[llvm-bugs] [Bug 39333] New: Redundant jump table index xoring

via llvm-bugs llvm-bugs at lists.llvm.org
Tue Oct 16 15:17:06 PDT 2018


            Bug ID: 39333
           Summary: Redundant jump table index xoring
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Scalar Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: david.bolvansky at gmail.com
                CC: llvm-bugs at lists.llvm.org

template<int> int foo();
typedef int (*int_fun)();

int_fun get_fun(unsigned int x)
        case 0: return foo<0>;
        case 1: return foo<1>;
        case 2: return foo<2>;
        case 3: return foo<3>;
    return 0;

Current (SVN, X86-64, -O3):
get_fun(unsigned int):                         
        and     edi, 3
        xor     edi, 2
        mov     rax, qword ptr [8*rdi + .Lswitch.table.get_fun(unsigned int)]

get_fun(unsigned int):
        and     edi, 3
        mov     rax, QWORD PTR CSWTCH.1[0+rdi*8]

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