Disable the code generation module by default

After this patch, users have to add -DCODE_GENERATION=ON to the
cmake command, if they want to compile the code generation module.
A warning is printed if they enable code generation.

This informs the users that the code generation is still under
development and should be used with care. Also, we don't break
the master branch immediately if one of the bigger codegen
patches fails to build on some platform.

When this system is finished, we can enable it by default or
remove this option again.

Change-Id: Ib26498f0d5bd8b3c165807ffd774c057c2d21d39

CMakeLists.txt

@@ -144,6 +144,10 @@ option(BUILD_DOCUMENTATION "Build User's Guide (html)" OFF)
 option(BUILD_EXAMPLES "Build examples" ON)
 option(BUILD_BENCHMARKS "Build Ceres benchmarking suite" ON)
 option(BUILD_SHARED_LIBS "Build Ceres as a shared library." OFF)
+option(CODE_GENERATION "Build the code generation module." OFF)
+if(CODE_GENERATION)
+    message(WARNING "The code generation module is still under development. The functionality and API of the current implementation might change in the future.")
+endif(CODE_GENERATION)
 set(SANITIZERS "" CACHE STRING "Semicolon-separated list of sanitizers to use (e.g address, memory, thread)")
 include(EnableSanitizer)
 enable_sanitizer(${SANITIZERS})

examples/CMakeLists.txt

@@ -72,8 +72,10 @@ target_link_libraries(robust_curve_fitting ceres)
 add_executable(simple_bundle_adjuster simple_bundle_adjuster.cc)
 target_link_libraries(simple_bundle_adjuster ceres)
 
-add_executable(autodiff_codegen autodiff_codegen.cc)
-target_link_libraries(autodiff_codegen ceres)
+if(CODE_GENERATION)
+  add_executable(autodiff_codegen autodiff_codegen.cc)
+  target_link_libraries(autodiff_codegen ceres)
+endif(CODE_GENERATION)
 
 if (GFLAGS)
   add_executable(powell powell.cc)

internal/ceres/CMakeLists.txt

@@ -60,7 +60,6 @@ set(CERES_INTERNAL_SRC
     canonical_views_clustering.cc
     cgnr_solver.cc
     callbacks.cc
-    code_generator.cc
     compressed_col_sparse_matrix_utils.cc
     compressed_row_jacobian_writer.cc
     compressed_row_sparse_matrix.cc
@@ -83,9 +82,6 @@ set(CERES_INTERNAL_SRC
     dynamic_sparse_normal_cholesky_solver.cc
     evaluator.cc
     eigensparse.cc
-    expression.cc
-    expression_graph.cc
-    expression_ref.cc
     file.cc
     float_suitesparse.cc
     float_cxsparse.cc
@@ -152,6 +148,16 @@ set(CERES_INTERNAL_SRC
     wall_time.cc
 )
 
+if(CODE_GENERATION)
+  set(CERES_CODEGEN_SRC
+      code_generator.cc
+      expression.cc
+      expression_graph.cc
+      expression_ref.cc
+  )
+  list(APPEND CERES_INTERNAL_SRC ${CERES_CODEGEN_SRC})
+endif(CODE_GENERATION)
+
 # Also depend on the header files so that they appear in IDEs.
 file(GLOB CERES_INTERNAL_HDRS *.h)
 if (MINIGLOG)
@@ -497,6 +503,7 @@ if (BUILD_TESTING AND GFLAGS)
 
   add_subdirectory(generated_bundle_adjustment_tests)
 
+  if(CODE_GENERATION)
   # Testing the AutoDiffCodegen system is more complicated, because function- and
   # constructor calls have side-effects. In C++ the evaluation order and
   # the elision of copies is implementation defined. Between different compilers,
@@ -524,6 +531,7 @@ if (BUILD_TESTING AND GFLAGS)
       ceres_test(expression_graph)
       set(CMAKE_CXX_FLAGS ${CXX_FLAGS_OLD})
   endif()
+  endif()
 
 endif (BUILD_TESTING AND GFLAGS)