[llvm-bugs] [Bug 38912] New: GVNHoist calls IDFCalculator::calculate() passing an non-empty vector

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=38912

            Bug ID: 38912
           Summary: GVNHoist calls IDFCalculator::calculate() passing an
                    non-empty vector
           Product: tools
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: opt
          Assignee: unassignedbugs at nondot.org
          Reporter: alexandros.lamprineas at arm.com
                CC: llvm-bugs at lists.llvm.org

The same underlying bug was initially addressed by rL340818: [GVNHoist] Prune
out useless CHI insertions, where we realized GVNHoist was using excessive
memory. It appears that this wasn't the right fix, since the ubsan buildbot
kept failing with timeouts. On my machine compiling SemaChecking.cpp with
clang++ and both GVNHoist/Ubsan enabled takes approximately half an hour. The
real problem is in GVNHoist::computeInsertionPoints(), where we iterate over
the Value Numbers and calculate the Iterated Dominance Frontiers without
clearing the IDFBlocks vector first.

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