arg: slightly reduce compilation time

[ngxson]

Tested on my Macbook M3 Max, time reduced from ~13.46s to ~11.33s

master

time cmake --build build --target common
[  0%] Built target build_info
[  6%] Built target ggml-base
[ 10%] Built target ggml-metal
[ 20%] Built target ggml-cpu
[ 20%] Built target ggml-blas
[ 20%] Built target ggml
[ 91%] Built target llama
[ 91%] Building CXX object common/CMakeFiles/common.dir/arg.cpp.o
[ 91%] Linking CXX static library libcommon.a
[100%] Built target common
cmake --build build --target common  13.46s user 0.53s system 97% cpu 14.291 total

Repeated 3 more times:
cmake --build build --target common  13.39s user 0.50s system 98% cpu 14.143 total
cmake --build build --target common  13.63s user 0.50s system 98% cpu 14.357 total
cmake --build build --target common  13.49s user 0.53s system 97% cpu 14.388 total

PR

time cmake --build build --target common
[  0%] Built target build_info
[  6%] Built target ggml-base
[ 10%] Built target ggml-metal
[ 20%] Built target ggml-cpu
[ 20%] Built target ggml-blas
[ 20%] Built target ggml
[ 91%] Built target llama
[ 91%] Building CXX object common/CMakeFiles/common.dir/arg.cpp.o
[ 91%] Linking CXX static library libcommon.a
[100%] Built target common
cmake --build build --target common  11.33s user 0.52s system 97% cpu 12.145 total

Repeated 3 more times:
cmake --build build --target common  11.16s user 0.50s system 97% cpu 11.984 total
cmake --build build --target common  11.26s user 0.50s system 97% cpu 12.048 total
cmake --build build --target common  11.65s user 0.50s system 97% cpu 12.438 total

[ngxson]

Quite an interesting thing, here is the breakdown of the compilation time of each object: https://gist.github.com/ngxson/5569a8ec8f380a2c51df029feab260da

Summarized by AI:

1. common/CMakeFiles/common.dir/chat.cpp.o - 20.8244 seconds
2. tools/server/CMakeFiles/llama-server.dir/server.cpp.o - 15.4134 seconds
3. src/CMakeFiles/llama.dir/llama-model.cpp.o - 10.6133 seconds
4. common/CMakeFiles/common.dir/arg.cpp.o - 10.7781 seconds
5. tests/CMakeFiles/test-chat.dir/test-chat.cpp.o - 10.0094 seconds
6. tests/CMakeFiles/test-backend-ops.dir/test-backend-ops.cpp.o - 11.353 seconds
7. vendor/cpp-httplib/CMakeFiles/cpp-httplib.dir/httplib.cpp.o - 11.5905 seconds
8. src/CMakeFiles/llama.dir/llama-quant.cpp.o - 3.4398 seconds
9. src/CMakeFiles/llama.dir/llama-sampling.cpp.o - 3.07033 seconds
10. src/CMakeFiles/llama.dir/unicode.cpp.o - 4.31515 seconds

[jeffbolznv]

Have you considered using precompiled headers? I've had good luck with them in other projects, at least with msvc.

[ngxson]

Have you considered using precompiled headers? I've had good luck with them in other projects, at least with msvc.

I tried to address #17329 using pre-compiled header but it doesn't resolve the problem. Free free to give it a try on your side.

For the current PR, I'm not quite sure if pre-compiled header can help, as much of the time spent on creating copy constructor of common_arg. I try to prevent as much copy as possible in the current PR (which also improve the runtime performance, though it's very negligible)

[ggerganov]

I don't observe a significant compile-time reduction with this change on M2 Ultra. The reported 15% speed-up is good, though it seems a bit too large for such a change. If you can confirm it then I guess it's ok. Otherwise, if the speedup is small, I'd recommend keeping the old idiomatic l-value reference implementation.