Micro-optimize performance and code size generated for longjmp in Emscripten invokes to avoid string comparisons

Author: juj

src/library.js

@@ -1409,7 +1409,7 @@ LibraryManager.library = {
   // ==========================================================================
 
   _emscripten_throw_longjmp__sig: 'v',
-  _emscripten_throw_longjmp: function() { throw 'longjmp'; },
+  _emscripten_throw_longjmp: function() { throw Infinity; },
 #if !SUPPORT_LONGJMP
 #if !INCLUDE_FULL_LIBRARY
   // These are in order to print helpful error messages when either longjmp of

src/library_dylink.js

@@ -235,7 +235,10 @@ var LibraryDylink = {
         return dynCall(sig, arguments[0], Array.prototype.slice.call(arguments, 1));
       } catch(e) {
         stackRestore(sp);
-        if (e !== e+0 && e !== 'longjmp') throw e;
+        // Exceptions thrown from C++ exception will be integer numbers.
+        // longjmp will throw the number Infinity. Re-throw other types of
+        // exceptions using a compact and fast check.
+        if (e !== e+0) throw e;
         _setThrew(1, 0);
       }
     }

src/preamble.js

@@ -707,7 +707,7 @@ 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'
-        || e === 'longjmp' // rethrow longjmp if enabled
+        || e === Infinity // rethrow longjmp if enabled (In Emscripten EH format longjmp will throw Infinity)
 #endif
       ) {
         throw e;

tools/shared.py

@@ -687,12 +687,11 @@ def make_invoke(sig, named=True):
     args = ['index'] + ['a' + str(i) for i in range(1, len(legal_sig))]
     ret = 'return ' if sig[0] != 'v' else ''
     body = '%s%s;' % (ret, JS.make_dynCall(sig, args))
-    # C++ exceptions are numbers, and longjmp is a string 'longjmp'
-    if settings.SUPPORT_LONGJMP:
-      rethrow = "if (e !== e+0 && e !== 'longjmp') throw e;"
-    else:
-      rethrow = "if (e !== e+0) throw e;"
-
+    # Exceptions thrown from C++ exception will be integer numbers.
+    # longjmp will throw the number Infinity.
+    # Create a try-catch guard that rethrows the exception if anything else
+    # than a Number was thrown. To do that quickly and in a code size conserving
+    # manner, use the compact test "e !== e+0" to check if e was not a Number.
     name = (' invoke_' + sig) if named else ''
     ret = '''\
 function%s(%s) {
@@ -701,10 +700,10 @@ def make_invoke(sig, named=True):
     %s
   } catch(e) {
     stackRestore(sp);
-    %s
+    if (e !== e+0) throw e;
     _setThrew(1, 0);
   }
-}''' % (name, ','.join(args), body, rethrow)
+}''' % (name, ','.join(args), body)
 
     return ret