Default to STACK_OVERFLOW_CHECK=2 in debug builds

Previously we defaulted to 1 in this case, but we want to improve the
error reporting around stack overflow in preparation for lowering the
default stack size.  See #14177

Author: sbc100

ChangeLog.md

@@ -20,6 +20,10 @@ See docs/process.md for more on how version tagging works.
 
 3.1.25 (in development)
 -----------------------
+- `STACK_OVERFLOW_CHECK` now defaults 2 in debug builds (when `ASSERTIONS` are
+  enabled). This is because we are planning on lowering the default stack size
+  and we want good reporting of stack overflow in debug builds.  See #14177.
+  This can be set of 1 or 0 if the performance overhead is a problem.
 
 3.1.24 - 10/11/22
 -----------------

emcc.py

@@ -1774,14 +1774,12 @@ def phase_linker_setup(options, state, newargs, user_settings):
   if '_main' in settings.EXPORTED_FUNCTIONS:
     settings.EXPORT_IF_DEFINED.append('__main_argc_argv')
 
-  # -sASSERTIONS implies basic stack overflow checks, and ASSERTIONS=2
-  # implies full stack overflow checks.
-  if settings.ASSERTIONS:
-    # However, we don't set this default in PURE_WASI, or when we are linking without standard
-    # libraries because STACK_OVERFLOW_CHECK depends on emscripten_stack_get_end which is defined
-    # in libcompiler-rt.
-    if not settings.PURE_WASI and '-nostdlib' not in newargs and '-nodefaultlibs' not in newargs:
-      default_setting(user_settings, 'STACK_OVERFLOW_CHECK', max(settings.ASSERTIONS, settings.STACK_OVERFLOW_CHECK))
+  # -sASSERTIONS implies stack overflow checks
+  # However, we don't set this default in PURE_WASI, or when we are linking without standard
+  # libraries (because STACK_OVERFLOW_CHECK depends on emscripten_stack_get_end which is defined
+  # in compiler-rt).
+  if settings.ASSERTIONS and not settings.PURE_WASI and '-nostdlib' not in newargs and '-nodefaultlibs' not in newargs:
+    default_setting(user_settings, 'STACK_OVERFLOW_CHECK', 2)
 
   if settings.LLD_REPORT_UNDEFINED or settings.STANDALONE_WASM:
     # Reporting undefined symbols at wasm-ld time requires us to know if we have a `main` function

src/settings.js

@@ -63,10 +63,10 @@ var ASSERTIONS = 1;
 var RUNTIME_LOGGING = false;
 
 // Chooses what kind of stack smash checks to emit to generated code:
-// Building with ASSERTIONS=1 causes STACK_OVERFLOW_CHECK default to 1.
+// Building with ASSERTIONS=1 causes STACK_OVERFLOW_CHECK default to 2.
 // Since ASSERTIONS=1 is the default at -O0, which itself is the default
 // optimization level this means that this setting also effectively
-// defaults 1, absent any other settings.
+// defaults 2, absent any other settings.
 // 0: Stack overflows are not checked.
 // 1: Adds a security cookie at the top of the stack, which is checked at end of
 //    each tick and at exit (practically zero performance overhead)