https://bugs.llvm.org/show_bug.cgi?id=36303

            Bug ID: 36303
           Summary: PGO leads to performance degradation
           Product: new-bugs
           Version: 6.0
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: new bugs
          Assignee: unassignedb...@nondot.org
          Reporter: vlesc...@gmail.com
                CC: llvm-bugs@lists.llvm.org

Created attachment 19840
  --> https://bugs.llvm.org/attachment.cgi?id=19840&action=edit
Code sample

I've been testing PGO with the code from the thread:
http://lists.llvm.org/pipermail/llvm-dev/2016-May/099395.html

(Code is attached).

And it appeared that using PGO (either -fprofile-instr or -fprofile) lead to
performance degradation. See the results of `{ for i in `seq 1 100`; do time
./test > /dev/null ; done ; } 2>> perf.log` below:

clang -O3: 3.12253
clang -O3 + -fprofile-use: 3.14982
clang -O3 + -fprofile-use + -mllvm -force-precise-rotation-cost : 3.14189
gcc -O3: 3.38019
gcc -O3 + -fprofile-use: 2.937

clang/llvm were built from release_60 branch, gcc version was 7.3.0.

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