[llvm-dev] RFC: Comprehensive Static Instrumentation

Thu Jun 16 15:27:50 PDT 2016

Hi TB,

Thanks for you answer.

> On Jun 16, 2016, at 2:50 PM, TB Schardl <neboat at mit.edu> wrote:
> 
> Hey Mehdi,
> 
> Thank you for your comments.  I've CC'd the CSI mailing list with your comments and put my responses inline.  Please let me know any other questions you have.
> 
> Cheers,
> TB
> 
> On Thu, Jun 16, 2016 at 3:48 PM, Mehdi Amini <mehdi.amini at apple.com <mailto:mehdi.amini at apple.com>> wrote:
> 
>> On Jun 16, 2016, at 9:01 AM, TB Schardl via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>> 
>> Hey LLVM-dev,
>> 
>> We propose to build the CSI framework to provide a comprehensive suite of compiler-inserted instrumentation hooks that dynamic-analysis tools can use to observe and investigate program runtime behavior.  Traditionally, tools based on compiler instrumentation would each separately modify the compiler to insert their own instrumentation.  In contrast, CSI inserts a standard collection of instrumentation hooks into the program-under-test.  Each CSI-tool is implemented as a library that defines relevant hooks, and the remaining hooks are "nulled" out and elided during link-time optimization (LTO), resulting in instrumented runtimes on par with custom instrumentation.  CSI allows many compiler-based tools to be written as simple libraries without modifying the compiler, greatly lowering the bar for
>> developing dynamic-analysis tools.
>> 
>> ================
>> Motivation
>> ================
>> 
>> Key to understanding and improving the behavior of any system is visibility -- the ability to know what is going on inside the system.  Various dynamic-analysis tools, such as race detectors, memory checkers, cache simulators, call-graph generators, code-coverage analyzers, and performance profilers, rely on compiler instrumentation to gain visibility into the program behaviors during execution.  With this approach, the tool writer modifies the compiler to insert instrumentation code into the program-under-test so that it can execute behind the scene while the program-under-test runs.  This approach, however, means that the development of new tools requires compiler work, which many potential tool writers are ill equipped to do, and thus raises the bar for building new and innovative tools.
>> 
>> The goal of the CSI framework is to provide comprehensive static instrumentation through the compiler, in order to simplify the task of building efficient and effective platform-independent tools.  The CSI framework allows the tool writer to easily develop analysis tools that require
>> compiler instrumentation without needing to understand the compiler internals or modifying the compiler, which greatly lowers the bar for developing dynamic-analysis tools.
>> 
>> ================
>> Approach
>> ================
>> 
>> 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...
> 
> Here's the idea.  When a tool user, who has a program they want to instrument, compiles their program source into an object/bitcode, he can turn on the CSI compile-time pass to insert instrumentation hooks (function calls to instrumentation routines) throughout the IR of the program.  Separately, a tool writer implements a particular tool by writing a library that defines the subset of instrumentation hooks she cares about.  At link time, the object/bitcode of the program source is linked with the object file/bitcode of the tool, resulting in a tool-instrumented executable.  When LTO is used at link time, unused instrumentation is elided, and additional optimizations can run on the instrumented program.  (I'm happy to send you a nice picture that we have of this flow, if the mailing list doesn't mind.)

Ok this is roughly what I had in mind.

I still believe it is not great to rely on LTO, and better, it is not needed to achieve this result.

For instance, I don't see why the "library" that defines the subset of instrumentation hooks used by this tool can't be fed during a regular compile, and the useless hook be eliminated at this point. 
Implementation detail, but in practice, instead of feeding the library itself, the "framework" that allows to generate the library for the tool writer can output a "configuration file" along side the library, and this configuration file is what is fed to the compiler and tells the instrumentation pass which of the hooks to generate. It sounds more efficient to me, and remove the dependency on LTO. 
I imagine there is a possible drawback that I'm missing right now...

