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Our C++ code base has been using Clang modules and all those optimizations (explicit template instantiations, incremental linking, dynamic libraries, etc.) sinc

Our C++ code base has been using Clang modules and all those optimizations (explicit template instantiations, incremental linking, dynamic libraries, etc.) since 2011.. It also supports unity builds, and C++ modules TS, so we can compare the performance of all 3 approaches Rust consistently beats all of them by at least an order of magnitude in apples to apples comparisons, with at least an order of magnitude less fiddling in terms of modifying the source code, fiddling with build system and compiler options, etc. So in our experience, Rust gives you 10x faster compile times with 10x less effort than C++ if you use similarly sized compilation units. Rust makes it easy to write compilation units that are 10x-100x faster than C++ ones and still compile reasonably fast for their size, but we don’t allow this by convention (we have a clippy lint that rejects crates that are too large).

pjmlp 5y ago
Which happens to be known to not be as good as MSVC in incremental linking and pre-compiled headers. So do you also have something comparable to Incredibuild and Clearcase derived object files in place?
- volta83 5y ago
  Pre Compiled Headers are 1:1 perf wise identical to Clang modules for our 1 million Loc C++ project modulo system noise. There was no difference in compilation speed, neither for clean nor incremental builds. We supported them for a while (until 2015 or so; no perf difference over 3 years), but Clang modules gave the exact same perf and was easier to use, and unity builds were much faster for clean builds anyways, so there was no point in maintaining PCH in parallel to these two other options. For people that are not aware, PCH, modules, etc. bought us 20-40% compile time reduction. I expect 30% or so to be typical for projects that are using TUs correctly. A 30% compile time reduction or even a 2x compile time reduction is not “life changing”. It’s peanuts. Particularly compared with the 10-100x compile time reductions that Rust gives you. From 100min to 1 min, that’s a huge difference. From 100 min to 50-70min? That’s an irrelevant difference.

pjmlp 5y ago

Which happens to be known to not be as good as MSVC in incremental linking and pre-compiled headers. So do you also have something comparable to Incredibuild and Clearcase derived object files in place?

volta83 5y ago
Pre Compiled Headers are 1:1 perf wise identical to Clang modules for our 1 million Loc C++ project modulo system noise. There was no difference in compilation speed, neither for clean nor incremental builds. We supported them for a while (until 2015 or so; no perf difference over 3 years), but Clang modules gave the exact same perf and was easier to use, and unity builds were much faster for clean builds anyways, so there was no point in maintaining PCH in parallel to these two other options. For people that are not aware, PCH, modules, etc. bought us 20-40% compile time reduction. I expect 30% or so to be typical for projects that are using TUs correctly. A 30% compile time reduction or even a 2x compile time reduction is not “life changing”. It’s peanuts. Particularly compared with the 10-100x compile time reductions that Rust gives you. From 100min to 1 min, that’s a huge difference. From 100 min to 50-70min? That’s an irrelevant difference.