PATCH: Replace @PLT with @GOTPCREL(%rip) in call
Commit Message
Since both PREINIT_FUNCTION and __libc_start_main are defined externally,
they are called via PLT. Use "call *func@GOTPCREL(%rip)" removes the
extra branch to PLT entry.
[BZ #19745]
* sysdeps/x86_64/crti.S (_init): Replace @PLT with
@GOTPCREL(%rip) in call.
* sysdeps/x86_64/start.S (_start): Likewise.
---
sysdeps/x86_64/crti.S | 2 +-
sysdeps/x86_64/start.S | 2 +-
sysdeps/x86_64/sysdep.h | 14 +++++++++++---
3 files changed, 13 insertions(+), 5 deletions(-)
Comments
> --- a/sysdeps/x86_64/crti.S
> +++ b/sysdeps/x86_64/crti.S
> @@ -66,7 +66,7 @@ _init:
> movq PREINIT_FUNCTION@GOTPCREL(%rip), %rax
> testq %rax, %rax
> je .Lno_weak_fn
> - call PREINIT_FUNCTION@PLT
> + call *PREINIT_FUNCTION@GOTPCREL(%rip)
Can't this just be "call *%rax"?
> --- a/sysdeps/x86_64/start.S
> +++ b/sysdeps/x86_64/start.S
> @@ -105,7 +105,7 @@ ENTRY (_start)
>
> /* Call the user's main function, and exit with its value.
> But let the libc call main. */
> - call __libc_start_main@PLT
> + call *__libc_start_main@GOTPCREL(%rip)
> #else
> /* Pass address of our own entry points to .fini and .init. */
> mov $__libc_csu_fini, %R8_LP
In the general case, foo@PLT is preferable to foo@GOTPCREL(%rip) because it
allows for lazy resolution. That's not worthwhile here because _start is
presumably always run and run very early. But it merits a comment saying
why it's not following the normal pattern for PIC calls.
Thanks,
Roland
@@ -66,7 +66,7 @@ _init:
movq PREINIT_FUNCTION@GOTPCREL(%rip), %rax
testq %rax, %rax
je .Lno_weak_fn
- call PREINIT_FUNCTION@PLT
+ call *PREINIT_FUNCTION@GOTPCREL(%rip)
.Lno_weak_fn:
#else
call PREINIT_FUNCTION
@@ -105,7 +105,7 @@ ENTRY (_start)
/* Call the user's main function, and exit with its value.
But let the libc call main. */
- call __libc_start_main@PLT
+ call *__libc_start_main@GOTPCREL(%rip)
#else
/* Pass address of our own entry points to .fini and .init. */
mov $__libc_csu_fini, %R8_LP