Significant Increase in Fat LTO Time Post Update to rustc 1.75.0-nightly

[0xtyls]

perf result for c23cf1f1ae4b4f214b1f56682785fe158af01c1c

[nikic]

Thanks for testing! This confirms that the fix works, so I've submitted it upstream as [GlobalOpt] Cache whether CC is changeable by nikic · Pull Request #71381 · llvm/llvm-project · GitHub.

For the remaining SLPVectorizer issue, it's not really possible to understand the root cause without a test case. Something you could try is to add -Cllvm-args=-slp-max-vf=200 to RUSTFLAGS and see whether that helps.

[0xtyls]

After adding -Cllvm-args=-slp-max-vf=200:

===-------------------------------------------------------------------------===
                          Pass execution timing report
===-------------------------------------------------------------------------===
  Total Execution Time: 1175.9350 seconds (1175.8385 wall clock)

   ---User Time---   --System Time--   --User+System--   ---Wall Time---  --- Name ---
  551.0560 ( 47.9%)   0.6698 (  2.5%)  551.7258 ( 46.9%)  551.7250 ( 46.9%)  SLPVectorizerPass
  152.6149 ( 13.3%)   3.6148 ( 13.6%)  156.2297 ( 13.3%)  156.2006 ( 13.3%)  InstCombinePass
  105.5270 (  9.2%)   1.6281 (  6.1%)  107.1551 (  9.1%)  107.1473 (  9.1%)  GVNPass
  48.7491 (  4.2%)   2.7861 ( 10.5%)  51.5352 (  4.4%)  51.5276 (  4.4%)  InlinerPass
  35.7035 (  3.1%)   3.5199 ( 13.2%)  39.2235 (  3.3%)  39.2242 (  3.3%)  IPSCCPPass
  31.6451 (  2.8%)   0.3663 (  1.4%)  32.0115 (  2.7%)  32.0074 (  2.7%)  DSEPass
  29.9858 (  2.6%)   1.8889 (  7.1%)  31.8747 (  2.7%)  31.8752 (  2.7%)  GlobalOptPass
  23.2393 (  2.0%)   1.4679 (  5.5%)  24.7073 (  2.1%)  24.7075 (  2.1%)  CalledValuePropagationPass

[0xtyls]

Thank you for your efforts and for submitting the fix upstream. I'm eager to know, is there a typical schedule or estimated timeframe for when this PR might be merged into the Rust nightly channel? Any information on this would be greatly appreciated.

[Eh2406]

If it was just merged it will be in tomorrow's nightly. If I am reading https://forge.rust-lang.org/ correctly nightly will have version 1.75. It will be on the stable channel December 28 2023.

[the8472]

It's a patch to llvm master. It won't be available in rust until the next major llvm version unless it gets backported.

[nikic]

The change is part of the LLVM update at Update to LLVM 17.0.5 by nikic · Pull Request #117907 · rust-lang/rust · GitHub. So once that lands it will be in the next nightly.