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codegen-units + ThinLTO is not as good as codegen-units = 1 #47745
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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.C-optimizationCategory: An issue highlighting optimization opportunities or PRs implementing suchCategory: An issue highlighting optimization opportunities or PRs implementing suchI-slowIssue: Problems and improvements with respect to performance of generated code.Issue: Problems and improvements with respect to performance of generated code.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.C-optimizationCategory: An issue highlighting optimization opportunities or PRs implementing suchCategory: An issue highlighting optimization opportunities or PRs implementing suchI-slowIssue: Problems and improvements with respect to performance of generated code.Issue: Problems and improvements with respect to performance of generated code.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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We recently had a fair amount of reports about code generation quality drop. One of the recent causes for the quality drop is the enablement of codegen-units and ThinLTO.
It seems that ThinLTO is not capable of producing results matching those obtained by compiling without codegen-units in the first place.
The list of known reports follows:
Improvements to ThinLTO quality are inbound with the soon-to-happen LLVM upgrade(s), however those do not help sufficiently, it would be nice to figure out why ThinLTO is not doing good enough job.
cc @alexcrichton @nikomatsakis