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The hidden compile-time cost of C++26 reflection

(vittorioromeo.com)

31 points

by SuperV12343 days ago |
10 comments
upvoteby r2vcap14 minutes ago|
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Yuck. I’ve already noticed compilation times increasing from C++17 to C++20, and this feature makes it much worse. I guess I’ll need to audit any reflection usage in third-party dependencies.
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upvoteby ralferoo29 minutes ago|
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"I can recompile the entire thing from scratch in ~4.3s. That’s around ~900 TUs, including external dependencies, tests, and examples"

In 30 years of using C++ this is the first time I've ever come across "translation unit" being abbreviated to TU and it took a bit of effort to figure out what the author was trying to say. Not sure why they felt the need to abbreviate this when they explain PCH for instance, which is a far more commonly used term.

Thought I'd add the context here to help anyone else out.

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upvoteby pjmlp20 minutes ago|
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It is quite common when being around WG21 stuff, just as info.
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upvoteby wjakob35 minutes ago|
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I am very worried by feature creep in libc++ and libstdc++ and the harm that this inflicts on the wider C++ ecosystem. Transitive inclusion of large parts of the STL, and entangling of STL with core language features are both extremely bad. This should IMO be topic #1 of the committee but is barely even noticed. The mantra "It's okay, modules will save us" is naive and will not work.
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upvoteby leni5361 hours ago|
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libstdc++'s <print> is very heavy, reflection or not. AFAIK there is no inherent reason for it to be that heavy, fmtlib compiles faster.

<meta> is another question, it depends on string_view, vector, and possibly other parts. Maybe it's possible to make it leaner with more selective internal deps.

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upvoteby craftit49 minutes ago|
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I don't know the exact details, but I have heard (on C++ Weekly, I believe) that it offers some advantages when linking code compiled with different compiler versions. That said, I normally avoid it and use fmtlib to avoid the extra compile time. So it isn't clear if it is a win to me. Header-only libraries are great on small projects, but on large codebases with 1000's of files, it really hits you.
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upvoteby dalvrosa34 minutes ago|
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Nice to learn about hyperfine

Compilation speed is a huge part of productivity and enjoying writing C++

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upvoteby SuperV12343 days ago|
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I ran some more measurements using import std; with a properly built module that includes reflection.

I first created the module via:

  g++ -std=c++26 -fmodules -freflection -fsearch-include-path -fmodule-only -c bits/std.cc
And then benchmarked with:

  hyperfine "g++ -std=c++26 -fmodules -freflection ./main.cpp"
The only "include" was import std;, nothing else.

These are the results:

- Basic struct reflection: 352.8 ms

- Barry's AoS -> SoA example: 1.077 s

Compare that with PCH:

- Basic struct reflection: 208.7 ms

- Barry's AoS -> SoA example: 1.261 s

So PCH actually wins for just <meta>, and modules are not that much better than PCH for the larger example. Very disappointing.

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upvoteby jstimpfle1 hours ago|
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The hidden compile-time cost of <insert almost any C++ feature>
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upvoteby SuperV123413 minutes ago|
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Nah, many great features are extremely cheap. E.g. constexpr, templates, fold expressions, equality operator defaulting, concepts...
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