wasm: Update asyncify instructions

It is now link time enabled

Change-Id: I4e2c091614d38fef80e2cc051e207f163cd85a0c
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>
(cherry picked from commit 8a9ec5504abdecd7bbe88f85deff1919221f7309)
(cherry picked from commit e37a546ff844e6a7ff8faeb484c88f114b19955b)
Reviewed-by: Lorn Potter <lorn.potter@gmail.com>

1 files changed, 27 insertions, 0 deletions

diff --git a/doc/src/platforms/wasm.qdoc b/doc/src/platforms/wasm.qdoc
index eddb66cc..3b1ae356 100644
--- a/doc/src/platforms/wasm.qdoc
+++ b/doc/src/platforms/wasm.qdoc
@@ -509,6 +509,33 @@ int main(int argc, char **argc)
 }
 \endcode
 
+\section3 Asyncify
+
+The default build of Qt for WebAssembly does not support reentering the event loop,
+for example by calling QEventLoop::exec() or QDialog::exec(), due to resitrictions
+of the web platform.
+
+Emscripten's \l{https://emscripten.org/docs/porting/asyncify.html}{asyncify} feature lifts
+these restrictions by allowing sychronous calls (like QEventLoop::exec() and QDialog::exec())
+to yield to the event loop. Nested calls are not supported, and for this reason asyncify is
+not used for the top-level QApplication::exec() call.
+
+As of Qt 6.4, Asyncify support is enabled in the binary package, but needs to be enabled for
+applications by adding -sASYNCIFY -Os to linker options:
+
+CMake:
+\badcode
+set target_link_options(<target> PUBLIC -sASYNCIFY -Os)
+\endcode
+
+qmake:
+\badcode
+QMAKE_LFLAGS += -sASYNCIFY -Os)
+\endcode
+
+Enabling asyncify adds overhead in the form of increased binary sizes and increased CPU
+usage. Build with optimizations enabled to minimize the overhead.
+
 \section2 Debugging and Profiling
 
 Wasm debugging is done on browser javascript console, debugging applications on Wasm