[rabit] Drop support for MPI backend.

CMakeLists.txt

@@ -47,7 +47,6 @@ option(USE_OPENMP "Build with OpenMP support." ON)
 option(BUILD_STATIC_LIB "Build static library" OFF)
 option(BUILD_DEPRECATED_CLI "Build the deprecated command line interface" OFF)
 option(FORCE_SHARED_CRT "Build with dynamic CRT on Windows (/MD)" OFF)
-option(RABIT_BUILD_MPI "Build MPI" OFF)
 ## Bindings
 option(JVM_BINDINGS "Build JVM bindings" OFF)
 option(R_LIB "Build shared library for R package" OFF)
@@ -106,12 +105,6 @@ if (R_LIB AND GOOGLE_TEST)
   message(WARNING "Some C++ unittests will fail with `R_LIB` enabled,
  as R package redirects some functions to R runtime implementation.")
 endif (R_LIB AND GOOGLE_TEST)
-if (USE_AVX)
-  message(SEND_ERROR  "The option 'USE_AVX' is deprecated as experimental AVX features have been removed from XGBoost.")
-endif (USE_AVX)
-if (PLUGIN_LZ4)
-  message(SEND_ERROR  "The option 'PLUGIN_LZ4' is removed from XGBoost.")
-endif (PLUGIN_LZ4)
 if (PLUGIN_RMM AND NOT (USE_CUDA))
   message(SEND_ERROR "`PLUGIN_RMM` must be enabled with `USE_CUDA` flag.")
 endif (PLUGIN_RMM AND NOT (USE_CUDA))
@@ -144,6 +137,26 @@ if (PLUGIN_FEDERATED)
   endif ()
 endif ()
 
+#-- Removed options
+if (USE_AVX)
+  message(SEND_ERROR  "The option `USE_AVX` is deprecated as experimental AVX features have been removed from XGBoost.")
+endif (USE_AVX)
+if (PLUGIN_LZ4)
+  message(SEND_ERROR  "The option `PLUGIN_LZ4` is removed from XGBoost.")
+endif (PLUGIN_LZ4)
+if (RABIT_BUILD_MPI)
+  message(SEND_ERROR "The option `RABIT_BUILD_MPI` has been removed from XGBoost.")
+endif (RABIT_BUILD_MPI)
+if (USE_S3)
+  message(SEND_ERROR "The option `USE_S3` has been removed from XGBoost")
+endif (USE_S3)
+if (USE_AZURE)
+  message(SEND_ERROR "The option `USE_AZURE` has been removed from XGBoost")
+endif (USE_AZURE)
+if (USE_HDFS)
+  message(SEND_ERROR "The option `USE_HDFS` has been removed from XGBoost")
+endif (USE_HDFS)
+
 #-- Sanitizer
 if (USE_SANITIZER)
   include(cmake/Sanitizer.cmake)
@@ -222,9 +235,6 @@ endif (MSVC)
 
 # rabit
 add_subdirectory(rabit)
-if (RABIT_BUILD_MPI)
-  find_package(MPI REQUIRED)
-endif (RABIT_BUILD_MPI)
 
 # core xgboost
 add_subdirectory(${xgboost_SOURCE_DIR}/src)

cmake/Utils.cmake

@@ -295,10 +295,6 @@ macro(xgboost_target_link_libraries target)
     target_link_libraries(${target} PRIVATE CUDA::nvToolsExt)
   endif (USE_NVTX)
 
-  if (RABIT_BUILD_MPI)
-    target_link_libraries(${target} PRIVATE MPI::MPI_CXX)
-  endif (RABIT_BUILD_MPI)
-
   if (MINGW)
     target_link_libraries(${target} PRIVATE wsock32 ws2_32)
   endif (MINGW)

rabit/CMakeLists.txt

@@ -6,9 +6,7 @@ set(RABIT_SOURCES
   ${CMAKE_CURRENT_LIST_DIR}/src/allreduce_base.cc
   ${CMAKE_CURRENT_LIST_DIR}/src/rabit_c_api.cc)
 
-if (RABIT_BUILD_MPI)
-  list(APPEND RABIT_SOURCES ${CMAKE_CURRENT_LIST_DIR}/src/engine_mpi.cc)
-elseif (RABIT_MOCK)
+if (RABIT_MOCK)
   list(APPEND RABIT_SOURCES ${CMAKE_CURRENT_LIST_DIR}/src/engine_mock.cc)
 else ()
   list(APPEND RABIT_SOURCES ${CMAKE_CURRENT_LIST_DIR}/src/engine.cc)