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Bounds check not optimized out after {r}position on a slice #45964

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#78252
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Bounds check not optimized out after {r}position on a slice#45964
#78252
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A-LLVMArea: Code generation parts specific to LLVM. Both correctness bugs and optimization-related issues.C-enhancementCategory: An issue proposing an enhancement or a PR with one.E-needs-testCall for participation: An issue has been fixed and does not reproduce, but no test has been added.I-slowIssue: 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.

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@dtolnay
dtolnay
opened on Nov 13, 2017

All the unrolling makes it hard for llvm to optimize it out. Sample: https://godbolt.org/g/R9nLvC

#45501 has a fix that involves assume(index < len) but we couldn't quite get it to pass tests. Will need to give it another try with some fresh eyes.

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    A-LLVMArea: Code generation parts specific to LLVM. Both correctness bugs and optimization-related issues.C-enhancementCategory: An issue proposing an enhancement or a PR with one.E-needs-testCall for participation: An issue has been fixed and does not reproduce, but no test has been added.I-slowIssue: 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.

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