[Bug tree-optimization/93521] New: 40% slower in O2 than O1 (tree-pre)

[hehaochen at hotmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93521

            Bug ID: 93521
           Summary: 40% slower in O2 than O1 (tree-pre)
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hehaochen at hotmail dot com
  Target Milestone: ---

Created attachment 47747
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=47747&action=edit
testcase

The attached heap sort is ~40% slower in O2 than O1 (slow with tree-pre)


gcc-9 (Homebrew GCC 9.2.0_2) 9.2.0:

  g++-9 -O1 stepanov_vector.cpp -o a.out && ./a.out
    >> 7.48 sec 

  g++-9 -O1 -ftree-pre stepanov_vector.cpp -o a.out && ./a.out
    >> 12.21 sec

  g++-9 -O2 stepanov_vector.cpp -o a.out && ./a.out
    >> 12.25 sec

  g++-9 -O2 -fno-inline-small-functions stepanov_vector.cpp -o a.out && ./a.out
    >> 10.72 sec

  g++-9 -O2 -fno-tree-pre stepanov_vector.cpp -o a.out && ./a.out
    >> 9.63 sec


while in clang:
Apple clang version 11.0.0 (clang-1100.0.33.17)

  clang++ -O1 stepanov_vector.cpp
    >> 38.04 sec

  clang++ -O2 stepanov_vector.cpp
    >> 4.85 sec