Summary:
This diff changes the analysis exploration algorithm from considering
only sequential executions to considering executions of the
interleaving semantics. As part of this, symbolic states are changed
so that each thread has its own registers, while all memory is shared
between them.
Currently only a simple threads interface is supported: they can be
created with `thread_create(&thread_routine)`, they can exit by
returning from `thread_routine`, and they can be joined with
`thread_join`. Current simplifications include that newly created
threads are already runnable, thread routines accept no arguments and
return no result, and no failures are possible.
The concurrent exploration algorithm gives preference to executions
which have fewer context switches, thereby performing an incremental
form of context-bounded analysis.
A form of partial-order reduction is performed, where the symbolic
states are joined across (prefixes of) executions with the same number
of context switches which reach a point where the instruction pointers
and call stacks of all threads are the same. This has the effect of
"dagifying" the concurrent execution tree by merging points after
e.g. threads perform actions that commute with each other. This is
unlike traditional partial-order reduction in that it relies on the
symbolic join to combine the results of commuting operations in a way
that the following symbolic execution can take advantage of, rather
than performing some up-front analysis to identify commuting
operations and quotienting the space of executions. The current state
of the symbolic join and execution is significantly suboptimal in this
regard.
Differential Revision: D29441149
fbshipit-source-id: cf801a6b1
Josh Berdine