Hello again! I just wanted to drop a note detailing my experiences building
Telegram's Database Library (https://github.com/tdlib/td) with the new WASM
backend. Overall the experience was a bit rough, mainly due to errors that
did not occur using fastcomp, primarily the new strict pthreads checking
and lack of support for Position Independent Code.
To get TdLib building, I had to make the following changes:
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 3ec8c456..96736d4f 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -36,7 +36,7 @@ set(CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/CMake"
"${CMAKE_MODULE_PATH}"
set(CMAKE_EXPORT_COMPILE_COMMANDS 1)
-set(CMAKE_POSITION_INDEPENDENT_CODE ON)
+# set(CMAKE_POSITION_INDEPENDENT_CODE ON)
enable_testing()
@@ -98,8 +98,8 @@ if (EMSCRIPTEN)
set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -s WASM=0 -Wno-almost-asm")
else()
set(TD_EMSCRIPTEN td_wasm)
- set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -s WASM=1")
- set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -s WASM=1")
+ set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -s WASM=1 -s
ERROR_ON_UNDEFINED_SYMBOLS=0")
+ set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -s WASM=1 -s
ERROR_ON_UNDEFINED_SYMBOLS=0")
endif()
endif()
diff --git a/example/web/build-openssl.sh b/example/web/build-openssl.sh
index faf4f657..0031bef8 100755
--- a/example/web/build-openssl.sh
+++ b/example/web/build-openssl.sh
@@ -12,7 +12,7 @@ echo "Unpacking OpenSSL sources..."
tar xzf $OPENSSL.tar.gz || exit 1
cd openssl-$OPENSSL
-emconfigure ./Configure linux-generic32 no-shared no-dso no-engine
no-unit-test no-ui || exit 1
+emconfigure ./Configure linux-generic32 no-threads no-pic no-asm no-shared
no-dso no-engine no-unit-test no-ui || exit 1
sed -i.bak 's/CROSS_COMPILE=.*/CROSS_COMPILE=/g' Makefile || exit 1
sed -i.bak 's/-ldl //g' Makefile || exit 1
echo "Building OpenSSL..."
These changes disabled threading and PIC when building OpenSSL, and also
disabled the error on undefined symbols. The latter change was necessary as
I was running into "error: undefined symbol: __cxa_uncaught_exception".
Once those changes were made, I was able to successfully compile TdLib to
WASM! Sadly I didn't see any significant size changes: the original
fastcomp WASM module was 7.7 MB, and the new module was 7.5 MB. While the
module seems to load and perform some actions correctly, the WebSocket
connections are closed before a connection is established, so I wasn't able
to make any calls to the Telegram API. I'm not a C++ developer, so I
probably can't debug that particular issue and will have to wait for the
Telegram developers to take a look.
I just wanted to share my experiences trying to compile a non-trivial WASM
library with the new upstream backend. The only suggestions I can make are
to add warnings to the fastcomp compiler for things that won't be supported
in the new backend, such as linking a library built with threads support
with a library that doesn't support it.
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