[llvm] [CMake] Factor check- target deps to reduce build.ninja size by 45% (PR #223592)

[Reid Kleckner via llvm-commits]

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@@ -2495,6 +2502,31 @@ function(add_lit_testsuites project directory)
       set(ARG_FOLDER "${subproject_title}/Tests/LIT Testsuites")
     endif()
 
+    # CMake expands the transitive closure of a custom target's dependencies
+    # onto every custom command that depends on it. For large lit test suites,
+    # repeating the full tool dependency list on every per-directory check
+    # target makes the generated Ninja file unnecessarily large. A symbolic
+    # custom command output can be named directly on Ninja's command line, so
+    # use one shared stamp to represent the dependency closure and make each
+    # check command depend only on that stamp.
+    set(use_factored_lit_dependencies OFF)
+    # TODO: Remove the CMake version fallback once LLVM requires CMake 3.31.
+    if (CMAKE_GENERATOR STREQUAL "Ninja" AND
+        CMAKE_VERSION VERSION_GREATER_EQUAL 3.27)
+      set(use_factored_lit_dependencies ON)
+      string(SHA1 lit_testsuites_id
+        "${CMAKE_CURRENT_BINARY_DIR};${project};${directory}")
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rnk wrote:

Yeah, this isn't actually necessary. This is now called `check-${proj}-test-depends.stamp`.

https://github.com/llvm/llvm-project/pull/223592