[cmake] Stop putting the revision info in LLVM_VERSION_STRING

[rnk]

Summary

This reduces the number of build actions after a no-op commit from
thousands to about six, which should be acceptable. If six actions is
still too many, developers can disable the LLVM_APPEND_VC_REV cmake
option.

llvm-config.h is a widely included header that should rarely change.
Before this patch, it would change after every re-configure. Very few
users of llvm-config.h need to know the precise version, and those that
do can migrate to incorporating LLVM_REVISION as provided by
llvm/Support/VCSRevision.h.

This should bring LLVM back to the behavior that it had before r306858
from June 30 2017. Most LLVM tools will now print a version string like
"6.0.0svn" instead of "6.0.0-git-c40c2a23de4".

Fixes PR34308

[hans]

llvm/CMakeLists.txt

207

I had to do a double-take to understand how this affects llvm-config.h.. but I see the function here appends to PACKAGE_VERSION which is then used in the header. ok.

llvm/docs/CMake.rst

249

I suppose it's not actually appending anything anymore, though.

[rnk]

llvm/docs/CMake.rst

249

Yeah, it's really "embedding" the revision somewhere. Let's do a rename in a follow-up. Using a new option name will ensure everyone gets the default build behavior again.

[thakis]

Should we guard clang's SVNVersion.inc and LLD's LLD_REVISION behind this too?

[rnk]

In D37272#1375557, @thakis wrote:

Should we guard clang's SVNVersion.inc and LLD's LLD_REVISION behind this too?

+1, make it so. I'm not sure how popular this option is in practice. I think mainly @espindola used it, but I'm sure there are a small handful of other developers that know about it.

llvm/CMakeLists.txt

if ( LLVM_USE_FOLDERS )
   set_property(GLOBAL PROPERTY USE_FOLDERS ON)
 endif()
 
 include(VersionFromVCS)
 
 option(LLVM_APPEND_VC_REV
   "Embed the version control system revision id in LLVM" ON)
 
-if( LLVM_APPEND_VC_REV )
-  add_version_info_from_vcs(PACKAGE_VERSION)
-endif()
-
 set(PACKAGE_NAME LLVM)
 set(PACKAGE_STRING "${PACKAGE_NAME} ${PACKAGE_VERSION}")
 set(PACKAGE_BUGREPORT "http://llvm.org/bugs/")
 
 set(BUG_REPORT_URL "${PACKAGE_BUGREPORT}" CACHE STRING
   "Default URL where bug reports are to be submitted.")
 
 # Configure CPack.

llvm/docs/CMake.rst

**LLVM_BUILD_TESTS**:BOOL
   for a complete list of unit tests.) It is possible to build all unit tests
   with the target *UnitTests*.
 
 **LLVM_INCLUDE_TESTS**:BOOL
   Generate build targets for the LLVM unit tests. Defaults to ON. You can use
   this option to disable the generation of build targets for the LLVM unit
   tests.
 
 **LLVM_APPEND_VC_REV**:BOOL
   Embed version control revision info (svn revision number or Git revision id).
-  This is used among other things in the LLVM version string (stored in the
-  PACKAGE_VERSION macro). For this to work cmake must be invoked before the
-  build. Defaults to ON.
+  The version info is provided by the ``LLVM_REVISION`` macro in
+  ``llvm/include/llvm/Support/VCSRevision.h``. Developers using git who don't
+  need revision info can disable this option to avoid re-linking most binaries
+  after a branch switch. Defaults to ON.
 
 **LLVM_ENABLE_THREADS**:BOOL
   Build with threads support, if available. Defaults to ON.
 
 **LLVM_ENABLE_CXX1Y**:BOOL
   Build in C++1y mode, if available. Defaults to OFF.
 
 **LLVM_ENABLE_ASSERTIONS**:BOOL

llvm/include/llvm/Support/CMakeLists.txt

 file(WRITE "${version_inc}.empty" "")
 if((DEFINED llvm_vc) AND LLVM_APPEND_VC_REV)
 
   execute_process(COMMAND ${CMAKE_COMMAND} -E compare_files
       "${version_inc}.empty" "${version_inc}"
       RESULT_VARIABLE files_not_equal
       OUTPUT_QUIET
       ERROR_QUIET)
-  # Remove ${version_inc} if it's empty -- toggling LLMV_APPEND_VC_REV
+  # Remove ${version_inc} if it's empty -- toggling LLVM_APPEND_VC_REV
   # from OFF to ON.
   if(NOT files_not_equal)
     file(REMOVE "${version_inc}")
   endif()
 
   # Create custom target to generate the VC revision include.
   add_custom_command(OUTPUT "${version_inc}"
     DEPENDS "${llvm_vc}" "${get_svn_script}"