Always indirect branch to __libc_start_main via GOT

Since __libc_start_main in libc.so is called very early, lazy binding
isn't relevant.  Always call __libc_start_main with indirect branch via
GOT to avoid extra branch to PLT slot.  In case of static executable,
ld in binutils 2.26 or above can convert indirect branch into direct
branch:

0000000000400a80 <_start>:
  400a80:       31 ed                   xor    %ebp,%ebp
  400a82:       49 89 d1                mov    %rdx,%r9
  400a85:       5e                      pop    %rsi
  400a86:       48 89 e2                mov    %rsp,%rdx
  400a89:       48 83 e4 f0             and    $0xfffffffffffffff0,%rsp
  400a8d:       50                      push   %rax
  400a8e:       54                      push   %rsp
  400a8f:       49 c7 c0 20 1b 40 00    mov    $0x401b20,%r8
  400a96:       48 c7 c1 90 1a 40 00    mov    $0x401a90,%rcx
  400a9d:       48 c7 c7 c0 03 40 00    mov    $0x4003c0,%rdi
  400aa4:       67 e8 96 09 00 00       addr32 callq 401440 <__libc_start_main>
  400aaa:       f4                      hlt

	* sysdeps/x86_64/start.S (_start): Always indirect branch to
	__libc_start_main via GOT.

diff --git a/ChangeLog b/ChangeLog
index 9c417f2..f72f22e 100644 (file)
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,10 @@
 2016-06-09  H.J. Lu  <hongjiu.lu@intel.com>
 
+       * sysdeps/x86_64/start.S (_start): Always indirect branch to
+       __libc_start_main via GOT.
+
+2016-06-09  H.J. Lu  <hongjiu.lu@intel.com>
+
        * sysdeps/x86_64/memcopy.h: New file.
        * sysdeps/x86_64/wordcopy.c: Likewise.
 

diff --git a/sysdeps/x86_64/start.S b/sysdeps/x86_64/start.S
index 2369b69..f1b961f 100644 (file)
--- a/sysdeps/x86_64/start.S
+++ b/sysdeps/x86_64/start.S
@@ -102,23 +102,22 @@ ENTRY (_start)
        mov __libc_csu_init@GOTPCREL(%rip), %RCX_LP
 
        mov main@GOTPCREL(%rip), %RDI_LP
-
-       /* Call the user's main function, and exit with its value.
-          But let the libc call main.  Since __libc_start_main is
-          called very early, lazy binding isn't relevant here.  Use
-          indirect branch via GOT to avoid extra branch to PLT slot.  */
-       call *__libc_start_main@GOTPCREL(%rip)
 #else
        /* Pass address of our own entry points to .fini and .init.  */
        mov $__libc_csu_fini, %R8_LP
        mov $__libc_csu_init, %RCX_LP
 
        mov $main, %RDI_LP
+#endif
 
        /* Call the user's main function, and exit with its value.
-          But let the libc call main.    */
-       call __libc_start_main
-#endif
+          But let the libc call main.  Since __libc_start_main in
+          libc.so is called very early, lazy binding isn't relevant
+          here.  Use indirect branch via GOT to avoid extra branch
+          to PLT slot.  In case of static executable, ld in binutils
+          2.26 or above can convert indirect branch into direct
+          branch.  */
+       call *__libc_start_main@GOTPCREL(%rip)
 
        hlt                     /* Crash if somehow `exit' does return.  */
 END (_start)