[LLVMbugs] [Bug 6009] New: Improved switch conversion

bugzilla-daemon at cs.uiuc.edu bugzilla-daemon at cs.uiuc.edu
Tue Jan 12 04:31:29 PST 2010


           Summary: Improved switch conversion
           Product: new-bugs
           Version: 2.6
          Platform: PC
        OS/Version: Windows XP
            Status: NEW
          Keywords: code-quality
          Severity: normal
          Priority: P2
         Component: new bugs
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: bearophile at mailas.com
                CC: llvmbugs at cs.uiuc.edu

<baldrick> on the IRC channel has suggested me to put this here, despite this
doesn't come from a performance problem in real code (usually I report things
here after I have seen an actual performance problem in some of my code).

In the attach there's a small C program, compiled with gcc 4.3, gcc 4.4 and
llvm-gcc 2.6, it shows an improvement in the compilation of switch tables done
by gcc 4.4 and absent in llvm 2.6.

It looks like a cute optimization, but:
1) It's easy to manually do this optimization (using a constant mapping array
that generally doesn't start from index 0, shifting the starting pointer)
2) I don't think this situation happens often enough in real code.

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