[Bug other/55354] [asan] by default, the asan run-time should be linked statically, not dynamically

[dvyukov at google dot com]

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

--- Comment #17 from Dmitry Vyukov <dvyukov at google dot com> 2012-11-19 10:53:04 UTC ---
>When building libtsan as a shared library (for which I had to hack our assembly
>blobs a bit) we get two sources of slowdown: 
>  1. __tsan_read8 and friends are called through PLT
>  2. __tsan_read8 and friends use one extra load to get to TLS

> I bet 9.5% or more of that is due to the PLT call.

That's not the overhead you are looking for, Luke.

We currently compile with -fPIC and link statically, linker inserts only 1
memory dereference in this case. However, -fPIC affects code generation in
compiler, it has to reserve more registers for tls access code and has to
allocate stack frame because of the potential call. Only that causes *20%*
slowdown on a real application (not a synthetic benchmark).

Kostya, to evaluate initial-exec you need to insure that code characteristics
of __tsan_read/write are not affected, i.e. 0 stack spills and analyze script
passes. Everything else we have w/o initial-exec.