[android] Add libopencilk-abi.mk file to build the OpenCilk bitcode ABI for Android, and update the Android README.

Author: neboat

android/README.md

@@ -8,12 +8,30 @@ These instructions describe how to build the OpenCilk runtime system for Android
 ## Setup Android Studio and NDK
 
 1. Install [Android Studio](https://developer.android.com/studio) with [NDK](https://developer.android.com/studio/projects/install-ndk) onto your system.
-2. Replace NDK's copy of `clang-<version>` with OpenCilk's `clang-<version>`.  For example, for NDK version 28, which includes a prebuilt LLVM toolchain based on LLVM 19, replace the `clang-19` binary in that NDK with OpenCilk's `clang-19` binary.
+2. Replace [NDK's copy of `clang-<version>`](https://developer.android.com/ndk/guides/other_build_systems#overview) with OpenCilk's `clang-<version>` binary.  For example, for NDK version 28, which includes a prebuilt LLVM toolchain based on LLVM 19, replace the `clang-19` binary in that NDK with OpenCilk's `clang-19` binary.
     > [!TIP]
     > Save the distributed `clang-<version>` binary in the NDK somewhere before replacing it with the OpenCilk version, so you can restore it if anything goes wrong.
 
 ## Build the runtime system
 
-1. Run `ndk-build` inside the `android/jni` subdirectory of this repository.
-2. Make sure your Android project contains a `jniLibs` subdirectory in the [correct place](https://developer.android.com/studio/projects/gradle-external-native-builds#jniLibs), for example, as a subdirectory within `src/main`.
-3. Copy `android/libs/*` into your Android project under the `jniLibs` subdirectory.
+1. Export the `NDK` environment variable set to the path to the NDK in Android Studio.
+2. Run `$NDK/ndk-build` inside the `android/jni` subdirectory of this repository.
+3. Run `make -f libopencilk-abi.mk` inside the `android/jni` subdirectory.
+
+## Use OpenCilk in your Android project
+
+1. Make sure your Android project contains a `jniLibs` subdirectory in the [correct place](https://developer.android.com/studio/projects/gradle-external-native-builds#jniLibs), for example, as a subdirectory within `src/main`.
+2. Copy `android/libs/*` in your copy of this repository into the `jniLibs` subdirectory of your Android project.
+3. To compile the Cilk parts of your Android project, add these flags to `CMAKE_C_FLAGS` or `CMAKE_CXX_FLAGS`:
+
+    ```cmake
+    -fopencilk --opencilk-abi-bitcode=${CMAKE_SOURCE_DIR}/../jniLibs/${CMAKE_ANDROID_ARCH_ABI}/libopencilk-abi.bc -femulated-tls
+    ```
+
+4. To link your Android project with Cilk code, add these flags to `CMAKE_SHARED_LINKER_FLAGS`:
+
+    ```cmake
+    -fopencilk -L${CMAKE_SOURCE_DIR}/../jniLibs/${CMAKE_ANDROID_ARCH_ABI}
+    ```
+
+5. [Optional] To use OpenCilk runtime headers, first copy the contents of the `include/cilk` subdirectory in this repository into an `include/cilk` subdirectory in your Android project, such as in `app/src/main/cpp/include/cilk`.  Then use `include_directories()` to add that `include` directory, such as via `include_directories(${CMAKE_SOURCE_DIR}/include)`.

android/jni/libopencilk-abi.mk

@@ -0,0 +1,59 @@
+DIR?=$(shell pwd)
+LOCAL_PATH := $(DIR)
+LIBOPENCILK_ROOT_REL := ../..
+LIBOPENCILK_ROOT_ABS := $(LOCAL_PATH)/../..
+
+UNAME := $(shell uname -s)
+ifneq (,$(findstring Linux,$(UNAME)))
+	HOST_OS := linux
+endif
+ifneq (,$(findstring Darwin,$(UNAME)))
+	HOST_OS := darwin
+endif
+
+TOOLCHAIN=${NDK}/toolchains/llvm/prebuilt/$(HOST_OS)-x86_64
+CC=$(TOOLCHAIN)/bin/clang
+CXX=$(TOOLCHAIN)/bin/clang++
+
+# libopencilk-abi.bc
+
+MODULE := libopencilk-abi
+OUT_DIR := $(LOCAL_PATH)/../libs
+TARGET_AARCH64_DIR := $(OUT_DIR)/arm64-v8a
+TARGET_X86_64_DIR := $(OUT_DIR)/x86_64
+TARGET_AARCH64 := $(TARGET_AARCH64_DIR)/$(MODULE).bc
+TARGET_X86_64 := $(TARGET_X86_64_DIR)/$(MODULE).bc
+API := 28
+
+all: $(TARGET_AARCH64) $(TARGET_X86_64)
+
+SRC_FILES := $(LIBOPENCILK_ROOT_ABS)/runtime/cilk2c_inlined.c
+CFLAGS += \
+	-DCHEETAH_API="" \
+	-DCHEETAH_INTERNAL_NORETURN='__attribute__((noreturn))' \
+	-DCHEETAH_INTERNAL="" \
+	-DCILK_DEBUG=0 \
+	-g -gdwarf-4
+
+C_INCLUDES := -I$(LIBOPENCILK_ROOT_ABS)/include
+
+ifeq (${DEBUG},1)
+	CFLAGS += -O0
+else
+	CFLAGS += -O3
+endif
+
+$(TARGET_AARCH64_DIR) $(TARGET_X86_64_DIR):
+	@mkdir -p $@
+
+$(TARGET_AARCH64) : $(TARGET_AARCH64_DIR)
+$(TARGET_X86_64) : $(TARGET_X86_64_DIR)
+
+$(TARGET_AARCH64) : $(SRC_FILES)
+	$(CC) --target=aarch64-linux-android$(API) $(CFLAGS) $(C_INCLUDES) -c -emit-llvm -o $@ $<
+
+$(TARGET_X86_64) : $(SRC_FILES)
+	$(CC) --target=x86_64-linux-android$(API) $(CFLAGS) $(C_INCLUDES) -c -emit-llvm -o $@ $<
+
+clean:
+	rm $(TARGET_AARCH64) $(TARGET_X86_64)