> 
> The final binary is specialized to a given tool.  One advantage of CSI, however, is that a single set of instrumentation covers the needs of a wide variety of tools, since different tools provide different implementations of the same hooks.  The specialization of a binary to a given tool happens at link time.
> 
> 
> 
>> instrumented program-under-test is run.  Furthermore, LTO can optimize a tool's instrumentation within a program using traditional compiler optimizations.  Our initial study indicates that the use of LTO does not unduly slow down the build time
> 
> This is a false claim: LTO has a very large overhead, and especially is not parallel, so the more core you have the more the difference will be. We frequently observes builds that are 3 times slower. Moreover, LTO is not incremental friendly and during debug (which is very relevant with sanitizer) rebuilding involves waiting for the full link to occur again.
> 
> 
> Can you please point us towards some projects where LTO incurs a 3x slowdown?  We're interested in the overhead of LTO on build times, and although we've found LTO to incur more overhead on parallel build times than serial build times, as you mentioned, the overheads we've measured on serial or parallel builds have been less than 40% (which we saw when building the Apache HTTP server).

I expect this to be reproducible on most non-trivial C/C++ programs.
But taking clang as an example, just running `ninja clang` on OS X a not-so-recent 12-cores machine takes 970s with LTO and 252s without (and I believe this is without debug info...).
Running just `ninja` to build all of llvm/clang here would take *a lot* longer with LTO, and not so much without.

The LTO builds without assert

Best,

-- 
Mehdi

> 
> We've also designed CSI such that it does not depend on LTO for correctness; the program and tool will work correctly with ordinary ld.  Of course, the downside of not using LTO is that instrumentation is not optimized, and in particular, unused instrumentation will incur overhead.
>  
> 
> -- 
> Mehdi
> 
>> , and the LTO can indeed optimize away unused hooks.  One of our experiments with Apache HTTP server shows that, compiling with CSI and linking with the "null" CSI-tool (which consists solely of empty hooks) slows down the build time of the Apache HTTP server by less than 40%, and the resulting tool-instrumented executable is as fast as the original uninstrumented code.
>> 
>> ================
>> CSI version 1
>> ================
>> 
>> The initial version of CSI supports a basic set of hooks that covers the following categories of program objects: functions, function exits (specifically, returns), basic blocks, call sites, loads, and stores.  We prioritized instrumenting these IR objects based on the need of seven example CSI tools, including a race detector, a cache-reuse analyzer, and a code-coverage analyzer.  We plan to evolve the CSI API over time to be more comprehensive, and we have designed the CSI API to be extensible, allowing new instrumentation to be added as needs grow.  We chose to initially implement a minimal "core" set of hooks, because we felt it was best to add new instrumentation on an as-needed basis in order to keep the interface simple.
>> 
>> There are three salient features about the design of CSI.  First, CSI assigns each instrumented program object a unique integer identifier within one of the (currently) six program-object categories.  Within each category, the ID's are consecutively numbered from 0 up to the number of such objects minus 1.  The contiguous assignment of the ID's allows the tool writer to easily keep track of IR objects in the program and iterate through all objects in a category (whether the object is encountered during execution or not).  Second, CSI provides a platform-independent means to relate a given program object to locations in the source code.  Specifically, CSI provides "front-end-data (FED)" tables, which provide file name and source lines for each program object given the object's ID.  Third, each CSI hook takes in as a parameter a "property": a 64-bit unsigned integer that CSI uses to export the results of compiler analyses and other information known at compile time.  The use of properties allow the tool to rely on compiler analyses to optimize instrumentation and decrease overhead.  In particular, since the value of a property is known at compile time, LTO can constant-fold the conditional test around a property to elide unnecessary instrumentation.
>> 
>> ================
>> Future plan
>> ================
>> 
>> We plan to expand CSI in future versions by instrumenting additional program objects, such as atomic instructions, floating-point instructions, and exceptions.  We are also planning to provide additional static information to tool writers, both through information encoded in the properties passed to hooks and by other means.  In particular, we are also looking at mechanisms to present tool writers with more complex static information, such as how different program objects relate to each other, e.g., which basic blocks belong to a given function.
>> 
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