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

Michael Kruse via flang-dev flang-dev at lists.llvm.org
Fri Jul 31 12:01:29 PDT 2020

Support for out-of-tree builds, like BUILD_SHARED_LIBS or gn builds, are
helpful for developers, but not relevant for release builds. Requiring to
maintain all combinations of build configurations is not feasible before
every commit, this is why they are only best effort. When you notice a
out-of-tree build breaking, you are free to fix the problem (and assuming
the fix is straightforward, low-risk, without review).

However, I am wondering, why isn't an iterative build sufficient? Assuming
you are not touching any non-flang files, make/ninja should not try to
rebuild them.


Am Fr., 31. Juli 2020 um 13:41 Uhr schrieb Peter Steinfeld via flang-dev <
flang-dev at lists.llvm.org>:

> 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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