Is there a reason why not to use link-time optimization (LTO)?

c++ c performance compilation compiler-optimization
Honza picture Honza · May 19, 2014 · Viewed 27.1k times · Source

GCC, MSVC, LLVM, and probably other toolchains have support for link-time (whole program) optimization to allow optimization of calls among compilation units.

Is there a reason not to enable this option when compiling production software?

Answer

Ali picture Ali · May 19, 2014

I assume that by "production software" you mean software that you ship to the customers / goes into production. The answers at Why not always use compiler optimization? (kindly pointed out by Mankarse) mostly apply to situations in which you want to debug your code (so the software is still in the development phase -- not in production).

6 years have passed since I wrote this answer, and an update is necessary. Back in 2014, the issues were:

  • Link time optimization occasionally introduced subtle bugs, see for example Link-time optimization for the kernel. I assume this is less of an issue as of 2020. Safeguard against these kinds of compiler and linker bugs: Have appropriate tests to check the correctness of your software that you are about to ship.
  • Increased compile time. There are claims that the situation has significantly improved since 2014, for example thanks to slim objects.
  • Large memory usage. This post claims that the situation has drastically improved in recent years, thanks to partitioning.

As of 2020, I would try to use LTO by default on any of my projects.

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