I had been eagerly awaiting the releases of Clang 18 and GCC 14 for the last few weeks. Now that they are both out, I figured I would check how fast they are for compiling PostgreSQL.

(The actual reasons why I was awaiting these compiler releases are related to the warning option -Wmissing-variable-declarations. GCC 14 now supports this option and PostgreSQL might want to use it. On the Clang side, the new release of LLVM enables a new release of IWYU that handles global variable declarations correctly.)

For the speed tests, I’m reusing the methodology from my previous articles about this last year (here and here). The host machine has been upgraded over time in various ways, so the timings are not necessarily comparable across these articles. The tests are done on tag REL_16_3 this time.

OS Compiler time meson compile -j1
macOS 14 gcc-14 4:57.73
  gcc-13 4:45.41
  gcc-12 4:30.17
  clang-18 2:18.10
  clang-17 4:22.19
  clang-16 4:00.87
  clang-15 2:56.41
  Apple clang (15) 2:18.48

Observations:

  • GCC is getting progressively slower with every release, continuing the trend already noticeable last year.

  • Clang had some trouble with versions 16 and 17, but it turned out that this was a compiler bug triggered by a specific file in PostgreSQL. This was apparently fixed in version 18, and now the compile performance is better than version 15 even.

  • Normal Clang has now reached parity with Apple Clang, which was previously mysteriously significantly faster.

The above tests were intentionally done with -j1, but just for illustration I also ran a few tests with -j8:

OS Compiler time meson compile -j8
macOS 14 gcc-14 44.577 s
  clang-18 20.021 s

This shows that this scales pretty well linearly.

Clang 18 is now, on this platform, clearly the best choice.1 Roughly twice as fast as GCC 14!

  1. I had in the past been hesitant to switch to Clang completely because I was worried that Clang would issue different kinds of compiler warnings and I would end up producing code that compiled without warnings on Clang but might produce warnings on GCC that everybody else was using. After investigating this, if you switch from GCC 14 to Clang 18, you lose this two warnings: -Wimplicit-fallthrough=3 -Wshadow=compatible-local. That seems tolerable. ↩