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Subject: [Bug other/55354] [asan] by default, the asan run-time should be linked statically, not dynamically
Date: Sun, 18 Nov 2012 19:54:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55354

--- Comment #10 from Jakub Jelinek <jakub at gcc dot gnu.org> 2012-11-18 19:54:37 UTC ---
(In reply to comment #9)
> 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. 

Except obviously you can't use the last code sequence if you want to link it
into a shared library.  The extra indirection is the standard cost of
relocatable code, especially if there are just a few TLS vars in libtsan and
they are accessed a lot, that memory (the .got section entry) is in caches most
likely and so the indirection can be just a cycle or at most a few of them.
No idea how would you plan to compile libtsan with -fPIE flag, for libtsan.so.0
you obviously can't, it would fail to link or load, and for libtsan.a it would
make the shared library
only usable in executables or PIEs, not from shared libraries.