Exploring Swift build strategies.

1. Incremental builds
2. Pipelined (two phase) builds

Pipelined builds run two swiftc commands, one that generates a .swiftmodule, and one that generates the binaries. Generating a .swiftmodule is a subset of compilation, generating just a .swiftmodule is faster than full compilation. As an example, module B depends on A. With pipelined builds, B depends on A.swiftmodule, not any of A's binaries. By dividing A's compilation into two phases, the first to generate the A.swiftmodule, B can start compiling sooner, and thus finish sooner.

The strategies live in rules/. To try one, edit Sources/BUILD and pick the desired Swift build strategy to load().

To build the sample project, run:

bazel build Sources/C

or

bazel run Sources/runme

To visualize the build sequence, build with these extra flags:

--experimental_generate_json_trace_profile --profile=trace.json

Then in Chrome open a tab to chrome://tracing, and load or drag in the Bazel generated .json event file.