From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <gcc-bugs-return-407004-listarch-gcc-bugs=gcc.gnu.org@gcc.gnu.org>
Received: (qmail 4346 invoked by alias); 18 Nov 2012 19:36:11 -0000
Received: (qmail 1602 invoked by uid 48); 18 Nov 2012 19:35:46 -0000
From: "konstantin.s.serebryany at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug other/55354] [asan] by default, the asan run-time should be linked statically, not dynamically
Date: Sun, 18 Nov 2012 19:36:00 -0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: changed
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: other
X-Bugzilla-Keywords:
X-Bugzilla-Severity: normal
X-Bugzilla-Who: konstantin.s.serebryany at gmail dot com
X-Bugzilla-Status: RESOLVED
X-Bugzilla-Priority: P3
X-Bugzilla-Assigned-To: unassigned at gcc dot gnu.org
X-Bugzilla-Target-Milestone: ---
X-Bugzilla-Changed-Fields:
Message-ID: <bug-55354-4-izAWMNN2vX@http.gcc.gnu.org/bugzilla/>
In-Reply-To: <bug-55354-4@http.gcc.gnu.org/bugzilla/>
References: <bug-55354-4@http.gcc.gnu.org/bugzilla/>
X-Bugzilla-URL: http://gcc.gnu.org/bugzilla/
Auto-Submitted: auto-generated
Content-Type: text/plain; charset="UTF-8"
MIME-Version: 1.0
Mailing-List: contact gcc-bugs-help@gcc.gnu.org; run by ezmlm
Precedence: bulk
List-Id: <gcc-bugs.gcc.gnu.org>
List-Archive: <http://gcc.gnu.org/ml/gcc-bugs/>
List-Post: <mailto:gcc-bugs@gcc.gnu.org>
List-Help: <mailto:gcc-bugs-help@gcc.gnu.org>
Sender: gcc-bugs-owner@gcc.gnu.org
X-SW-Source: 2012-11/txt/msg01666.txt.bz2


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55354

--- Comment #9 from Konstantin Serebryany <konstantin.s.serebryany at gmail dot com> 2012-11-18 19:35:43 UTC ---
As dvyuokv@ pointed out, 
-ftls-model=initial-exec improves the situation, but does not fully help. 

Experiment: 

% cat x.c 
__thread int a;
int foo() {
  return a;
}


HORRIBLE: -fPIC -shared
% gcc x.c -O2 -fPIC -shared -o x.so  ; objdump -d x.so  | grep foo.: -A 5 
00000000000006e0 <foo>:
 6e0:   66 48 8d 3d f0 08 20    lea    0x2008f0(%rip),%rdi        # 200fd8
<_DYNAMIC+0x1b8>
 6e7:   00 
 6e8:   66 66 48 e8 10 ff ff    callq  600 <__tls_get_addr@plt>
 6ef:   ff 
 6f0:   8b 00                   mov    (%rax),%eax


NOT-SO-BAD: -fPIC -shared  -ftls-model=initial-exec
% gcc x.c -O2 -fPIC -shared -o x.so  -ftls-model=initial-exec ; objdump -d x.so
 | grep foo.: -A 5 
0000000000000630 <foo>:
 630:   48 8b 05 a9 09 20 00    mov    0x2009a9(%rip),%rax        # 200fe0
<_DYNAMIC+0x1b8>
 637:   64 8b 00                mov    %fs:(%rax),%eax
 63a:   c3                      retq   


GOOD: -fPIE 
% gcc -c x.c -O2 -fPIE -o x.o  ; objdump -d x.o  | grep foo.: -A 5 
0000000000000000 <foo>:
   0:   64 8b 04 25 00 00 00    mov    %fs:0x0,%eax
   7:   00 
   8:   c3                      retq   


So, while -ftls-model=initial-exec improves the TLS performance, it is still 
2x slower than -fPIE. 

For tsan, which does this for *every* memory access in the original program, 
this will cost 5%-10% slowdown. 

For our users this is a big deal, so they will link the static library whenever
possible. Which default is used in gcc -- I don't care that much.