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MIR optimization results in significantly increased compile time for a rustc-perf benchmark #68105

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ktrianta opened this issue Jan 10, 2020 · 1 comment · Fixed by #99762
Closed

MIR optimization results in significantly increased compile time for a rustc-perf benchmark #68105

ktrianta opened this issue Jan 10, 2020 · 1 comment · Fixed by #99762
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A-LLVM Area: Code generation parts specific to LLVM. Both correctness bugs and optimization-related issues. A-MIR Area: Mid-level IR (MIR) - https://blog.rust-lang.org/2016/04/19/MIR.html A-mir-opt Area: MIR optimizations I-compiletime Issue: Problems and improvements with respect to compile times. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.

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@ktrianta
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In PR #66329 we created a MIR optimization pass that optimizes away unreachable basic blocks, i.e., basic blocks whose terminator kind is Unreachable.

Running on rustc_perfwe observed improvement in all benchmarks except unicode-normalization (avg ~3%) and deeply-nested-opt (avg: 740644.6%). A more complete analysis of the compile times can be found here and here. As explained by @bjorn3, for deeply-nested-opt the LLVM related queries are the ones slowing down, not anything inside rustc.

Until the root cause of the increase in compile times is found and fixed, we have made the optimization available only as a level 3 optimization.
@jonas-schievink jonas-schievink added A-LLVM Area: Code generation parts specific to LLVM. Both correctness bugs and optimization-related issues. A-MIR Area: Mid-level IR (MIR) - https://blog.rust-lang.org/2016/04/19/MIR.html I-compiletime Issue: Problems and improvements with respect to compile times. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Jan 10, 2020
@mati865
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mati865 commented Jan 11, 2020

Could be interesting to test this PR with #67954 once it's more polished.

@wesleywiser wesleywiser mentioned this issue Jan 15, 2020
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@jonas-schievink jonas-schievink added the A-mir-opt Area: MIR optimizations label May 23, 2020
@jonas-schievink jonas-schievink mentioned this issue Oct 7, 2020
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@Noratrieb Noratrieb mentioned this issue Jul 26, 2022
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@bors bors closed this as completed in 015a824 Aug 22, 2022
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A-LLVM Area: Code generation parts specific to LLVM. Both correctness bugs and optimization-related issues. A-MIR Area: Mid-level IR (MIR) - https://blog.rust-lang.org/2016/04/19/MIR.html A-mir-opt Area: MIR optimizations I-compiletime Issue: Problems and improvements with respect to compile times. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.
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UnreachableProp: Preserve unreachable branches for multiple targets Noratrieb/rust
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@mati865 @ktrianta @jonas-schievink