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Build WASM

Build for Browsers​

Build WebAssembly from prime.cpp using Emscripten.

This is the minimal command to build .wasm from .c for browsers. This command generate prime.js and prime.wasm files. We need both to run the WebAssembly on browsers.

build for browsers
em++ src/prime.cpp -o prime.js -O3 -flto -sWASM=1 -sEXIT_RUNTIME=1 -sINVOKE_RUN=0 -sEXPORTED_FUNCTIONS=_prime -sMODULARIZE=1
build for browsers with more options
em++ src/prime.cpp -o prime.js -O3 -flto -sWASM=1 -sEXIT_RUNTIME=1 -sINVOKE_RUN=0 -sEXPORTED_FUNCTIONS=_prime,_malloc -sINITIAL_MEMORY=32mb -sALLOW_MEMORY_GROWTH=1 -sMODULARIZE=1
  • -O3 : optimize from the standpoint of speed
  • -flto : enable link time optimization
  • -sWASM=1 : compile targeting wasm
  • -sEXIT_RUNTIME=1 : do not exit wasm at the exit of the function because we want to read the memory of the wasm
  • -sINVOKE_RUN=0 : do not invoke main() function on instantiation
  • -sEXPORTED_FUNCTIONS=_prime,_malloc : make the functions available from javascript
  • -sINITIAL_MEMORY=32mb : enlarge initialmemory size (default is 16 MB)
  • -sMODULARIZE=1 : modularize javascript to enable reuse. if not set, module is initialize globally in the emitted javascript

If optimization options is set to -O3 or higher, function names will be broken. This is not the big problem when using the glue javascript code emitted by compiler.
Other important options like -sEXPORTED_RUNTIME_METHODS=ccall,cwrap or -sALLOW_MEMORY_GROWTH=1 can be found at Emscripten Compiler Frontend (emcc).

Build for Standalone​

When building the program for standalone runtime, -sSTANDALONE_WASM=1 option is used. Because main function is not contained, --no-entry option is used here. We need to tell the commands what function should be exported so that we can call them.

build for standalone
em++ src/prime.cpp -o prime_standalone.wasm -O3 -flto -sWASM=1 -sEXPORTED_FUNCTIONS=_prime -sSTANDALONE_WASM=1 --no-entry

or, when using WAVM runtime

build for standalone WAVM runtime
em++ src/print_prime.cpp src/prime.cpp -o print_prime_standalone.wasm -O3 -flto -sWASM=1 -sEXPORTED_FUNCTIONS=_prime,_print_prime -sSTANDALONE_WASM=1 --no-entry
  • --no-entry : this is required when not setting _main in the EXPORTED_FUNCTIONS option