5 ms·

> Phoronix even have GCC faster than Clang at building the Linux Kernel, even in an (according to a comment) biased sample where GCC wasn't built with lto enabl

> Phoronix even have GCC faster than Clang at building the Linux Kernel, even in an (according to a comment) biased sample where GCC wasn't built with lto enabled. I don't see that this tells us anything negative about Clang. How is it a bad thing for the compiler to give you the option of a very slow build using additional optimisations? If you don't want that, then don't use that feature.

deleted 6y ago
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ndesaulniers 6y ago
That's not what's going on though. Clang's not slow because LLVM is using additional optimizatons, it's slow because Clang is spending a ton of time in the frontend mostly lexing and keeping track of source locations for macro tokens. (I literally just looked at a perf record of a whole build of the linux kernel with Clang; and know more about the subject than...anyone). ToT LLVM, linux-next x86_64 defconfig: https://photos.app.goo.gl/hNwwy2K1Gh7Uc2D67 https://photos.app.goo.gl/hNwwy2K1Gh7Uc2D67 (yes, I know about disabling kptr_restrict) Our next big project is to look into lazy parsing.
- gsnedders 6y ago
  Who's "our" here?
  - gfxgirl 6y ago
    the linked image has a name associated with it. Googling that name should answer your question
    - ralfd 6y ago
      To save people the click and search: Someone from Google who works on compiling Linux.
  - ndesaulniers 6y ago
    "People who care about improving compile times of the Linux kernel with Clang." "There are literally dozens of us!" /s
mhh__ 6y ago
The performance is basically neck and neck (GCC can be measured to be faster at a few SPEC benchmarks, but they can be contrived and the kernel is often hyper-optimized so the difference may not be down to the optimizer). GCC in this case gives you equal or as-near-as-makes-no-difference up or down performance in less compilation. Some rigs are now posting 17s Kernel build times, reasons to be cheerful

deleted 6y ago

[deleted]

ndesaulniers 6y ago

That's not what's going on though. Clang's not slow because LLVM is using additional optimizatons, it's slow because Clang is spending a ton of time in the frontend mostly lexing and keeping track of source locations for macro tokens. (I literally just looked at a perf record of a whole build of the linux kernel with Clang; and know more about the subject than...anyone). ToT LLVM, linux-next x86_64 defconfig: https://photos.app.goo.gl/hNwwy2K1Gh7Uc2D67 https://photos.app.goo.gl/hNwwy2K1Gh7Uc2D67 (yes, I know about disabling kptr_restrict) Our next big project is to look into lazy parsing.

gsnedders 6y ago
Who's "our" here?
- gfxgirl 6y ago
  the linked image has a name associated with it. Googling that name should answer your question
  - ralfd 6y ago
    To save people the click and search: Someone from Google who works on compiling Linux.
- ndesaulniers 6y ago
  "People who care about improving compile times of the Linux kernel with Clang." "There are literally dozens of us!" /s

mhh__ 6y ago

The performance is basically neck and neck (GCC can be measured to be faster at a few SPEC benchmarks, but they can be contrived and the kernel is often hyper-optimized so the difference may not be down to the optimizer). GCC in this case gives you equal or as-near-as-makes-no-difference up or down performance in less compilation. Some rigs are now posting 17s Kernel build times, reasons to be cheerful