[Bug c/56446] New: Generate one fewer relocation when calling a checked weakref function

[thiago at kde dot org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56446



             Bug #: 56446

           Summary: Generate one fewer relocation when calling a checked

                    weakref function

    Classification: Unclassified

           Product: gcc

           Version: 4.8.0

            Status: UNCONFIRMED

          Severity: enhancement

          Priority: P3

         Component: c

        AssignedTo: unassig...@gcc.gnu.org

        ReportedBy: thi...@kde.org





When you have code like:



static int f() __attribute__((weakref("foo"))); 

void g() 

{ 

    int (*ptr)() = f; 

    if (ptr) 

        ptr(); 

}



which is typical for weakref functions, when compiled in PIC/PIE mode, gcc sees

through the variable and generates:



        cmpq    $0, f@GOTPCREL(%rip)

        je      .L1

        xorl    %eax, %eax

        jmp     f@PLT

.L1:

        ret



That means there will be two GOT entries for the "foo" symbol: one in the

actual GOT and one in the .plt.got (lazily initialised). Since the actual GOT

needs to have the address filled in at load time, there's no gain in lazy

initialisation -- in fact, there's a loss.



GCC could do exactly what the code is suggesting and load the actual address

onto a register and then use it. This would save one relocation, the indirect

PLT jumps and the loss in the lazy resolution.