4 ms·

I learned about LTO a few years ago, and it gave a significant runtime speed boost to my hobby project, but as far as I can tell, CMake still just has `CMAKE_IN

I learned about LTO a few years ago, and it gave a significant runtime speed boost to my hobby project, but as far as I can tell, CMake still just has `CMAKE_INTERPROCEDURAL_OPTIMIZATION` as a boolean on/off flag, it doesn't seem to let you specify parallel jobs.

menaerus 3y ago
> it doesn't seem to let you specify parallel jobs. You don't need to if you're using the LLVM linker (lld) - by default it will use all of your HW threads (nr of cores). lld --help | grep thread --threads=<value> Number of threads. '1' disables multi-threading. By default all available hardware threads are used
- mgaunard 3y ago
  Why not just use mold?
  - eru 3y ago
    Mold is only available on Linux.
    - mgaunard 3y ago
      If you insist on using inferior OSes like Mac OS X, there is sold.
      - eru 3y ago
        Yes. But lots of people also have to use eg Windows.
        mgaunard 3y ago
        Windows has WSL.
- BearOso 3y ago
  Yes, but that CMake option also doesn't specify thin LTO, so you're only going to get 1 thread with LLVM.
  - menaerus 3y ago
    lld will launch as many threads by default unless it is explicitly told not to. This is with and without ThinLTO switch. From https://clang.llvm.org/docs/ThinLTO.html#id10 https://clang.llvm.org/docs/ThinLTO.html#id10 By default, the ThinLTO link step will launch as many threads in parallel as there are cores. If the number of cores can’t be computed for the architecture, then it will launch std::thread::hardware_concurrency number of threads in parallel. For machines with hyper-threading, this is the total number of virtual cores. But maybe I misunderstood you.
    - BearOso 3y ago
      Yeah, regular LTO, not thin LTO, isn't currently threaded. CMake uses the former.
      - menaerus 3y ago
        Right. It wasn't clear at the beginning but now I understand your point. Regular LTO (by design) doesn't support concurrency nor does it support incremental builds. OTOH ThinLTO supports both. CMake CMAKE_INTERPROCEDURAL_OPTIMIZATION variable, when set, opts in for LTO so, yes, due to LTO design you will inherently lose concurrent linkage.

menaerus 3y ago

> it doesn't seem to let you specify parallel jobs. You don't need to if you're using the LLVM linker (lld) - by default it will use all of your HW threads (nr of cores). lld --help | grep thread --threads=<value> Number of threads. '1' disables multi-threading. By default all available hardware threads are used

mgaunard 3y ago
Why not just use mold?
- eru 3y ago
  Mold is only available on Linux.
  - mgaunard 3y ago
    If you insist on using inferior OSes like Mac OS X, there is sold.
    - eru 3y ago
      Yes. But lots of people also have to use eg Windows.
      - mgaunard 3y ago
        Windows has WSL.
BearOso 3y ago
Yes, but that CMake option also doesn't specify thin LTO, so you're only going to get 1 thread with LLVM.
- menaerus 3y ago
  lld will launch as many threads by default unless it is explicitly told not to. This is with and without ThinLTO switch. From https://clang.llvm.org/docs/ThinLTO.html#id10 https://clang.llvm.org/docs/ThinLTO.html#id10 By default, the ThinLTO link step will launch as many threads in parallel as there are cores. If the number of cores can’t be computed for the architecture, then it will launch std::thread::hardware_concurrency number of threads in parallel. For machines with hyper-threading, this is the total number of virtual cores. But maybe I misunderstood you.
  - BearOso 3y ago
    Yeah, regular LTO, not thin LTO, isn't currently threaded. CMake uses the former.
    - menaerus 3y ago
      Right. It wasn't clear at the beginning but now I understand your point. Regular LTO (by design) doesn't support concurrency nor does it support incremental builds. OTOH ThinLTO supports both. CMake CMAKE_INTERPROCEDURAL_OPTIMIZATION variable, when set, opts in for LTO so, yes, due to LTO design you will inherently lose concurrent linkage.