[LLVMbugs] [Bug 6551] New: C++ constructors not completely eliminated from function-level static constants

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=6551

           Summary: C++ constructors not completely eliminated from
                    function-level static constants
           Product: libraries
           Version: trunk
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: Scalar Optimizations
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: stoklund at 2pi.dk
                CC: llvmbugs at cs.uiuc.edu


llvm-g++ compiles this into a nice constant table:

#include <utility>

typedef std::pair<int,int> P;

static const P table[] = {
  P(1,2),
  P(3,4),
  P(5,6)
};

P f(int n) {
  return table[n];
}

Produces the table:

    .section    __TEXT,__const
    .align    4
__ZL5table:
    .long    1
    .long    2
    .long    3
    .long    4
    .long    5
    .long    6

If the static const table is moved into the function

P f(int n) {
  static const P table[] = {
    P(1,2),
    P(3,4),
    P(5,6)
  };
  return table[n];
}

We get horrible dynamic initialization:

Llabel3:
    movl    %edi, %ebx
    cmpb    $0, __ZGVZ1fiE5table(%rip)
    jne    LBB1_3
    leaq    __ZGVZ1fiE5table(%rip), %rdi
    callq    ___cxa_guard_acquire
    testl    %eax, %eax
    je    LBB1_3
    leaq    __ZGVZ1fiE5table(%rip), %rdi
    movl    $1, __ZZ1fiE5table(%rip)
    movl    $2, __ZZ1fiE5table+4(%rip)
    movl    $3, __ZZ1fiE5table+8(%rip)
    movl    $4, __ZZ1fiE5table+12(%rip)
    movl    $5, __ZZ1fiE5table+16(%rip)
    movl    $6, __ZZ1fiE5table+20(%rip)
    callq    ___cxa_guard_release

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