[CMAKE] Speedup developer builds when passing LLVM_APPEND_VC_REV = OFF

[hintonda]

Summary

Make sure multiple targets don't get rebuilt unnecessarily when LLVM_APPEND_VC_REV = OFF.

In https://reviews.llvm.org/D34560, the default value for LLVM_APPEND_VC_REV was changed from OFF to ON, and subsequently used to control how VCSRevision.h was created.

While this is a good idea in general, it exposed an existing problem where VCRevision.h was always rewritten if the version control file is not found.

Currently, if LLVM_APPEND_VC_REV = OFF, an empty VCSRevision.h will always get rewritten every time cmake is (re)run,
which causes multiple dependencies to get rebuilt unnecessarily. Please see https://bugs.llvm.org/show_bug.cgi?id=33717 for more info.

This change unifies how VCRevision.h is maintained, and only creates an empty VCRevision.h once if LLVM_APPEND_VC_REV = OFF, minimizing the need to rebuild dependencies.

[hintonda]

• Fix typo -- replace tab with spaces.

[hintonda]

• Handle VCSRevision.h correctly when toggling LLVM_APPEND_VC_REV.

[dblaikie]

I don't really have enough cmake-fu to understand what's going on here. (CC'd chandlerc since I think he still technically owns the cmake build... )

[vitalybuka]

Could we also make some bot to check that "cmake/ninja" has nothing to do on incremental build without sync?

[hintonda]

Could we also make some bot to check that "cmake/ninja" has nothing to do on incremental build without sync?

If you create a new bot, you'll get the new default value and get the pre r306858.

This change does 2 things:

• obviate unnecessary work when rerunning cmake if LLVM_APPEND_VC_REV=OFF (which was the previous default and probably still in your CMakeCache if your build directory was setup prior to r306858)
• allows users to toggle LLVM_APPEND_VC_REV and get correct behavior without needing to run git pull, etc...

[hintonda]

Please see https://reviews.llvm.org/D34560 which changed the default value for LLVM_APPEND_VC_REV.

[hintonda]

• Simplify VCRevision.h creation.

[hintonda]

Simplify VCRevision.h creation when LLVM_APPEN_VC_REV = OFF.

[hintonda]

Fix comments -- NFC.

[hintonda]

Only add custom commands if LLVM_APPEND_VC_REV = ON.

[hintonda]

Only add custom commands if LLVM_APPEND_VC_REV = ON -- minimize diff.

[hintonda]

• Remove phony target so rebuilds with nothing to do report 'no work to do.'

[hintonda]

ping...

llvm/trunk/include/llvm/Support/CMakeLists.txt

 # Figure out if we can track VC revisions.
 function(find_first_existing_file out_var)
   foreach(file ${ARGN})
     if(EXISTS "${file}")
       set(${out_var} "${file}" PARENT_SCOPE)
       return()
     endif()
   endforeach()
 endfunction()
 
 macro(find_first_existing_vc_file out_var path)
-  if ( LLVM_APPEND_VC_REV )
     find_program(git_executable NAMES git git.exe git.cmd)
     # Run from a subdirectory to force git to print an absolute path.
     execute_process(COMMAND ${git_executable} rev-parse --git-dir
       WORKING_DIRECTORY ${path}/cmake
       RESULT_VARIABLE git_result
       OUTPUT_VARIABLE git_dir
       ERROR_QUIET)
     if(git_result EQUAL 0)
       string(STRIP "${git_dir}" git_dir)
       set(${out_var} "${git_dir}/logs/HEAD")
       # some branchless cases (e.g. 'repo') may not yet have .git/logs/HEAD
       if (NOT EXISTS "${git_dir}/logs/HEAD")
         file(WRITE "${git_dir}/logs/HEAD" "")
       endif()
     else()
       find_first_existing_file(${out_var}
         "${path}/.svn/wc.db"   # SVN 1.7
         "${path}/.svn/entries" # SVN 1.6
       )
     endif()
-  endif()
 endmacro()
 
 find_first_existing_vc_file(llvm_vc "${LLVM_MAIN_SRC_DIR}")
 
 # The VC revision include that we want to generate.
 set(version_inc "${CMAKE_CURRENT_BINARY_DIR}/VCSRevision.h")
 
 set(get_svn_script "${LLVM_CMAKE_PATH}/GenerateVersionFromCVS.cmake")
 
-if(DEFINED llvm_vc)
+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 ${version_inc} if it's empty -- toggling LLMV_APPEND_VC_REV
+  # from OFF to ON.
+  if(NOT files_not_equal)
+    file(REMOVE "${version_inc}")
+  endif()
+
   # Create custom target to generate the VC revision include.
   add_custom_command(OUTPUT "${version_inc}"
     DEPENDS "${llvm_vc}" "${get_svn_script}"
     COMMAND
     ${CMAKE_COMMAND} "-DSOURCE_DIR=${LLVM_MAIN_SRC_DIR}"
                      "-DNAME=LLVM_REVISION"
                      "-DHEADER_FILE=${version_inc}"
                      -P "${get_svn_script}")
+else()
+  # Make sure ${version_inc} is an empty file.
+  execute_process(COMMAND ${CMAKE_COMMAND} -E copy_if_different
+    "${version_inc}.empty" "${version_inc}")
+endif()
+file(REMOVE "${version_inc}.empty")
 
-  # Mark the generated header as being generated.
-  set_source_files_properties("${version_inc}"
-    PROPERTIES GENERATED TRUE
-               HEADER_FILE_ONLY TRUE)
-else()
-  file(WRITE "${version_inc}" "")
-endif()
 
 add_custom_target(llvm_vcsrevision_h DEPENDS "${version_inc}")