Disable unnecessary rebuilds on git changes #813
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Currently LLVM LTO and other parts depend on the git HEAD file because they pull in the current git revision (and should update when the git revision changes). This is causing some unnecessary rebuilding everytime someone is touching his git HEAD file. This commit is disabling this in future LLVM versions by setting the appropriate flag in LLVM to OFF (doesn't do a lot by now as we first need to get this revision in the ROOT llvm version https://reviews.llvm.org/D35377).
This removes the file dependency between LLVM and the git HEAD file, which currently causes that we rebuild LLVM/Cling/Dictionaries once someone does anything meaningful in git.
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Starting build on |
vgvassilev
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Jul 26, 2017
| # Ugly hack to prevent rebuilding LLVM whenever the git HEAD timestamp | ||
| # changes. This is properly solved by setting LLVM_APPEND_VC_REV to OFF | ||
| # but this only really works once this review is included in our LLVM | ||
| # version: https://reviews.llvm.org/D35377 |
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It's multiple commits that refactored this thing, D35377 is only the one that actually makes it fully work as intended (we don't even have the setting variable he's using in our CMake code yet), and backporting bigger pieces of CMake code isn't a lot of fun.
It's also just until the next LLVM upgrade where the other commit fixes this properly and we can drop this hack.
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Sounds reasonable to me, thanks for explaining.
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This fixes that we often rebuild parts of ROOT by just doing simple git things. Before this change
we had this dependency on the header VCSRevision.h which is used by different parts of LLVM:
After this change we no longer have the git head in here:
I added two commits: One that fixes the current builds in a hacky way and one that is doing it properly but only goes into effect once we have the next LLVM upgrade (as the flag for configuring this is just added recently).