# Significant Increase in Fat LTO Time Post Update to rustc 1.75.0-nightly **URL:** \ **Category:** compiler\ **Created:** [October 26, 2023, 7:37am UTC](https://internals.rust-lang.org/t/significant-increase-in-fat-lto-time-post-update-to-rustc-1-75-0-nightly/19765 "2023-10-26T07:37:18Z")\ **Posts on this page:** 8\ **Page:** 2
**Author:** ![0xtyls](https://sea2.discourse-cdn.com/flex002/user_avatar/internals.rust-lang.org/0xtyls/32/11456_2.png) [@0xtyls](https://internals.rust-lang.org/u/0xtyls)\ **Post date:** [November 6, 2023, 6:31am UTC](https://internals.rust-lang.org/t/significant-increase-in-fat-lto-time-post-update-to-rustc-1-75-0-nightly/19765/21 "2023-11-06T06:31:17Z")
perf result for `c23cf1f1ae4b4f214b1f56682785fe158af01c1c` ![image](https://us1.discourse-cdn.com/flex002/uploads/rustlang/original/2X/7/7bb89a88cbd091b14dc14dad320b556e5f0fcb21.jpeg) ---
**Author:** ![nikic](https://sea2.discourse-cdn.com/flex002/user_avatar/internals.rust-lang.org/nikic/32/4836_2.png) [@nikic](https://internals.rust-lang.org/u/nikic)\ **Post date:** [November 6, 2023, 11:49am UTC](https://internals.rust-lang.org/t/significant-increase-in-fat-lto-time-post-update-to-rustc-1-75-0-nightly/19765/22 "2023-11-06T11:49:59Z")
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](https://github.com/llvm/llvm-project/pull/71381). 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. ---
**Author:** ![0xtyls](https://sea2.discourse-cdn.com/flex002/user_avatar/internals.rust-lang.org/0xtyls/32/11456_2.png) [@0xtyls](https://internals.rust-lang.org/u/0xtyls)\ **Post date:** [November 7, 2023, 2:45pm UTC](https://internals.rust-lang.org/t/significant-increase-in-fat-lto-time-post-update-to-rustc-1-75-0-nightly/19765/23 "2023-11-07T14:45:38Z")
After adding `-Cllvm-args=-slp-max-vf=200`: ```plaintext ===-------------------------------------------------------------------------=== 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 ``` ![image](https://us1.discourse-cdn.com/flex002/uploads/rustlang/original/2X/5/5317d4e4c1b5da661aaf1908bb7619bb0341c061.jpeg) ---
**Author:** ![0xtyls](https://sea2.discourse-cdn.com/flex002/user_avatar/internals.rust-lang.org/0xtyls/32/11456_2.png) [@0xtyls](https://internals.rust-lang.org/u/0xtyls)\ **Post date:** [November 10, 2023, 2:13am UTC](https://internals.rust-lang.org/t/significant-increase-in-fat-lto-time-post-update-to-rustc-1-75-0-nightly/19765/24 "2023-11-10T02:13:21Z")
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. ---
**Author:** ![Eh2406](https://sea2.discourse-cdn.com/flex002/user_avatar/internals.rust-lang.org/eh2406/32/4839_2.png) [@Eh2406](https://internals.rust-lang.org/u/Eh2406)\ **Post date:** [November 10, 2023, 3:22am UTC](https://internals.rust-lang.org/t/significant-increase-in-fat-lto-time-post-update-to-rustc-1-75-0-nightly/19765/25 "2023-11-10T03:22:39Z")
If it was just merged it will be in tomorrow's nightly. If I am reading [https://forge.rust-lang.org/](https://forge.rust-lang.org/) correctly nightly will have version 1.75. It will be on the stable channel December 28 2023. ---
**Author:** ![the8472](https://avatars.discourse-cdn.com/v4/letter/t/0ea827/32.png) [@the8472](https://internals.rust-lang.org/u/the8472)\ **Post date:** [November 10, 2023, 11:13am UTC](https://internals.rust-lang.org/t/significant-increase-in-fat-lto-time-post-update-to-rustc-1-75-0-nightly/19765/26 "2023-11-10T11:13:04Z")
It's a patch to llvm master. It won't be available in rust until the next major llvm version unless it gets backported. ---
**Author:** ![nikic](https://sea2.discourse-cdn.com/flex002/user_avatar/internals.rust-lang.org/nikic/32/4836_2.png) [@nikic](https://internals.rust-lang.org/u/nikic)\ **Post date:** [November 14, 2023, 11:21am UTC](https://internals.rust-lang.org/t/significant-increase-in-fat-lto-time-post-update-to-rustc-1-75-0-nightly/19765/27 "2023-11-14T11:21:37Z")
The change is part of the LLVM update at [Update to LLVM 17.0.5 by nikic · Pull Request #117907 · rust-lang/rust · GitHub](https://github.com/rust-lang/rust/pull/117907). So once that lands it will be in the next nightly. ---
**Author:** ![system](https://sea2.discourse-cdn.com/flex002/user_avatar/internals.rust-lang.org/system/32/14092_2.png) [@system](https://internals.rust-lang.org/u/system)\ **Post date:** [February 12, 2024, 11:22am UTC](https://internals.rust-lang.org/t/significant-increase-in-fat-lto-time-post-update-to-rustc-1-75-0-nightly/19765/28 "2024-02-12T11:22:23Z")
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