Improve SUPPORT_LONGJMP=0 (#12394)

Don't run the backend pass if longjmp support is disabled. This allows setting the
flag at compile time to disable invoke generation for longjmps, when that is not
desired.

Also, before this change, the error would just be "undefined symbol: longjmp" which is
not that clear, and worse, if errors on undefined symbols were disabled, it would
just be a warning. This emits a clear error at link time if the flag is set to not support
longjmp but the code requires it.

Author: kripken

ChangeLog.md

@@ -17,6 +17,11 @@ See docs/process.md for how version tagging works.
 
 Current Trunk
 -------------
+- When `-s SUPPORT_LONGJMP=0` is passed to disable longjmp, do not run the LLVM
+  wasm backend path that handles longjmp. Before this only affected linking, and
+  now the flag gives you the ability to affect codegen at compile time too. This
+  is necessary if one does not want any invokes generated for longjmp at all.
+  (#12394)
 
 2.0.6: 10/02/2020
 -----------------

src/library.js

@@ -1766,19 +1766,27 @@ LibraryManager.library = {
   // setjmp.h
   // ==========================================================================
 
-#if SUPPORT_LONGJMP
   longjmp__sig: 'vii',
+#if SUPPORT_LONGJMP
   longjmp: function(env, value) {
     _setThrew(env, value || 1);
     throw 'longjmp';
   },
+#else
+  longjmp__deps: [function() {
+    error('longjmp support was disabled (SUPPORT_LONGJMP=0), but it is required by the code (either set SUPPORT_LONGJMP=1, or remove uses of it in the project)');
+  }],
+  // will never be emitted, as the dep errors at compile time
+  longjmp: function() {
+    abort('longjmp not supported');
+  },
+#endif
   // TODO: remove these aliases if/when the LLVM backend can stop emitting them
   // (it emits them atm as they are generated by an IR pass, at at that time
   // they each have a different signature - it is only at the wasm level that
   // they become identical).
   emscripten_longjmp: 'longjmp',
   emscripten_longjmp_jmpbuf: 'longjmp',
-#endif
 
   // ==========================================================================
   // sys/wait.h

src/settings.js

@@ -1497,6 +1497,9 @@ var RUNTIME_FUNCS_TO_IMPORT = ['abort', 'setTempRet0', 'getTempRet0']
 // not available.  If you are using C++ exceptions, but do not need
 // setjmp()+longjmp() API, then you can set this to 0 to save a little bit of
 // code size and performance when catching exceptions.
+// [compile+link] - at compile time this enables the transformations needed for
+// longjmp support at codegen time, while at link it allows linking in the
+// library support.
 var SUPPORT_LONGJMP = 1;
 
 // If set to 1, disables old deprecated HTML5 API event target lookup behavior.

tests/test_other.py

@@ -9407,3 +9407,15 @@ def test_SYSCALL_DEBUG(self):
   def test_LIBRARY_DEBUG(self):
     self.set_setting('LIBRARY_DEBUG')
     self.do_runf(path_from_root('tests', 'hello_world.c'), '[library call:_fd_write: 0x1')
+
+  def test_SUPPORT_LONGJMP_executable(self):
+    stderr = self.run_process([EMCC, path_from_root('tests', 'core', 'test_longjmp.c'), '-s', 'SUPPORT_LONGJMP=0'], stderr=PIPE, check=False).stderr
+    self.assertContained('error: longjmp support was disabled (SUPPORT_LONGJMP=0), but it is required by the code (either set SUPPORT_LONGJMP=1, or remove uses of it in the project)',
+                         stderr)
+
+  def test_SUPPORT_LONGJMP_object(self):
+    # compile the object *with* support, but link without
+    self.run_process([EMCC, path_from_root('tests', 'core', 'test_longjmp.c'), '-c', '-s', 'SUPPORT_LONGJMP=1', '-o', 'a.o'])
+    stderr = self.run_process([EMCC, 'a.o', '-s', 'SUPPORT_LONGJMP=0'], stderr=PIPE, check=False).stderr
+    self.assertContained('error: longjmp support was disabled (SUPPORT_LONGJMP=0), but it is required by the code (either set SUPPORT_LONGJMP=1, or remove uses of it in the project)',
+                         stderr)

tools/building.py

@@ -438,8 +438,9 @@ def llvm_backend_args():
     allowed = ','.join(Settings.EXCEPTION_CATCHING_ALLOWED or ['__fake'])
     args += ['-emscripten-cxx-exceptions-allowed=' + allowed]
 
-  # asm.js-style setjmp/longjmp handling
-  args += ['-enable-emscripten-sjlj']
+  if Settings.SUPPORT_LONGJMP:
+    # asm.js-style setjmp/longjmp handling
+    args += ['-enable-emscripten-sjlj']
 
   # better (smaller, sometimes faster) codegen, see binaryen#1054
   # and https://bugs.llvm.org/show_bug.cgi?id=39488