These changes speed up my build (default config/options/targets) by roughly 10%. I suspect the difference may be more significant in other build configs.
Before:
$ time cmake –build build -j24 real 3m26.932s
After:
$ time cmake –build build -j24 real 3m7.556s
Generally they allow for jobservers (either make -jX or ninja) to be better utilized. This can be verified using top while building and looking at the number of compiles running at any given time before/after these changes. Before, it’s easy to observe periods of stalling when only one or two compiles are happening. After these changes, the compiler process count should mostly match the number of jobs given (-jX) until it falls off at the end.
The first commit sets DEPENDS_EXPLICIT_ONLY for commands which generate our test header files. Without this option, test_bitcoin’s generated headers won’t be built until all of its other dependencies have been built. This introduces a significant stall in the build, though currently only Ninja benefits from this being set, and only CMake >= 3.27 understands it.
Example from a generated build.ninja:
Before:
# Custom command for src/test/data/base58_encode_decode.json.h
build src/test/data/base58_encode_decode.json.h | ${cmake_ninja_workdir}src/test/data/base58_encode_decode.json.h: CUSTOM_COMMAND /home/cory/dev/bitcoin/src/test/data/base58_encode_decode.json /home/cory/dev/bitcoin/cmake/script/GenerateHeaderFromJson.cmake || libcrc32c.a libcrc32c_sse42.a libleveldb.a libminisketch.a minisketch_clmul src/bitcoin_clientversion src/crypto/libbitcoin_crypto.a src/crypto/libbitcoin_crypto_avx2.a src/crypto/libbitcoin_crypto_sse41.a src/crypto/libbitcoin_crypto_x86_shani.a src/generate_build_info src/libbitcoin_cli.a src/libbitcoin_common.a src/libbitcoin_consensus.a src/libbitcoin_node.a src/secp256k1/src/libsecp256k1.a src/secp256k1/src/secp256k1_precomputed src/test/util/libtest_util.a src/univalue/libunivalue.a src/util/libbitcoin_util.a src/wallet/libbitcoin_wallet.a src/zmq/libbitcoin_zmq.a
After:
# Custom command for src/test/data/base58_encode_decode.json.h
build src/test/data/base58_encode_decode.json.h | ${cmake_ninja_workdir}src/test/data/base58_encode_decode.json.h: CUSTOM_COMMAND /home/cory/dev/bitcoin/src/test/data/base58_encode_decode.json /home/cory/dev/bitcoin/cmake/script/GenerateHeaderFromJson.cmake
The second commit is more significant. It sets CMAKE_OPTIMIZE_DEPENDENCIES globally, which allows the objects of static libs to be built in parallel when one lib depends on the other. This can be set as a per-lib property, ~but I don’t see any need for that as we don’t currently have any edge-cases where this wouldn’t be ok. If those should arise, we could always disable on a per-lib basis~.
Edit: turns out this triggers an upstream bug, which I guess can be considered an edge-case until fixed in CMake. I’ve added 2 per-lib opt-outs as a result.
Example:
Before:
# Link the static library src/libbitcoin_cli.a
build src/libbitcoin_cli.a: CXX_STATIC_LIBRARY_LINKER__bitcoin_cli_RelWithDebInfo src/CMakeFiles/bitcoin_cli.dir/compat/stdin.cpp.o src/CMakeFiles/bitcoin_cli.dir/rpc/client.cpp.o || src/univalue/libunivalue.a
After:
# Link the static library src/libbitcoin_cli.a
build src/libbitcoin_cli.a: CXX_STATIC_LIBRARY_LINKER__bitcoin_cli_RelWithDebInfo src/CMakeFiles/bitcoin_cli.dir/compat/stdin.cpp.o src/CMakeFiles/bitcoin_cli.dir/rpc/client.cpp.o
