Comparing C/C++ unity build with regular build on a large codebase (2024)

If you're doing single-core builds, you will get impressive speedups from unity builds.

This is because C++ compilers spends a lot of time redundantly parsing the same headers included in different .cpp files.

Normally, you get enough gains from compiling each .cpp file in parallel that it outweighs the parsing, but if you're artificially limited in parallelism then unity builds can pay for themselves very quickly as they did in the article.

C++20 modules try to split the difference by parsing each header once into a precompiled module, allowing it to reuse work across different .cpp files.

Unfortunately, it means C++ compilation isn't embarrassingly parallel, which is why we have slow buildsystem adoption.

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torginus a day ago | parent [-]

The problem is that due to how templates work, each compilation unit will end up with its own copy of templated function, which creates extra work, code bloat etc.

The compiler also doesn't really inline or optimize functions as well across object boundaries without link-time optimization.

But the linker is single threaded and notoriously slow - with LTO, I wouldnt be surprised it would take up as much time as the whole unity build, and the results are often suboptimal.

Also, C++ syntax is notoriously hard and slow to parse, the clang frontend takes almost as much time to run as LLVM itself.

So probably modules would help a lot with parallel parsing, but that would help unity builds just as much.

	▲	jjmarr 16 hours ago \| parent [-]
		> each compilation unit will end up with its own copy of templated function, which creates extra work, code bloat etc. Yes, that's what causes the parsing bottleneck. Unity builds don't need to create multiple copies of templated functions. C++20 modules could fix that because the function is parsed before substitution. Tbd on if that optimization works yet, I tried it on Clang 18 and it didn't. > But the linker is single threaded and notoriously slow I think most linkers have parallel LTO and `mold` provides actual parallel linking.

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pstomi 2 days ago | parent | prev | next [-]

By reading your initial script,I see that there was absolutely no parallelisation in the initial build.

Was it a choice because you wanted to compare only single core performances?

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saharazara a day ago | parent | prev [-]

3 minutes to process 500k lines of text, it really is pathetic how slow C++ is to compile.

Any new modern language that can't compile 1 million lines in a couple seconds (without optimizations) should be considered a failure.