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MIR optimization results in significantly increased compile time for a rustc-perf benchmark #68105
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A-LLVMArea: Code generation parts specific to LLVM. Both correctness bugs and optimization-related issues.Area: Code generation parts specific to LLVM. Both correctness bugs and optimization-related issues.A-MIRArea: Mid-level IR (MIR) - https://blog.rust-lang.org/2016/04/19/MIR.htmlArea: Mid-level IR (MIR) - https://blog.rust-lang.org/2016/04/19/MIR.htmlA-mir-optArea: MIR optimizationsArea: MIR optimizationsI-compiletimeIssue: Problems and improvements with respect to compile times.Issue: Problems and improvements with respect to compile times.T-compilerRelevant to the compiler team, which will review and decide on the PR/issue.Relevant to the compiler team, which will review and decide on the PR/issue.
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A-LLVMArea: Code generation parts specific to LLVM. Both correctness bugs and optimization-related issues.Area: Code generation parts specific to LLVM. Both correctness bugs and optimization-related issues.A-MIRArea: Mid-level IR (MIR) - https://blog.rust-lang.org/2016/04/19/MIR.htmlArea: Mid-level IR (MIR) - https://blog.rust-lang.org/2016/04/19/MIR.htmlA-mir-optArea: MIR optimizationsArea: MIR optimizationsI-compiletimeIssue: Problems and improvements with respect to compile times.Issue: Problems and improvements with respect to compile times.T-compilerRelevant to the compiler team, which will review and decide on the PR/issue.Relevant to the compiler team, which will review and decide on the PR/issue.
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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.
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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, fordeeply-nested-optthe 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.