[llvm-dev] Advice on memory copy instrumentation

[Matt Fysh via llvm-dev]

Thanks Hiroshi, this is exactly what I was looking for! I've been looking
into it for a few days, and I've just found the MemorySSA (virtual IR)
which is probably gonna help me out a bunch. Thanks again :)

On Sat, 22 Feb 2020 at 03:23, Hiroshi Yamauchi <yamauchi at google.com> wrote:

> I'm not sure if I understand exactly what you would like to do, but I
> would imagine one could write a pass that looks for the pattern of "%x =
> load ... store %x ..." and insert a function call that logs the copy. I
> don't know of any existing analysis or utility code that you could build on
> to make it easy to do that, though.
>
> It might be helpful if there is an example of the annotations you have in
> mind.
>
> On Wed, Feb 19, 2020 at 3:07 PM Matt Fysh via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Hi all,
>>
>> Given a couple of lines of C++ code `int x = 42; int y = x`, we end up
>> with the following LLVM IR instructions:
>>
>> %x = alloca i32, align 4
>> %y = alloca i32, align 4
>> store i32 42, i32* %x, align 4
>> %0 = load i32, i32* %x, align 4
>> store i32 %0, i32* %y, align 4
>>
>> Is it possible to instrument the IR to perform a value trace?
>>
>> What I'd like to do is stream a log of memory copies (reads then writes),
>> such that in this example, the final instruction will produce a log event
>> along the lines of:
>> "COPY: Value at <address of x> copied to <address of y>"?
>>
>> Essentially what I'd like to do is annotate particular values, so that
>> when these same values are encountered again later in the program, I can
>> retrieve the annotation. I will also need the annotation to survive copies,
>> moves, etc. This could be considered a lightweight, parallel symbolic trace
>> performed at runtime on a very small subset of program values.
>>
>> I am hoping to implement this tooling at the LLVM IR level, so that it
>> can be useful beyond C++, but if it's easier to instrument the CLang AST
>> instead then I guess I can start there. Looking forward to your responses :)
>>
>>
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>
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