ninja: break unnecessary target dependencies

Previously, given two libraries, X and Y where X depends on Y, all
object compilations of X would require the Y library to have been linked
before being compiled. This is not necessary and can instead be loosened
such that object compilations of X only depend on the order-only
dependencies of Y to be completed. This is to ensure that generated
sources, headers, custom commands, etc. are completed before X starts to
compile its objects.

This should help build performance in projects with many libraries which
cause a deep library dependency chain. Previously, a library at the
bottom would not start compilation until after all other libraries
completed, but now only its link step needs to wait and its compilation
jobs can be run in parallel with other tasks.

Fixes: #15555

4 files changed, 54 insertions, 0 deletions

diff --git a/Tests/RunCMake/Ninja/LooseObjectDepends.cmake b/Tests/RunCMake/Ninja/LooseObjectDepends.cmake
new file mode 100644
index 0000000..360c7ba
--- /dev/null
+++ b/Tests/RunCMake/Ninja/LooseObjectDepends.cmake
@@ -0,0 +1,26 @@
+cmake_minimum_required(VERSION 3.8)
+project(LooseObjectDepends C)
+
+add_custom_command(
+  OUTPUT  "${CMAKE_CURRENT_BINARY_DIR}/command.h"
+  COMMAND "${CMAKE_COMMAND}" -E touch
+          "${CMAKE_CURRENT_BINARY_DIR}/command.h"
+          COMMENT "Creating command.h")
+add_custom_target(create-command.h
+  DEPENDS
+    "${CMAKE_CURRENT_BINARY_DIR}/command.h")
+
+add_custom_target(create-target.h
+  BYPRODUCTS  "${CMAKE_CURRENT_BINARY_DIR}/target.h"
+  COMMAND "${CMAKE_COMMAND}" -E touch
+          "${CMAKE_CURRENT_BINARY_DIR}/target.h"
+  COMMENT "Creating target.h")
+
+add_library(dep SHARED dep.c)
+add_dependencies(dep create-command.h create-target.h)
+target_include_directories(dep
+  PUBLIC
+    "${CMAKE_CURRENT_BINARY_DIR}")
+
+add_library(top top.c)
+target_link_libraries(top PRIVATE dep)
diff --git a/Tests/RunCMake/Ninja/RunCMakeTest.cmake b/Tests/RunCMake/Ninja/RunCMakeTest.cmake
index 063a6d1..fb7798d 100644
--- a/Tests/RunCMake/Ninja/RunCMakeTest.cmake
+++ b/Tests/RunCMake/Ninja/RunCMakeTest.cmake
@@ -95,6 +95,23 @@ ${ninja_stderr}
   endif()
 endfunction(run_ninja)
 
+function (run_LooseObjectDepends)
+  set(RunCMake_TEST_BINARY_DIR ${RunCMake_BINARY_DIR}/LooseObjectDepends-build)
+  run_cmake(LooseObjectDepends)
+  run_ninja("${RunCMake_TEST_BINARY_DIR}" "CMakeFiles/top.dir/top.c${CMAKE_C_OUTPUT_EXTENSION}")
+  if (EXISTS "${RunCMake_TEST_BINARY_DIR}/${CMAKE_SHARED_LIBRARY_PREFIX}dep${CMAKE_SHARED_LIBRARY_SUFFIX}")
+    message(FATAL_ERROR
+      "The `dep` library was created when requesting an object file to be "
+      "built; this should no longer be necessary.")
+  endif ()
+  if (EXISTS "${RunCMake_TEST_BINARY_DIR}/CMakeFiles/dep.dir/dep.c${CMAKE_C_OUTPUT_EXTENSION}")
+    message(FATAL_ERROR
+      "The `dep.c` object file was created when requesting an object file to "
+      "be built; this should no longer be necessary.")
+  endif ()
+endfunction ()
+run_LooseObjectDepends()
+
 function(sleep delay)
   execute_process(
     COMMAND ${CMAKE_COMMAND} -E sleep ${delay}
diff --git a/Tests/RunCMake/Ninja/dep.c b/Tests/RunCMake/Ninja/dep.c
new file mode 100644
index 0000000..728f031
--- /dev/null
+++ b/Tests/RunCMake/Ninja/dep.c
@@ -0,0 +1,4 @@
+int dep()
+{
+  return 0;
+}
diff --git a/Tests/RunCMake/Ninja/top.c b/Tests/RunCMake/Ninja/top.c
new file mode 100644
index 0000000..4a88eb2
--- /dev/null
+++ b/Tests/RunCMake/Ninja/top.c
@@ -0,0 +1,7 @@
+#include "command.h"
+#include "target.h"
+
+int top()
+{
+  return 0;
+}