[LLVMbugs] [Bug 17581] New: Unnecessary use of local dynamic on x86_64

bugzilla-daemon at llvm.org bugzilla-daemon at llvm.org
Mon Oct 14 15:13:30 PDT 2013


            Bug ID: 17581
           Summary: Unnecessary use of local dynamic on x86_64
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: Backend: X86
          Assignee: unassignedbugs at nondot.org
          Reporter: rafael.espindola at gmail.com
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

Local dynamic is not any better than general dynamic when there is only one
access. Given

__attribute__((visibility("hidden")))  __thread int obj;
int test() {
  return ((int)&obj) > 0;

clang produces

    leaq    obj at TLSLD(%rip), %rdi
    callq    __tls_get_addr at PLT
    movq    $obj at DTPOFF, %rcx
    addl    %eax, %ecx

which is 21 bytes 

without the hidden attribute clang itself produces

    leaq    obj at TLSGD(%rip), %rdi
    callq    __tls_get_addr at PLT

which is 16 bytes.

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