[llvm-bugs] [Bug 26564] New: Performance regression in AA

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=26564

            Bug ID: 26564
           Summary: Performance regression in AA
           Product: new-bugs
           Version: 3.8
          Hardware: PC
                OS: Linux
            Status: NEW
          Keywords: performance
          Severity: normal
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: bsteinbr at gmail.com
                CC: chandlerc at gmail.com, llvm-bugs at lists.llvm.org
    Classification: Unclassified

Since upgrading to a more recent LLVM version for the Rust compiler, there has
been a large increase in compile time spent in LLVM optimizations. The
corresponding Rust bug report can be found here:
https://github.com/rust-lang/rust/issues/31435

At least part of that seems to come from the new AA handling, which exhibits
O(n^2) behavior in some cases with n being the number of active AA passes.

This originates from using the AAResultProxy in the AAResultBase class. For
example, calling AAResults::getModRefBehavior(CallSite) will iterate over all
registered AA results which all may eventually call into
AAResultBase::getModRefBehavior(CallSite) which calls
AAResultProxy::getModRefBehavior(Function*) which will then iterate over all
registered passes again.

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