Set pthread stack size when -sSTACK_SIZE is set

This was always my intention with #18191 but somehow it got left out.  I
think it makes sense that user to override `STACK_SIZE` also, by
default, override the pthread stack size.  Such users can always
explicitly set the pthread stack size if they don't want this behaviour.

See #18472

ChangeLog.md

@@ -20,6 +20,9 @@ See docs/process.md for more on how version tagging works.
 
 3.1.30 (in development)
 -----------------------
+- As a followup to #18191, the default pthread stack size will now be set to
+  match `-sSTACK_SIZE` by default.  Setting `DEFAULT_PTHREAD_STACK_SIZE`
+  explicitly will override this.
 - The `buffer` JavaScript variable was removed.  This underlying buffer is
   still accessible via `wasmMemory.buffer` or `HEAPXX.buffer`.  In debug builds,
   a clear error is shown if you try to use it.  (#18454)

emcc.py

@@ -1627,6 +1627,8 @@ def setup_pthreads(target):
   if settings.ALLOW_MEMORY_GROWTH:
     diagnostics.warning('pthreads-mem-growth', 'USE_PTHREADS + ALLOW_MEMORY_GROWTH may run non-wasm code slowly, see https://github.com/WebAssembly/design/issues/1271')
 
+  default_setting('DEFAULT_PTHREAD_STACK_SIZE', settings.STACK_SIZE)
+
   # Functions needs to be exported from the module since they are used in worker.js
   settings.REQUIRED_EXPORTS += [
     'emscripten_dispatch_to_thread_',

src/settings.js

@@ -1581,18 +1581,14 @@ var PTHREAD_POOL_SIZE_STRICT = 1;
 // [link] - affects generated JS runtime code at link time
 var PTHREAD_POOL_DELAY_LOAD = false;
 
-// If not explicitly specified, this is the stack size to use for newly created
-// pthreads.  According to
-// http://man7.org/linux/man-pages/man3/pthread_create.3.html, default stack
-// size on Linux/x86-32 for a new thread is 2 megabytes, so follow the same
-// convention. Use pthread_attr_setstacksize() at thread creation time to
-// explicitly specify the stack size, in which case this value is ignored. Note
-// that the wasm function call control flow stack is separate from this
-// stack, and this stack only contains certain function local variables, such as
-// those that have their addresses taken, or ones that are too large to fit as
-// local vars in wasm code.
-// [link]
-var DEFAULT_PTHREAD_STACK_SIZE = 64*1024;
+// Default stack size to use for newly created pthreads.  When not set, this
+// defaults to STACK_SIZE (which in turn defaults to 64k).  Can also be set at
+// runtime using pthread_attr_setstacksize().  Note that the wasm control flow
+// stack is separate from this stack.  This stack only contains certain function
+// local variables, such as those that have their addresses taken, or ones that
+// are too large to fit as local vars in wasm code.
+// [link]
+var DEFAULT_PTHREAD_STACK_SIZE = 0;
 
 // True when building with --threadprofiler
 // [link]