[Bug 259789] clang 9 and later performance regression using pointers to traverse arrays

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=259789

            Bug ID: 259789
           Summary: clang 9 and later performance regression using
                    pointers to traverse arrays
           Product: Base System
           Version: Unspecified
          Hardware: amd64
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: bin
          Assignee: bugs@FreeBSD.org
          Reporter: jwb@freebsd.org

This may be an upstream issue, but I'm starting here in hopes that someone
would know for sure. I have not yet tried to reproduce this on other platforms.

1. The clang man page states that -O is equivalent to -O2, which is clearly not
the case for clang11 per the benchmark data below.

2. I wrote a very simple benchmark suite using selection sort to compare the
performance of various compilers and interpreters.  There are subscript and
pointer versions for C.  I just noticed that the pointer implementation takes
more than twice as long as the subscript implementations using clang90 or
later.  On amd64 with a modern compiler, I would expect very little difference,
maybe a slight advantage to the pointer implementation, which is what I see
with clang80 and gcc10.

To reproduce:

git clone https://github.com/outpaddling/Lang-speed
cd Lang-speed
./clang-check

clang version 8.0.1 (tags/RELEASE_801/final)                                    
Target: x86_64-portbld-freebsd13.0                                              
Thread model: posix                                                             
InstalledDir: /usr/local/llvm80/bin                                             
-O                                                                              
Subscripts:         2.27 real         2.27 user         0.00 sys                
Pointers:           2.05 real         2.05 user         0.00 sys                
-O2                                                                             
Subscripts:         2.10 real         2.10 user         0.00 sys                
Pointers:           2.02 real         2.02 user         0.00 sys                
clang version 9.0.1                                                             
Target: x86_64-portbld-freebsd13.0                                              
Thread model: posix                                                             
InstalledDir: /usr/local/llvm90/bin                                             
-O                                                                              
Subscripts:         2.09 real         2.09 user         0.00 sys                
Pointers:           4.67 real         4.67 user         0.00 sys                
-O2                                                                             
Subscripts:         2.05 real         2.05 user         0.00 sys                
Pointers:           4.62 real         4.62 user         0.00 sys                
FreeBSD clang version 11.0.1 (git@github.com:llvm/llvm-project.git
llvmorg-11.0.1-0-g43ff75f2c3fe)                                                 
Target: x86_64-unknown-freebsd13.0                                              
Thread model: posix                                                             
InstalledDir: /usr/bin                                                          
-O                                                                              
Subscripts:         4.62 real         4.62 user         0.00 sys                
Pointers:           4.67 real         4.67 user         0.00 sys                
-O2                                                                             
Subscripts:         2.04 real         2.04 user         0.00 sys                
Pointers:           4.61 real         4.61 user         0.00 sys

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