On Friday, 17 June 2016 at 20:36:53 UTC, H. S. Teoh wrote:
- Assuming that a revision control system is in place, and a workspace is checked out on revision X with no further modifications, then invoking the build tool should ALWAYS, without any exceptions, produce exactly the same outputs, bit for bit. I.e., if your workspace faithfully represents revision X in the RCS, then invoking the build tool will produce the exact same binary products as anybody else who checks out revision X, regardless of their initial starting conditions.
Making builds bit-for-bit reproducible is really, really hard to do, particularly on Windows. Microsoft's C/C++ compiler embeds timestamps and other nonsense into the binaries so that every time you build, even when no source changed, you get a different binary. Google wrote a tool to help eliminate this non-determinism as a post-processing step called zap_timestamp[1]. I want to eventually include something like this with Button on Windows. I'll probably have to make a PE reader library first though.
Without reproducible builds, caching outputs doesn't work very well either.
Moral of the story is, if you're writing a compiler, for the sake of build systems everywhere, make the output deterministic! For consecutive invocations, without changing any source code, I want the hashes of the binaries to be identical every single time. DMD doesn't do this and it saddens me greatly.
[1] https://github.com/google/syzygy/tree/master/syzygy/zap_timestamp
