Put stack first when not optimizing for size

Also, when not using santizers (which currently rely on using the start
of memory).

Author: sbc100

emcc.py

@@ -1814,6 +1814,14 @@ def phase_linker_setup(options, state, newargs):
   if not settings.AUTO_JS_LIBRARIES:
     default_setting('USE_SDL', 0)
 
+  if 'GLOBAL_BASE' not in user_settings and not settings.SHRINK_LEVEL and not settings.OPT_LEVEL:
+    # When optimizing for size it helps to put static data first before
+    # the stack (sincs this makes instructions for accessing this data
+    # use a smaller LEB encoding).
+    # However, for debugability is better to have the stack come first
+    # (becuase stack overflows will trap rather than corrupting data).
+    settings.STACK_FIRST = True
+
   # Default to TEXTDECODER=2 (always use TextDecoder to decode UTF-8 strings)
   # in -Oz builds, since custom decoder for UTF-8 takes up space.
   # In pthreads enabled builds, TEXTDECODER==2 may not work, see
@@ -2150,7 +2158,7 @@ def phase_linker_setup(options, state, newargs):
     exit_with_error('cannot have both DYNAMIC_EXECUTION=0 and RELOCATABLE enabled at the same time, since RELOCATABLE needs to eval()')
 
   if settings.SIDE_MODULE and 'GLOBAL_BASE' in user_settings:
-    exit_with_error('Cannot set GLOBAL_BASE when building SIDE_MODULE')
+    exit_with_error('GLOBAL_BASE is not compatible with SIDE_MODULE')
 
   if options.use_preload_plugins or len(options.preload_files) or len(options.embed_files):
     if settings.NODERAWFS:
@@ -2498,6 +2506,7 @@ def check_memory_setting(setting):
     # We start our global data after the shadow memory.
     # We don't need to worry about alignment here.  wasm-ld will take care of that.
     settings.GLOBAL_BASE = shadow_size
+    settings.STACK_FIRST = False
 
     if not settings.ALLOW_MEMORY_GROWTH:
       settings.INITIAL_MEMORY = total_mem

src/runtime_stack_check.js

@@ -14,6 +14,11 @@ function writeStackCookie() {
 #if ASSERTIONS
   assert((max & 3) == 0);
 #endif
+  // If the stack ends at address zero we write our cookies 4 bytes after the
+  // prevent interferance with the (separate) address-zero check below.
+  if (max == 0) {
+    max += 4;
+  }
   // The stack grow downwards towards _emscripten_stack_get_end.
   // We write cookies to the final two words in the stack and detect if they are
   // ever overwritten.
@@ -33,6 +38,9 @@ function checkStackCookie() {
 #if RUNTIME_DEBUG
   dbg('checkStackCookie: ' + max.toString(16));
 #endif
+  if (max == 0) {
+    max += 4;
+  }
   var cookie1 = {{{ makeGetValue('max', 0, 'u32') }}};
   var cookie2 = {{{ makeGetValue('max', 4, 'u32') }}};
   if (cookie1 != 0x2135467 || cookie2 != 0x89BACDFE) {

src/settings_internal.js

@@ -244,3 +244,5 @@ var ALL_INCOMING_MODULE_JS_API = [];
 // when weak symbols are undefined.  Only applies in the case of dyanmic linking
 // (MAIN_MODULE).
 var WEAK_IMPORTS = [];
+
+var STACK_FIRST = false;

test/core/test_emmalloc_trim.out

@@ -1,35 +1,35 @@
 heap size: 134217728
 dynamic heap 0: 4096
 free dynamic memory 0: 4096
-unclaimed heap memory 0: 2142171136
-sbrk 0: 0x502000
+unclaimed heap memory 0: 2142175232
+sbrk 0: 0x501000
 1
 dynamic heap 1: 37752832
 free dynamic memory 1: 4096
-unclaimed heap memory 1: 2104422400
-sbrk 1: 0x2902000
+unclaimed heap memory 1: 2104426496
+sbrk 1: 0x2901000
 1st trim: 1
 dynamic heap 1: 37752832
 free dynamic memory 1: 0
-unclaimed heap memory 1: 2104422400
-sbrk 1: 0x2902000
+unclaimed heap memory 1: 2104426496
+sbrk 1: 0x2901000
 2nd trim: 0
 dynamic heap 2: 37752832
 free dynamic memory 2: 0
-unclaimed heap memory 2: 2104422400
-sbrk 2: 0x2902000
+unclaimed heap memory 2: 2104426496
+sbrk 2: 0x2901000
 3rd trim: 1
 dynamic heap 3: 33656832
 free dynamic memory 3: 102400
-unclaimed heap memory 3: 2104422400
-sbrk 3: 0x2902000
+unclaimed heap memory 3: 2104426496
+sbrk 3: 0x2901000
 4th trim: 0
 dynamic heap 4: 33656832
 free dynamic memory 4: 102400
-unclaimed heap memory 4: 2104422400
-sbrk 4: 0x2902000
+unclaimed heap memory 4: 2104426496
+sbrk 4: 0x2901000
 5th trim: 1
 dynamic heap 5: 33558528
 free dynamic memory 5: 0
-unclaimed heap memory 5: 2104422400
-sbrk 5: 0x2902000
+unclaimed heap memory 5: 2104426496
+sbrk 5: 0x2901000

test/core/test_stack_placement.c

@@ -22,7 +22,9 @@ int main(int argc, char* argv[]) {
   int* bss_address = &static_bss[BSS_BYTES-1];
   int* stack_address = &stack_var;
   printf("data: %p bss: %p stack: %p\n", data_address, bss_address, stack_address);
-  assert(stack_address > bss_address);
+  // Stack can either come after BSS or before data (In debug builds we link
+  // with --stack-first).
+  assert(stack_address > bss_address || stack_address < data_address);
   assert(bss_address > data_address);
   printf("success.\n");
   return 0;

test/other/metadce/test_metadce_hello_O0.jssize

@@ -1 +1 @@
-26256
+26310

test/other/metadce/test_metadce_hello_O0.size

@@ -1 +1 @@
-12058
+12135

test/other/metadce/test_metadce_minimal_O0.jssize

@@ -1 +1 @@
-22987
+23046

test/other/metadce/test_metadce_minimal_O0.size

@@ -1 +1 @@
-865
+882

test/other/test_unoptimized_code_size.js.size

@@ -1 +1 @@
-66886
+67108