[flang-dev] Out-of-tree flang builds

[Peter Steinfeld via flang-dev]

Out-of-tree builds are a way to build only the flang code using a pre-existing full build of llvm.  Out-of-tree flang builds are four times faster than full builds.  Thus, for those of us actively developing code and reviewing other people's changes, out-of-tree builds significantly improve our productivity.

Unfortunately, out-of-tree builds are not currently supported by the llvm buildbots.  Thus, people changing build files must test this feature themselves.  The process of doing an out-of-tree build is described in the flang overview document -- https://github.com/llvm/llvm-project/tree/master/flang.  Alternatively, people changing build files could ask someone familiar with out-of-tree builds to review their changes.

A recent update from David Truby broke out-of-tree builds -- https://reviews.llvm.org/D84022.  In the conversation in the Phabricator review, David states that he understands that "out-of-tree builds are considered a "best effort" feature that isn't guaranteed to work".  But since out-of-tree builds are critical for my development, I don't consider them optional.

I'm writing this note to bring this issue to the broader flang community.  I consider out-of-tree builds to be a critical feature that no change should break.  What do you think?
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