[dragonegg RFC/PATCH] Add low-level support for LLVM instrumentation passes

[Peter Collingbourne]

Hi,

This patch implements the low level functionality required to be
able to use LLVM's instrumentation passes in DragonEgg.  For example,
to instrument a TU using ThreadSanitizer, the following flag can be
used: -fplugin-arg-dragonegg-llvm-option=-tsan

To get things working at a high level, the appropriate runtime
libraries must be linked.  For example, TSan executables must be
linked with libclang_rt.tsan-x86_64.a.  To make things work for
myself, I've been hacking up spec files which emulate the Clang
driver's sanitizer functionality.  For example, here's one for TSan:

------------------------------------------------------------------------
*cc1_options:
+ %{!nostdlib:-fplugin-arg-dragonegg-llvm-option=-tsan} %{!fPIC:%{!fPIE:-fPIE}}

*mflib:
+ %{!shared:%{!nostdlib:-whole-archive /path/to/llvm-build/lib/clang/3.3/lib/linux/libclang_rt.tsan-x86_64.a -no-whole-archive -lpthread -ldl -export-dynamic} %{!pie:%{!r:-pie}}}

*endfile:
%{Ofast|ffast-math|funsafe-math-optimizations:crtfastmath.o%s}    %{mpc32:crtprec32.o%s}    %{mpc64:crtprec64.o%s}    %{mpc80:crtprec80.o%s}    crtendS.o%s crtn.o%s

*startfile:
%{!shared: %{pg|p|profile:gcrt1.o%s;:Scrt1.o%s}}    crti.o%s %{static:crtbeginT.o%s;:crtbeginS.o%s}
------------------------------------------------------------------------

I wanted to send this patch out to open up a discussion on the best
way to support instrumentation in DragonEgg.  I don't think there's
any way around using spec files, for example.  Perhaps we could
have a set of .spec.in files, one for each sanitizer, which have the
right paths substituted in at compile time.  (I asked a GCC developer
whether it would be possible to support custom flags in a spec file,
and his answer was "no".)

Thanks,
-- 
Peter
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