[LLVMbugs] [Bug 3148] New: unnecessary GOT lookup when calling visibility hidden or protected function

[bugzilla-daemon at cs.uiuc.edu]

http://llvm.org/bugs/show_bug.cgi?id=3148

           Summary: unnecessary GOT lookup when calling visibility hidden or
                    protected function
           Product: libraries
           Version: trunk
          Platform: PC
        OS/Version: Linux
            Status: NEW
          Severity: normal
          Priority: P2
         Component: Common Code Generator Code
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: nicholas at mxc.ca
                CC: llvmbugs at cs.uiuc.edu


GCC elides the GOT lookup when calling a hidden or protected function. Here's a
quick, silly example:

  class __attribute__((visibility("hidden"))) Foo {
  public:
    void f(void) __attribute__((noinline)) {
      for (int i = 0; i < 1000000; i += 2)
        if (i < 123) f();
      return;
    }
  };

  void g(void) {
    Foo foo;
    foo.f();
  }

Firstly, if f() is empty, GCC inlines it into g() despite the noinline tag. I
consider that naughty, and we shouldn't follow its lead there.

Secondly, GCC generates this code for g() with -O2 -fPIC:
_Z1gv:
        pushl   %ebp
        movl    %esp, %ebp
        subl    $24, %esp
        leal    -1(%ebp), %eax
        movl    %eax, (%esp)
        call    _ZN3Foo1fEv
        leave
        ret

while LLVM produces this:

_Z1gv:
        pushl   %ebp
        movl    %esp, %ebp
        pushl   %ebx
        subl    $12, %esp
        call    .Lllvm$1.$piclabel
.Lllvm$1.$piclabel:
        popl    %ebx
        addl    $_GLOBAL_OFFSET_TABLE_ + [.-.Lllvm$1.$piclabel], %ebx
        leal    -8(%ebp), %eax
        movl    %eax, (%esp)
        call    _ZN3Foo1fEv
        addl    $12, %esp
        popl    %ebx
        popl    %ebp
        ret

Because it's hidden, the GOT lookup is unnecessary. Please eliminate it. Same
with protected.


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