[LLVMbugs] [Bug 6891] New: clang 2.6 -O1 faster than clang 2.7

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=6891

           Summary: clang 2.6 -O1 faster than clang 2.7
           Product: new-bugs
           Version: unspecified
          Platform: PC
        OS/Version: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: new bugs
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: edwintorok at gmail.com
                CC: llvmbugs at cs.uiuc.edu


Timing results on the crc32 code in zlib, all code compiled as 64-bit, on an
Intel Core 2 Quad Q9550.
64/32 refers to the element size in the crc32 table.

     (64) O1    O2    O3    Os  (32) O1    O2    O3   Os
gcc-4.4   87.2  92.8  93.3  98.9     90.0  92.8  92.8 100.0
gcc-4.5   94.4  92.8  92.8  98.9     92.2  92.8  92.8  99.4
clang-2.6 82.2  88.9  88.9  88.9     85.6  87.2  86.7  86.7
clang-2.7 91.7  90.6  92.8  90.6     86.1  85.6  87.2  83.9
dragonegg 89.4  87.2  86.7  86.7     90.0  86.7  86.7  87.8

clang 2.6 -O1 / 64 is better than everything else, while LLVM usually prefers
32-bit elements, and -O2/-Os. (gcc tends to prefer -O1 and 64).

It would be interesting to investigate why clang 2.6 -O1 is faster, and fix
clang 2.7 (well trunk/2.8) to be faster too.

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