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From: "steven at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/56446] [4.6/4.7/4.8 Regression] Generate one fewer relocation when calling a checked weakref function
Date: Mon, 25 Feb 2013 22:22:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56446

Steven Bosscher <steven at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |missed-optimization
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2013-02-25
            Summary|Generate one fewer          |[4.6/4.7/4.8 Regression]
                   |relocation when calling a   |Generate one fewer
                   |checked weakref function    |relocation when calling a
                   |                            |checked weakref function
     Ever Confirmed|0                           |1

--- Comment #2 from Steven Bosscher <steven at gcc dot gnu.org> 2013-02-25 22:22:11 UTC ---
Slightly modified test case to avoid the relatively new weakref attribute:

int f() __attribute__((weak));  /* weakref(("foo")) */
void g() 
{ 
    int (*ptr)() = f; 
    if (ptr) 
        ptr(); 
}

GCC 4.8 r196182 (ignoring cfi stuff) with "-O2 -fPIC":
g:
        cmpq    $0, f@GOTPCREL(%rip)
        je      .L1
        xorl    %eax, %eax
        jmp     f@PLT
.L1:
        ret

GCC 4.1.3, GCC 4.4.7 and GCC 4.6.3 produce the same code as above.


GCC 3.4.6 with "-O2 -fPIC":
g:
    movq    f@GOTPCREL(%rip), %rdx
    testq    %rdx, %rdx
    je    .L1
    xorl    %eax, %eax
    movq    %rdx, %r11
    jmp    *%r11
    .p2align 4,,7
.L1:
    rep ; ret


Arguably GCC4 produces better code, but GCC 3.4.6 re-uses the address
loaded in the register.  I think that is the right thing to do, too.

So, marking as a regression.