INITIAL_MEMORY, MAXIMUM_MEMORY cannot be set dynamically

[aharondavid]

INITIAL_MEMORY, MAXIMUM_MEMORY cannot be set dynamically like PTHREAD_POOL_SIZE for example.
it is crucial for support in different Platforms (e.g. iOS vs Windows)

[sbc100]

Normally with emscripten the memory is part of the wasm file and this is exported to JS, so the intial and max values are baked into the binary.

If you want to control them dynamically you can use the -sIMPORTED_MEMORY setting and then defined you own memory dynamiclally on the Module object. e.g.

Module.wasmMemory = new WebAssembly.Memory(...).

BTW there some reason is not enough to just always set a low INITIAL_MEMORY (small enough for your smallest device) and a high MAXIMUM_MEMORY (big enough for your biggest device)?

[aharondavid]

My goal is to create one binary wasm that fit to Windows (4g) and iOS device (less than 1G) as well.
In some iOS devices, MAXIMUM_MEMORY cannot be greater than 1G . the following Emscripten.JS code is crash in those devices:

wasmMemory = new WebAssembly.Memory({
   "initial": INITIAL_MEMORY / 65536,
   "maximum": 2147483648 / 65536,
   "shared": true
  });

About the IMPORTED_MEMORY flag, it is not clear who to work with this flag.
I don't see any change in Emscripten.js with or without this flag.

[sbc100]

I was not aware of the this limitation on MAXIMUM_MEMORY. It seems like the module should only fail to load if INITIAL_MEMORY is too big... so that sounds like a bug in iOS.

You can use IMPORTED_MEMORY in combination with Module['wasmMemory']. For example you could add the following to your --pre-js file:

emscripten/test/core/test_module_wasm_memory.js

Line 8 in 2bc5e31

Module['wasmMemory'] = new WebAssembly.Memory({ 'initial': 256, 'maximum': 256 });

See this test for a full example:

emscripten/test/test_core.py

Lines 2166 to 2174 in 2bc5e31

	def test_module_wasm_memory(self):
	if self.get_setting('MEMORY64') == 1:
	self.emcc_args += ['--pre-js', test_file('core/test_module_wasm_memory64.js')]
	else:
	self.emcc_args += ['--pre-js', test_file('core/test_module_wasm_memory.js')]
	self.set_setting('IMPORTED_MEMORY')
	self.set_setting('STRICT')
	self.set_setting('INCOMING_MODULE_JS_API', ['wasmMemory'])
	self.do_runf('core/test_module_wasm_memory.c', 'success')

[aharondavid]

• It seems that USE_PTHREADS disable the IMPORTED_MEMORY flag:

  Steps to reproduce (emcc 3.1.55):

  create test.cpp:
  int main() {
      //myFunction();
      return 0;
  };

2. emcc test.cpp -o output1.html -s WASM=1 -s USE_PTHREADS -s IMPORTED_MEMORY=1
3. emcc test.cpp -o output2.html -s WASM=1 -s USE_PTHREADS
4. result: output1.js is equal to output2.js
   only without USE_PTHREADS the IMPORTED_MEMORY is applied.

• What about MAXIMUM_MEMORY=4gb (4gb has special treatments of generating to the output Emscripten JS file).
  is 4gb can be set dynamically ?

[sbc100]

The code for handling memories larger than 2gb is baked into the output, but only when MAXIMUM_MEMORY is >2gb. Sadly there is no way to opt into this other than setting MAXIMUM_MEMORY > 2gb.

[sbc100]

Its actually the other way around: USE_PTHREADS forces the use of IMPORTED_MEMORY. This is a necessary detail of how pthreads are implemented.

[aharondavid]

Actually, MAXIMUM_MEMORY doesn't need be allocated dynamically.
This code in pre-js is works for all OS when MAXIMUM_MEMORY=4g:

////////////////////////////////////////////////////////////////////
// Fix iOS (17.0) bug:  WebAssembly.Memory.maximum cannot be greater than 1gb
/////////////////////////////////////////////////////
function GetMaximumMemory(){

	if (isMobileDevice()) {
		if (iOS())
			return 700*1024*1024;//700mb
	}
	return 4* 1024*1024*1024;//4gb
}


if (Module["wasmMemory"] == undefined)
	Module['wasmMemory'] = new WebAssembly.Memory({ 'initial': 300 * 1024 * 1024 / 65536, 'maximum': GetMaximumMemory() / 65536, "shared": true });

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