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Comment by ben-schaaf

15 hours ago

Every claim I've seen about unity builds being fast just never rings true to me. I just downloaded the rad debugger and ran the build script on a 7950x (about as fast as you can get). A debug build took 5s, a release build 34s with either gcc or clang.

Maybe it's a MSVC thing - it does seem to have some multi-threading stuff. In any case raddbg non-clean builds take longer than any of my rust projects.

4 comments

ben-schaaf

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maccard  9 hours ago

I use unity builds day in day out. The speed up is an order of magnitude on a 2m+ LOC project.

If you want to see the difference download unreal engine and compile the editor with and without unity builds enabled.

My experience has been the polar opposite of yours - similar size rust projects are an order of magnitude slower than C++ ones. Could you share an example of a project to compare with?

  • ben-schaaf  2 hours ago

    > If you want to see the difference download unreal engine and compile the editor with and without unity builds enabled.

    UE doesn't use a full unity build, it groups some files together into small "modules". I can see how this approach may have some benefits; you're trading off a faster clean build for a slower incremental build.

    I tested compiling UnrealFrontend, and a default setup with the hybrid unity build took 148s. I noticed it was only using half my cores due to memory constraints. I disabled unity and upped the parallelism and got 182s, so 22% slower while still using less memory. A similarly configured unity build was 108s, so best case is ~2x.

    On the other hand only changing the file TraceTools/SFilterPreset.cpp resulted in 10s compilation time under a unity build, and only 2s without unit.

    I can see how this approach has its benefits (and drawbacks). But to be clear this isn't what projects like raddbg and sqlite3 are doing. They're doing a single translation unit for the entire project. No parallelism, no incremental builds, just a single compiler invocation. This is usually what I've seen people mean by a unity build.

    > My experience has been the polar opposite of yours - similar size rust projects are an order of magnitude slower than C++ ones. Could you share an example of a project to compare with?

    I just did a release build of egui in 35s, about the same as raddbg's release build. This includes compiling dependencies like wgpu, serde and about 290 other dependencies which add up to well over a million lines of code.

    Note I do have mold configured as my linker, which speeds things up significantly.

  • almostgotcaught  2 hours ago

    How many LOC is unreal? I'm trying to estimate whether making LLVM compatible with UNITY_BUILD would be worth the effort.

    EDIT: i signed up to get access to unreal so take a look at how they do unity builds and turns out they have their own build tool (not CMake) that orchestrates the build. so does anyone know (can someone comment) whether unity builds for them (unreal) means literally one file for literally all project sources files or if it's "higher-granularity" like UNITY_BUILD in CMake (i.e., single file per object).