[llvm-dev] RFC: Comprehensive Static Instrumentation

TB Schardl via llvm-dev llvm-dev at lists.llvm.org
Fri Jun 17 11:27:57 PDT 2016

Hey Ben,

Thank you for your comments.  I've put my response inline.


On Fri, Jun 17, 2016 at 6:29 AM, Craig, Ben via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> On 6/16/2016 2:48 PM, Mehdi Amini via llvm-dev wrote:
> On Jun 16, 2016, at 9:01 AM, TB Schardl via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
> The CSI framework inserts instrumentation hooks at salient locations
> throughout the compiled code of a program-under-test, such as function
> entry and exit points, basic-block entry and exit points, before and after
> each memory operation, etc.  Tool writers can instrument a
> program-under-test simply by first writing a library that defines the
> semantics of relevant hooks
> and then statically linking their compiled library with the
> program-under-test.
> At first glance, this brute-force method of inserting hooks at every
> salient location in the program-under-test seems to be replete with
> overheads.  CSI overcomes these overheads through the use of
> link-time-optimization (LTO), which is now readily available in most major
> compilers, including GCC and LLVM.  Using LTO, instrumentation hooks that
> are not used by a particular tool can be elided, allowing the overheads of
> these hooks to be avoided when the
> I don't understand this flow: the front-end emits all the possible
> instrumentation but the useless calls to the runtime will be removed during
> the link?
> It means that the final binary is specialized for a given tool right? What
> is the advantage of generating this useless instrumentation in the first
> place then? I'm missing a piece here...
> Suppose I want to build a production build, and one build for each of
> With the current approach, I need to compile my source five different
> times, and link five different times.
> With the CSI approach (assuming it was the backing technology behind the
> sanitizers), I need to compile twice (once for production, once for
> instrumentation), then LTO-link five times.  I can reuse my .o files across
> the sanitizer types.

This reduction in the number of compile operations needed, and in the
number intermediate object/bitcode files produced, is indeed an advantage
of the CSI approach.

As an aside, we've been experimenting with linking CSI-instrumented
bitcodes against the "null tool," which implements every instrumentation
hook as a nop, and comparing the performance of those binaries against
production binaries.  Our preliminary tests have shown some promising
results.  For generating main executables, using LTO to link
CSI-instrumented bitcodes with the null tool produces executables that are
as fast as the production executables.  For generating dynamic libraries,
however, using LTO to link the CSI-instrumented bitcode of a dynamic
library with the null tool seems to produce a binary that is slower than
production.  (The Apache HTTP server benchmark we've tried runs roughly 30%
slower when using such null-tool-instrumented dynamic libraries.)  These
results suggest that using LTO to link CSI-instrumented bitcodes with the
null tool is almost, but not quite, able to produce binaries with
production performance, which would allow tool users to only compile their
sources once.

> It's possible that the math doesn't really work out in practice if the
> cost of the LTO-link dwarfs the compile times.
> --
> Employee of Qualcomm Innovation Center, Inc.
> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project
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