fix: Check for wasm-opt during configure & run on post_build (AztecPr…

…otocol/barretenberg#175)

circuits/cpp/barretenberg/README.md

@@ -8,6 +8,7 @@
 - clang >= 10 or gcc >= 10
 - clang-format
 - libomp (if multithreading is required. Multithreading can be disabled using the compiler flag `-DMULTITHREADING 0`)
+- wasm-opt (part of the [Binaryen](https://github.com/WebAssembly/binaryen) toolkit)
 
 ### Installing openMP (Linux)
 

circuits/cpp/barretenberg/cpp/src/aztec/CMakeLists.txt

@@ -62,7 +62,7 @@ if(WASM)
     # that as functions in 'env' should be implemented in JS itself.
     # It turns out that just explicitly telling the wasm module which object files to include was easiest.
     add_executable(
-        barretenberg-step1.wasm
+        barretenberg.wasm
         $<TARGET_OBJECTS:srs_objects>
         $<TARGET_OBJECTS:numeric_objects>
         $<TARGET_OBJECTS:crypto_sha256_objects>
@@ -88,22 +88,23 @@ if(WASM)
     # Presumably the -O3 when compiling the object files is fine as it's llvms IR optimiser.
     # The backend optimiser is presumably triggered after linking.
     target_link_options(
-        barretenberg-step1.wasm
+        barretenberg.wasm
         PRIVATE
         -nostartfiles -O2 -Wl,--no-entry -Wl,--export-dynamic -Wl,--import-memory -Wl,--allow-undefined -Wl,--stack-first -Wl,-z,stack-size=1048576
     )
 
+    find_program(WASM_OPT wasm-opt)
+    if(NOT WASM_OPT)
+        message(FATAL_ERROR "wasm-opt executable not found. Please install binaryen.")
+    endif()
+
     add_custom_command(
-        OUTPUT ${CMAKE_RUNTIME_OUTPUT_DIRECTORY}/barretenberg.wasm
-        COMMAND wasm-opt ${CMAKE_RUNTIME_OUTPUT_DIRECTORY}/barretenberg-step1.wasm -O2 --asyncify -o ${CMAKE_RUNTIME_OUTPUT_DIRECTORY}/barretenberg.wasm
-        DEPENDS barretenberg-step1.wasm
+        TARGET barretenberg.wasm
+        POST_BUILD
+        COMMAND wasm-opt "$<TARGET_FILE:barretenberg.wasm>" -O2 --asyncify -o "$<TARGET_FILE:barretenberg.wasm>"
         VERBATIM
     )
 
-    add_custom_target(
-        barretenberg.wasm
-        DEPENDS ${CMAKE_RUNTIME_OUTPUT_DIRECTORY}/barretenberg.wasm
-    )
     # For use when compiling dependent cpp projects for WASM
     message(STATUS "Compiling all-in-one barretenberg WASM archive")
     add_library(