[LLVMbugs] [Bug 6212] New: Clang misses optimisation on if vs else if

[bugzilla-daemon at cs.uiuc.edu]

http://llvm.org/bugs/show_bug.cgi?id=6212

           Summary: Clang misses optimisation on if vs else if
           Product: libraries
           Version: trunk
          Platform: PC
        OS/Version: All
            Status: NEW
          Severity: normal
          Priority: P2
         Component: Scalar Optimizations
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: arplynn at gmail.com
                CC: llvmbugs at cs.uiuc.edu


int test1(int mainType, int subType)
{
        if(mainType == 7)
        {
                subType = 4;
        }

        else if(mainType == 9)
        {
                subType = 6;
        }

        else if(mainType == 11)
        {
                subType = 9;
        }
        return subType;
}

int test2(int mainType, int subType)
{
        if(mainType == 7)
        {
                subType = 4;
        }

        if(mainType == 9)
        {
                subType = 6;
        }

        if(mainType == 11)
        {
                subType = 9;
        }
        return subType;
}

Clang generates more efficient code in the second case than the first, because
the first is transformed into a switch which then gets expanded to longer code
by the code generator.


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