a hack to do -- jitmaps

[Andy Wingo]

Hey :)

I thought of a thing that I don't have time to implement right now:
perf jitmap support in Guile.

Basically, the "perf" Linux tool is a widely-available instruction-level
and microarchitectural profiler.  It's great: you run "perf record guile
foo.scm", and then you run "perf report", and you get instruction-level
info on the profile.  You can grab call graphs, see performance
counters, all kinds of things.  Good stuff.

With run-time code generation though, perf needs some help from the
program generating code.  The most basic step is what's known as "perf
maps", placed in /tmp/perf-$PID.map.  These are super-basic and just
identify code ranges with functions.  I just pushed a patch doing this.

The more useful thing is what's known as "jitmaps".  See
https://lwn.net/Articles/633846/, and
https://chromium.googlesource.com/v8/v8/+/refs/heads/master/src/diagnostics/perf-jit.cc.
It's totally underdocumented, but it actually saves the JIT code to a
file, which is needed to have instruction-level profiling.  It also adds
support for unwinding frames and source info.

Anyway if anyone wants to take this task in the next few weeks, LMK :)

Andy