[LLVMdev] GSoC pre-proposal (instrumentation pass)

[David Tweed]

Hi,

Note: I'm not in any way associated with the GSoC infrastructure in the LLVM
community; these are purely personal thoughts.

| I'd like to propose a GSoC project with the goal of implementing a library
for profiling instrumentation of LLVM IR.

| Currently, my idea is make the library general enough to insert arbitrary
code or a call to a void(*)(void) before or after reads/writes from a
specified variable or in the prologue/epilogue of a specified function.

| I would like to build more than this on top of the general interface, but
a lot of profiling specific code is also above LLVM's level, such as getting
timestamps to measure time spent in a function or outputting results to a
file (like gcc -pg).

One of the things that is obviously present in putting in many timing points
is a "Heisenberg-type effect": if you check too frequently you're perturbing
the program (either/both in terms of inhibiting optimizations and changing
run-time behaviour). So one thing that I think would be very interesting
would be a way to specify which useful positions to put timing points on
(not everywhere, but not just at function level granularity). For example,
it might be interesting to put timing points before and after loops (or
perhaps just outermost loops). Of course this is a very vague notion which
would need firming up significantly before beginning. For example, in trying
to determine which formulation has the best cache behaviour adding lots of
timing points may provide enough extra instructions that a smart
out-of-order processor could hide the issues that would manifest in the
non-instrumented code. This sort of stuff would, for me, be most interesting
if it was done at the LLVM rather than clang level, since not every
interesting program is written in a dialect of C.

Now I don't know if either this is of interest to other members of the LLVM
community or this sort of non-known stuff, with a consequently greater
uncertainty is suitable for a GSoC project; these are more my thoughts about
profiling instrumentation in general.

Cheers,

Dave
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