[CMake] Consider LLVM_APPEND_VC_REV when generating SVNVersion.inc

[GBuella]

Summary

This is merely a quickfix, due to being fustrated with waiting
for ~70 objects to be linked each time following git commit --amend.
When toggling LLVM_APPEND_VC_REV between ON and OFF, the
version inc file is still updated. Still, this is arguable
an improvement.

[smeenai]

I believe LLVM_APPEND_VC_REV controls whether the revision is appended to LLVM version string (e.g. the output of llvm-config --version), whereas the SVNRevision.inc file (which is what's causing the relink after amending) is used for e.g. the clang --version output, so I'm not sure this is the right thing to do. I would also love for an option to disable the SVNRevision stuff entirely though.

[GBuella]

In D47912#1125803, @smeenai wrote:

I believe LLVM_APPEND_VC_REV controls whether the revision is appended to LLVM version string (e.g. the output of llvm-config --version), whereas the SVNRevision.inc file (which is what's causing the relink after amending) is used for e.g. the clang --version output, so I'm not sure this is the right thing to do. I would also love for an option to disable the SVNRevision stuff entirely though.

Right, that is why I added the word "quickfix".
Perhaps we can add a CLANG_APPEND_VC_REV CMake option?

[hintonda]

Sorry for the late comment...

Are you amending an llvm commit or a clang commit? The reason I ask is that if I amend an llvm commit, I have to update around 78 targets (llvm + clang + libcxx + libcxxabi + libunwind), but if I amend a clang commit, I only have to update 18 targets.

Since tools/clang/lib/Basic/SVNVersion.inc only includes revision number and repository for clang and llvm, the contents don't change on 'git commit --amend' of the clang repo. Conversely, include/llvm/Support/VCSRevision.h includes the git hash, so it changes each time 'git commit --amend' is run on the llvm repo.

If the clang version is the one causing your problem, perhaps you could add a check to see if the file actually changed before overwriting it. See llvm/include/llvm/Support/CMakeLists.txt for an example of how to do this, i.e., the "undef" case.

lib/Basic/CMakeLists.txt

 set(LLVM_LINK_COMPONENTS
   Core
   MC
   Support
   )
 
 find_first_existing_vc_file(llvm_vc "${LLVM_MAIN_SRC_DIR}")
 find_first_existing_vc_file(clang_vc "${CLANG_SOURCE_DIR}")
 
 # The VC revision include that we want to generate.
 set(version_inc "${CMAKE_CURRENT_BINARY_DIR}/SVNVersion.inc")
 
 set(get_svn_script "${LLVM_CMAKE_PATH}/GetSVN.cmake")
 
-if(DEFINED llvm_vc AND DEFINED clang_vc)
+if(DEFINED llvm_vc AND DEFINED clang_vc AND LLVM_APPEND_VC_REV)
   # Create custom target to generate the VC revision include.
   add_custom_command(OUTPUT "${version_inc}"
     DEPENDS "${llvm_vc}" "${clang_vc}" "${get_svn_script}"
     COMMAND
     ${CMAKE_COMMAND} "-DFIRST_SOURCE_DIR=${LLVM_MAIN_SRC_DIR}"
                      "-DFIRST_NAME=LLVM"
                      "-DSECOND_SOURCE_DIR=${CLANG_SOURCE_DIR}"
                      "-DSECOND_NAME=SVN"