Patchwork PATCH: Replace @PLT with @GOTPCREL(%rip) in call

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Submitter H.J. Lu
Date March 3, 2016, 1:20 p.m.
Message ID <20160303132054.GA3822@intel.com>
Download mbox | patch
Permalink /patch/11173/
State New
Headers show

Comments

H.J. Lu - March 3, 2016, 1:20 p.m.
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(-)
Roland McGrath - March 4, 2016, 12:03 a.m.
> --- 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

Patch

diff --git a/sysdeps/x86_64/crti.S b/sysdeps/x86_64/crti.S
index a345259..3dff141 100644
--- 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)
 .Lno_weak_fn:
 #else
 	call PREINIT_FUNCTION
diff --git a/sysdeps/x86_64/start.S b/sysdeps/x86_64/start.S
index 1374974..22b18ae 100644
--- 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