swapcontext_light: An Assembly repository from alk

This is small .so library that provides replacement of Linux/amd64
swapcontext function. Stock swapcontext can be replaced by
LD_PRELOAD-ing this .so file.

Regular swapcontext is relatively slow because it handles signal mask
(so invokes sigprocmask syscall) and FPU control registers. Both are not very
frequently changed, so we're practically wasting hundreds of nanos per
call doing this.

This implementation only handles basic registers, skipping all FPU
state and signal mask manipulation. So it is a bit of hack, but hack
that should work very well in practice.

Ruby's fibers facility, exercised in particular by Enumerator, is
heavily relying on performance of swapcontext.

This 'light' implementation makes swapcontext about 10x faster on my
box. So ruby fibers switching becomes roughly 3.5x faster.

I.e.:

$ LD_PRELOAD=./swapcontext_light.so ruby -rfiber -rbenchmark -e'def ex_fibers(n); f = Fiber.new { n.times { Fiber.yield } }; while f.alive?; f.resume; end; end' -e 'puts Benchmark.measure { ex_fibers(50000000) }'
  6.141363   0.000151   6.141514 (  6.141386)
$ ruby -rfiber -rbenchmark -e'def ex_fibers(n); f = Fiber.new { n.times { Fiber.yield } }; while f.alive?; f.resume; end; end' -e 'puts Benchmark.measure { ex_fibers(50000000) }'
 14.878579   7.223100  22.101679 ( 22.101994)

I've built it as part of work on malloc tracer and I manually tested
it back then. Including correctness of unwind info at each
instruction. So given this is just few instructions, and not likely to
evolve, no tests are provided.

This work is offered under public domain.
alk/swapcontext_light
