Fixing CMake to avoid unnecessary git_version relinking (#594)

CMakeLists.txt

@@ -124,14 +124,6 @@ include_directories(src/include)
 include_directories(src/collectives)
 include_directories(src/collectives/device)
 
-# Create git version file
-# _git_version.cpp is a dummy output to force re-run prior to build
-ADD_CUSTOM_COMMAND(
-    OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/git_version.cpp
-           ${CMAKE_CURRENT_BINARY_DIR}/_git_version.cpp
-    COMMAND ${CMAKE_COMMAND} -P
-            ${CMAKE_CURRENT_SOURCE_DIR}/cmake/git_version.cmake)
-
 if (BUILD_ALLREDUCE_ONLY)
   add_definitions(-DBUILD_ALLREDUCE_ONLY)
   set(CU_SOURCES
@@ -223,6 +215,20 @@ endforeach(filename)
 
 add_library(rccl ${CPP_SOURCES})
 
+# Create a custom target that creates/updates git_version.cpp
+# that executes whenever rccl is built
+add_custom_target(git_version_check
+  COMMENT "Updating git_version.cpp if necessary"
+  COMMAND ${CMAKE_COMMAND} -P ${CMAKE_CURRENT_SOURCE_DIR}/cmake/git_version.cmake
+  VERBATIM
+)
+
+# Create a dummy git_version.cpp file in case it doesn't exist
+configure_file(src/nccl.h.in ${CMAKE_CURRENT_BINARY_DIR}/git_version.cpp)
+
+# Execute git_version_check whenever rccl library is built
+add_dependencies(rccl git_version_check)
+
 if(TRACE)
   add_definitions(-DENABLE_TRACE)
 endif()

cmake/git_version.cmake

@@ -21,17 +21,22 @@ else()
   string(STRIP "${GIT_DIFF}" GIT_DIFF)
   string(STRIP "${GIT_BRANCH}" GIT_BRANCH)
 
-  set(GIT_VERSION "const char *rcclGitHash =\"${GIT_BRANCH}:${GIT_REV}${GIT_DIFF}\";")
+  set(CURR_GIT_VERSION "const char *rcclGitHash =\"${GIT_BRANCH}:${GIT_REV}${GIT_DIFF}\";")
 endif()
 
-# Compare file with existing file (if any)
-if(EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/git_version.cpp)
-  file(READ ${CMAKE_CURRENT_SOURCE_DIR}/git_version.cpp GIT_VERSION_)
+# Compare file with older git version file (git_version.cpp)
+if (EXISTS ${CMAKE_CURRENT_BINARY_DIR}/git_version.cpp)
+  #MESSAGE(STATUS "Found ${CMAKE_CURRENT_BINARY_DIR}/git_version.cpp")
+  file(READ ${CMAKE_CURRENT_BINARY_DIR}/git_version.cpp PREV_GIT_VERSION)
+  #message(STATUS "CURR GIT version: ${CURR_GIT_VERSION}")
+  #message(STATUS "PREV GIT version: ${PREV_GIT_VERSION}")
+  if (NOT "${CURR_GIT_VERSION}" STREQUAL "${PREV_GIT_VERSION}")
+    message(STATUS "Updating git_version.cpp")
+    file(WRITE ${CMAKE_CURRENT_BINARY_DIR}/git_version.cpp "${CURR_GIT_VERSION}")
+  else()
+    message(STATUS "No changes to git_version.cpp required")
+  endif()
 else()
-  set(GIT_VERSION_ "")
-endif()
-
-# Write updated file
-if (NOT "${GIT_VERSION}" STREQUAL "${GIT_VERSION_}")
-  file(WRITE ${CMAKE_CURRENT_SOURCE_DIR}/git_version.cpp "${GIT_VERSION}")
+  # Create git_version.cpp if it doesn't exist yet
+  file(WRITE ${CMAKE_CURRENT_BINARY_DIR}/git_version.cpp "${GIT_VERSION}")
 endif()