[llvm-dev] Finding live values

[Sanjoy Das via llvm-dev]

On Sat, Mar 26, 2016 at 12:52 PM, Rob Lyerly <rlyerly at vt.edu> wrote:
>
> Hi Sanjoy,
>
> I'm interested in being able to reconstruct stack frames at runtime.
> In particular, I'd like to be able to unwind frames from a thread's
> stack and inspect the live values of each individual activation.  I'd
> like to be able to find all live values (whether they be
> programmer-defined variables or compiler-generated intermediates) at
> arbitrary, but statically-known, locations in the the code.  Right
> now, I need the locations of all live values at function call sites.

This sound very interesting!  What do you intend to use this mechanism
for, if you don't mind sharing?

The liveness analysis in the statepoint lowering code cannot be
directly used for this, but it should be possible to extract out a
"liveness analysis utility" that is usable both by the statepoint
lowering code and by whatever it is that you're trying to do.  I'll be
happy to review changes in this direction -- let me know if you're
interested in taking this on, and we can work out the details.

As an aside, have you thought about how you'd represent and lower this
information ("this information" == the set of values live over a
function call)?  LLVM has a notion called operand bundles which may be
useful for you here -- we use it to represent deoptimization state for
specific physical frames, that lets the runtime replace the stack
frames running compiled code with stack frames running interpreted
code (is this similar to what you're trying to do?).

-- Sanjoy