wasm: set ALLOW_MEMORY_GROWTH for multi-threaded builds

Unify the settings for single-threaded and multi-threaded builds;
Qt now always enables heap growth by default.

This means we don't have to reserve a large (1GB) fixed memory
size, but can instead set the smaller (50 MB) initial memory size,
like the single-threaded build does.

Enabling threads + memory growth can potentially cause
a performance regression when accessing heap memory from
JavaScript (https://github.com/WebAssembly/design/issues/1271).
We leave it for the application to decide if this applies,
and if the switch to fixed memory should be made.

Change-Id: I96988b072506456685086e55aca4007a146bd70f
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>
Reviewed-by: Mikołaj Boc <Mikolaj.Boc@qt.io>
Reviewed-by: Lorn Potter <lorn.potter@gmail.com>

cmake/QtWasmHelpers.cmake

@@ -35,10 +35,10 @@ function (qt_internal_setup_wasm_target_properties wasmTarget)
     if (QT_FEATURE_thread)
         target_compile_options("${wasmTarget}" INTERFACE "SHELL:-pthread")
         target_link_options("${wasmTarget}" INTERFACE "SHELL:-pthread")
-    else()
-        target_link_options("${wasmTarget}" INTERFACE "SHELL:-s ALLOW_MEMORY_GROWTH=1")
     endif()
 
+    target_link_options("${wasmTarget}" INTERFACE "SHELL:-s ALLOW_MEMORY_GROWTH")
+
     # debug add_compile_options
     if ("QT_WASM_SOURCE_MAP=1" IN_LIST QT_QMAKE_DEVICE_OPTIONS)
         set(WASM_SOURCE_MAP_BASE "http://localhost:8000/")

mkspecs/features/wasm/wasm.prf

@@ -29,29 +29,16 @@ exists($$QMAKE_QT_CONFIG) {
 
         message("Setting PTHREAD_POOL_SIZE to" $$POOL_SIZE)
         EMCC_LFLAGS += -s PTHREAD_POOL_SIZE=$$POOL_SIZE
-    } else {
-        EMCC_LFLAGS += -s ALLOW_MEMORY_GROWTH=1
     }
 
+    # Set memory options
+    EMCC_LFLAGS += -sALLOW_MEMORY_GROWTH
     isEmpty(QT_WASM_INITIAL_MEMORY) {
-        # Hardcode wasm memory size.
+        INITIAL_MEMORY = 50MB # emscripten default is 16MB, we need slightly more
-
-        qtConfig(thread) {
-        # Pthreads and ALLOW_MEMORY_GROWTH can cause javascript wasm memory access to
-        # be slow. Instead, we specify the memory size
-        # at build time. Further, browsers limit the maximum initial memory size to 1GB.
-        # https://github.com/WebAssembly/design/issues/1271
-            INITIAL_MEMORY = 1GB
-        } else {
-            INITIAL_MEMORY = 50MB # emscripten default is 16MB, we need slightly more
-        }
     } else {
-        # QT_WASM_INITIAL_MEMORY must be a multiple of 64KB (i.e. 65536)
         INITIAL_MEMORY = $$QT_WASM_INITIAL_MEMORY
-        message("Setting INITIAL_MEMORY to" $$INITIAL_MEMORY)
     }
     EMCC_LFLAGS += -s INITIAL_MEMORY=$$INITIAL_MEMORY
-    message("Setting INITIAL_MEMORY to" $$INITIAL_MEMORY)
 
     qtConfig(sse2) {
         QMAKE_CFLAGS += -O2 -msimd128 -msse -msse2

src/corelib/Qt6WasmMacros.cmake

@@ -48,29 +48,14 @@ function(_qt_internal_wasm_add_target_helpers target)
             message(DEBUG "Setting PTHREAD_POOL_SIZE to ${POOL_SIZE} for ${target}")
         endif()
 
-        # Hardcode wasm memory size.
+        # Set initial memory size, either from user setting or to a minimum amount required by Qt.
         get_target_property(_tmp_initialMemory "${target}" QT_WASM_INITIAL_MEMORY)
         if(_tmp_initialMemory)
             set(QT_WASM_INITIAL_MEMORY "${_tmp_initialMemory}")
         elseif(NOT DEFINED QT_WASM_INITIAL_MEMORY)
-            if(QT_FEATURE_thread)
+            set(QT_WASM_INITIAL_MEMORY "50MB")
-                # Pthreads and ALLOW_MEMORY_GROWTH can cause javascript wasm memory access to
-                # be slow and having to update HEAP* views. Instead, we specify the memory size
-                # at build time. Further, browsers limit the maximum initial memory size to 1GB.
-                # https://github.com/WebAssembly/design/issues/1271
-                set(QT_WASM_INITIAL_MEMORY "1GB")
-            else()
-                # emscripten default is 16MB, we need slightly more sometimes
-                set(QT_WASM_INITIAL_MEMORY "50MB")
-            endif()
-        endif()
-
-        if(DEFINED QT_WASM_INITIAL_MEMORY)
-            # QT_WASM_INITIAL_MEMORY must be a multiple of 65536
-            target_link_options("${target}"
-                PRIVATE "SHELL:-s INITIAL_MEMORY=${QT_WASM_INITIAL_MEMORY}")
-            message(DEBUG "-- Setting INITIAL_MEMORY to ${QT_WASM_INITIAL_MEMORY} for ${target}")
         endif()
+        target_link_options("${target}" PRIVATE "SHELL:-s INITIAL_MEMORY=${QT_WASM_INITIAL_MEMORY}")
 
     endif()
 endfunction()