Add CI step to build WebAssembly version of Slang (#5164)

Closes #5117

This adds a build step of WebAssembly version of Slang.
Currently it is enabled only for Linux/Release to reduce the time cost
of CI, but the builds were all successful on all platforms and
configurations when tested.

2 files changed, 66 insertions, 25 deletions

diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index e4d9fb06b..162998102 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -25,12 +25,15 @@ jobs:
         os: [linux, macos, windows]
         config: [debug, release]
         compiler: [gcc, clang, cl]
-        platform: [x86_64, aarch64]
+        platform: [x86_64, aarch64, wasm]
         exclude:
           # Default to x64, but aarch64 on osx
           - { os: linux, platform: aarch64 }
           - { os: windows, platform: aarch64 }
           - { os: macos, platform: x86_64 }
+          - { os: linux, config: debug, platform: wasm }
+          - { os: windows, platform: wasm }
+          - { os: macos, platform: wasm }
           # Unused compiler configs
           - { os: linux, compiler: clang }
           - { os: linux, compiler: cl }
@@ -96,26 +99,44 @@ jobs:
           build-llvm: true
       - name: Build Slang
         run: |
-          if [[ "${{ matrix.os }}" =~ "windows" && "${{ matrix.config }}" != "release" ]]; then
-            # Doing a debug build will try to link against a release built llvm, this
-            # is a problem on Windows, so make slang-llvm in release build and use
-            # that as though it's a fetched binary via these presets.
-            cmake --workflow --preset slang-llvm
-            # Configure, pointing to our just-generated slang-llvm archive
-            cmake --preset default --fresh \
-              -DSLANG_SLANG_LLVM_FLAVOR=FETCH_BINARY \
-              "-DSLANG_SLANG_LLVM_BINARY_URL=$(pwd)/build/dist-release/slang-llvm.zip" \
-              "-DCMAKE_COMPILE_WARNING_AS_ERROR=${{matrix.warnings-as-errors}}"
-            cmake --workflow --preset "${{matrix.config}}"
+          if [[ "${{ matrix.platform }}" = "wasm" ]]; then
+              git clone https://github.com/emscripten-core/emsdk.git
+              pushd emsdk
+                ./emsdk install latest
+                ./emsdk activate latest
+                source ./emsdk_env.sh
+              popd
+              cmake --workflow --preset generators --fresh
+              mkdir generators
+              cmake --install build --prefix generators --component generators
+              emcmake cmake -DSLANG_GENERATORS_PATH=generators/bin --preset emscripten -G "Ninja"
+              cmake --build --preset emscripten --target slang
+              [ -f "build.em/Release/lib/libslang.a" ]
+              [ -f "build.em/Release/lib/libcompiler-core.a" ]
+              [ -f "build.em/Release/lib/libcore.a" ]
           else
-            # Otherwise, use the system llvm we have just build or got from the
-            # cache in the setup phase
-            cmake --preset default --fresh \
-              -DSLANG_SLANG_LLVM_FLAVOR=USE_SYSTEM_LLVM \
-              -DCMAKE_COMPILE_WARNING_AS_ERROR=${{matrix.warnings-as-errors}}
-            cmake --workflow --preset "${{matrix.config}}"
+            if [[ "${{ matrix.os }}" =~ "windows" && "${{ matrix.config }}" != "release" ]]; then
+              # Doing a debug build will try to link against a release built llvm, this
+              # is a problem on Windows, so make slang-llvm in release build and use
+              # that as though it's a fetched binary via these presets.
+              cmake --workflow --preset slang-llvm
+              # Configure, pointing to our just-generated slang-llvm archive
+              cmake --preset default --fresh \
+                -DSLANG_SLANG_LLVM_FLAVOR=FETCH_BINARY \
+                "-DSLANG_SLANG_LLVM_BINARY_URL=$(pwd)/build/dist-release/slang-llvm.zip" \
+                "-DCMAKE_COMPILE_WARNING_AS_ERROR=${{matrix.warnings-as-errors}}"
+              cmake --workflow --preset "${{matrix.config}}"
+            else
+              # Otherwise, use the system llvm we have just build or got from the
+              # cache in the setup phase
+              cmake --preset default --fresh \
+                -DSLANG_SLANG_LLVM_FLAVOR=USE_SYSTEM_LLVM \
+                -DCMAKE_COMPILE_WARNING_AS_ERROR=${{matrix.warnings-as-errors}}
+              cmake --workflow --preset "${{matrix.config}}"
+            fi
           fi
       - name: Test Slang
+        if: ${{ matrix.platform != 'wasm' }}
         run: |
           export SLANG_RUN_SPIRV_VALIDATION=1
           export SLANG_USE_SPV_SOURCE_LANGUAGE_UNKNOWN=1
@@ -140,7 +161,7 @@ jobs:
               -expected-failure-list tests/expected-failure-record-replay-tests.txt
           fi
       - name: Test Slang via glsl
-        if: ${{matrix.full-gpu-tests}}
+        if: ${{ matrix.full-gpu-tests && matrix.platform != 'wasm' }}
         run: |
           export SLANG_RUN_SPIRV_VALIDATION=1
           export SLANG_USE_SPV_SOURCE_LANGUAGE_UNKNOWN=1
diff --git a/docs/building.md b/docs/building.md
index 8bc20acb3..da55f57aa 100644
--- a/docs/building.md
+++ b/docs/building.md
@@ -49,17 +49,37 @@ The `vs2022-dev` preset turns on features that makes debugging easy.
 
 ### WebAssembly build
 
-First do a native build of the `all-generators` target, so that you get the generator executables in a path like `build\generators\Debug\bin` under the
-Slang source tree.
+In order to build WebAssembly build of Slang, Slang needs to be compiled with [Emscripten SDK](https://github.com/emscripten-core/emsdk).
+You can find more information about [Emscripten](https://emscripten.org/).
 
-For an [emscripten](https://emscripten.org) -based build targeting [WebAssembly](https://webassembly.org), run:
+You need to clone the EMSDK repo. And you need to install and activate the latest.
 ```bash
-emcmake cmake -DSLANG_GENERATORS_PATH=%SLANG_SRC_PATH%\build\generators\Debug\bin --preset emscripten -G "Ninja"
+git clone https://github.com/emscripten-core/emsdk.git
+cd emsdk
+./emsdk install latest  # For Windows, emsdk.bat install latest
+./emsdk activate latest # For Windows, emsdk.bat activate latest
+```
+
+After EMSDK is activated, Slang needs to be built in three steps: build "generators", configure the build with "emcmake" and build.
+For more information about "generators", please refer to the later part of the documentation about [cross-compiling](docs/building.md#cross-compiling).
+```bash
+# Build generators.
+cmake --workflow --preset generators --fresh
+mkdir generators
+cmake --install build --prefix generators --component generators
+
+# Configure the build with emcmake.
+# emcmake is available only when emsdk_env setup the environment correctly.
+pushd ../emsdk
+source ./emsdk_env # For Windows, emsdk_env.bat
+popd
+emcmake cmake -DSLANG_GENERATORS_PATH=generators/bin --preset emscripten -G "Ninja"
+
+# Build build.em/Release/bin/libslang.a
 cmake --build --preset emscripten --target slang
 ```
 
-**Note:** `emcmake` is a `cmake` wrapper that comes with [`emsdk`](https://emscripten.org/docs/getting_started/downloads.html), it will wrap your `cmake` calls with setup options that add support for emscripten.
-**Note:** If this fails, try running the command that `emcmake` outputs, directly.
+**Note:** If the last build step fails, try running the command that `emcmake` outputs, directly.
 
 ## Testing