors1.json

@@ -1,10 +1,10 @@
 {
-  "a.out.js": 19214,
-  "a.out.js.gz": 7981,
+  "a.out.js": 19193,
+  "a.out.js.gz": 7970,
   "a.out.nodebug.wasm": 132638,
   "a.out.nodebug.wasm.gz": 49927,
-  "total": 151852,
-  "total_gz": 57908,
+  "total": 151831,
+  "total_gz": 57897,
   "sent": [
     "__cxa_throw",
     "_abort_js",

test/codesize/test_codesize_cxx_ctors2.json

@@ -1,10 +1,10 @@
 {
-  "a.out.js": 19191,
-  "a.out.js.gz": 7966,
+  "a.out.js": 19170,
+  "a.out.js.gz": 7957,
   "a.out.nodebug.wasm": 132064,
   "a.out.nodebug.wasm.gz": 49586,
-  "total": 151255,
-  "total_gz": 57552,
+  "total": 151234,
+  "total_gz": 57543,
   "sent": [
     "__cxa_throw",
     "_abort_js",

test/codesize/test_codesize_cxx_except.json

@@ -1,10 +1,10 @@
 {
-  "a.out.js": 22875,
-  "a.out.js.gz": 8956,
+  "a.out.js": 23195,
+  "a.out.js.gz": 8968,
   "a.out.nodebug.wasm": 172516,
   "a.out.nodebug.wasm.gz": 57438,
-  "total": 195391,
-  "total_gz": 66394,
+  "total": 195711,
+  "total_gz": 66406,
   "sent": [
     "__cxa_begin_catch",
     "__cxa_end_catch",