[llvm-bugs] [Bug 34308] New: many files rebuilt after adding empty commit to llvm repo (cmake + ninja)

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=34308

            Bug ID: 34308
           Summary: many files rebuilt after adding empty commit to llvm
                    repo (cmake + ninja)
           Product: Build scripts
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: cmake
          Assignee: unassignedbugs at nondot.org
          Reporter: matthias.krueger at famsik.de
                CC: llvm-bugs at lists.llvm.org

This looks like the worse brother of #33717 .

My setup consists of llvm, clang, clang-tools-extra, compiler-rt, polly, lld,
lldb and openmp repos.
Using cmake and ninja to build only the targets clang LLVMgold llvm-ar
llvm-ranlib lld and afterwards check-llvm check-clang check-lld.

I noticed that ninja would always rebuild ~1900 files even if only a single cpp
file changed after the last pull.

Upon investigating, it turned out that an empty commit (git commit
--allow-empty) in the llvm repo is enough to make ninja rebuilt hundreds of
files. :/

However doing consecutive build without file changes/commits works fine and
ninja continues where it was last stopped.

My full bootstrap script can be found here:
https://github.com/matthiaskrgr/llvm_bootstrap/blob/master/build_git.sh

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