Turn off settings.LTO if -fno-lto is passed (#20309)

Fixes: #20308

Author: hoodmane

emcc.py

@@ -3454,6 +3454,8 @@ def consume_arg_file():
         settings.LTO = arg.split('=')[1]
       else:
         settings.LTO = 'full'
+    elif arg == "-fno-lto":
+      settings.LTO = 0
     elif check_arg('--llvm-lto'):
       logger.warning('--llvm-lto ignored when using llvm backend')
       consume_arg()

test/test_other.py

@@ -11148,6 +11148,24 @@ def test_setjmp_emulated_casts(self):
     ''')
     self.do_runf('src.c', 'ok\ndone\n', emcc_args=['-sEMULATE_FUNCTION_POINTER_CASTS'])
 
+  def test_no_lto(self):
+    # This used to fail because settings.LTO didn't reflect `-fno-lto`.
+    # See bug https://github.com/emscripten-core/emscripten/issues/20308
+    create_file('src.c', r'''
+      #include <stdio.h>
+      #include <setjmp.h>
+      int main() {
+        jmp_buf jb;
+        if (!setjmp(jb)) {
+          printf("ok\n");
+          longjmp(jb, 1);
+        } else {
+          printf("done\n");
+        }
+      }
+    ''')
+    self.do_runf('src.c', 'ok\ndone\n', emcc_args=['-flto', '-fno-lto'])
+
   def test_missing_stdlibs(self):
     # Certain standard libraries are expected to be useable via -l flags but
     # don't actually exist in our standard library path.  Make sure we don't