[llvm-dev] RFC: Comprehensive Static Instrumentation

[Craig, Ben via llvm-dev]

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 <mailto: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 
ASAN, MSAN, UBSAN, and TSAN.

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.

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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