This is the mail archive of the
libc-alpha@sourceware.org
mailing list for the glibc project.
Re: PATCH: Replace @PLT with @GOTPCREL(%rip) in call
- From: "H.J. Lu" <hjl dot tools at gmail dot com>
- To: Roland McGrath <roland at hack dot frob dot com>
- Cc: GNU C Library <libc-alpha at sourceware dot org>
- Date: Fri, 4 Mar 2016 15:40:52 -0800
- Subject: Re: PATCH: Replace @PLT with @GOTPCREL(%rip) in call
- Authentication-results: sourceware.org; auth=none
- References: <20160303132054 dot GA3822 at intel dot com> <20160304000323 dot AE59F2C3C13 at topped-with-meat dot com>
On Thu, Mar 3, 2016 at 4:03 PM, Roland McGrath <roland@hack.frob.com> wrote:
>> --- 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"?
Good idea.
>> --- 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.
>
Here are updated patches.
--
H.J.
From 976cfff7e4538503c70413d6746ccbcf87e395e1 Mon Sep 17 00:00:00 2001
From: "H.J. Lu" <hjl.tools@gmail.com>
Date: Fri, 4 Mar 2016 15:28:59 -0800
Subject: [PATCH 1/2] Replace @PLT with @GOTPCREL(%rip) in call
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.
[BZ #19745]
* sysdeps/x86_64/start.S (_start): __libc_start_main@PLT
with *__libc_start_main@GOTPCREL(%rip) in call.
---
ChangeLog | 6 ++++++
sysdeps/x86_64/start.S | 6 ++++--
2 files changed, 10 insertions(+), 2 deletions(-)
diff --git a/ChangeLog b/ChangeLog
index 143d4a6..af55529 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+2016-03-04 H.J. Lu <hongjiu.lu@intel.com>
+
+ [BZ #19745]
+ * sysdeps/x86_64/start.S (_start): Replace __libc_start_main@PLT
+ with *__libc_start_main@GOTPCREL(%rip) in call.
+
2016-03-04 Roland McGrath <roland@hack.frob.com>
* Makefile (tests-special): Add $(objpfx)c++-types-check.out only
diff --git a/sysdeps/x86_64/start.S b/sysdeps/x86_64/start.S
index 1374974..2369b69 100644
--- a/sysdeps/x86_64/start.S
+++ b/sysdeps/x86_64/start.S
@@ -104,8 +104,10 @@ ENTRY (_start)
mov main@GOTPCREL(%rip), %RDI_LP
/* Call the user's main function, and exit with its value.
- But let the libc call main. */
- call __libc_start_main@PLT
+ 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
--
2.5.0
From 9ff9ae1b792e3d986f68a127e0b3f8679e6bea17 Mon Sep 17 00:00:00 2001
From: "H.J. Lu" <hjl.tools@gmail.com>
Date: Fri, 4 Mar 2016 15:36:18 -0800
Subject: [PATCH 2/2] Replace PREINIT_FUNCTION@PLT with *%rax in call
Since we have loaded address of PREINIT_FUNCTION into %rax, we can
avoid extra branch to PLT slot.
[BZ #19745]
* sysdeps/x86_64/crti.S (_init): Replace PREINIT_FUNCTION@PLT
with *%rax in call.
---
ChangeLog | 6 ++++++
sysdeps/x86_64/crti.S | 2 +-
2 files changed, 7 insertions(+), 1 deletion(-)
diff --git a/ChangeLog b/ChangeLog
index af55529..f5b8d1e 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,6 +1,12 @@
2016-03-04 H.J. Lu <hongjiu.lu@intel.com>
[BZ #19745]
+ * sysdeps/x86_64/crti.S (_init): Replace PREINIT_FUNCTION@PLT
+ with *%rax in call.
+
+2016-03-04 H.J. Lu <hongjiu.lu@intel.com>
+
+ [BZ #19745]
* sysdeps/x86_64/start.S (_start): Replace __libc_start_main@PLT
with *__libc_start_main@GOTPCREL(%rip) in call.
diff --git a/sysdeps/x86_64/crti.S b/sysdeps/x86_64/crti.S
index a345259..e9d86ed 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 *%rax
.Lno_weak_fn:
#else
call PREINIT_FUNCTION
--
2.5.0