[llvm-dev] Advice on memory copy instrumentation

[Hiroshi Yamauchi via llvm-dev]

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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