[clang] [clang] Only build clang-tblgen if it is actually needed (PR #161952)

[Ross Burton via cfe-commits]

https://github.com/rossburton created https://github.com/llvm/llvm-project/pull/161952

It's possible to build clang with an existing clang-tblgen (common when cross-compiling, for instance) by setting CLANG_TABLEGEN_EXE.  If this is the case there's no need to build it, as it won't be used.

>From edbcf9a49d7770cfcd9ac950deb6884e6de712a8 Mon Sep 17 00:00:00 2001
From: Ross Burton <ross.burton at arm.com>
Date: Fri, 3 Oct 2025 20:34:02 +0100
Subject: [PATCH] [clang] Only build clang-tblgen if it is actually needed

It's possible to build clang with an existing clang-tblgen (common when
cross-compiling, for instance) by setting CLANG_TABLEGEN_EXE.  If this
is the case there's no need to build it, as it won't be used.
---
 clang/utils/TableGen/CMakeLists.txt | 61 +++++++++++++++--------------
 1 file changed, 32 insertions(+), 29 deletions(-)

diff --git a/clang/utils/TableGen/CMakeLists.txt b/clang/utils/TableGen/CMakeLists.txt
index 14f13824e9575..20a7b88ecdbee 100644
--- a/clang/utils/TableGen/CMakeLists.txt
+++ b/clang/utils/TableGen/CMakeLists.txt
@@ -1,31 +1,34 @@
-set(LLVM_LINK_COMPONENTS Support)
+# We only need to build clang-tblgen if we haven't been told to use an existing binary.
+if (NOT DEFINED CLANG_TABLEGEN_EXE)
+  set(LLVM_LINK_COMPONENTS Support)
 
-add_tablegen(clang-tblgen CLANG
-  DESTINATION "${CLANG_TOOLS_INSTALL_DIR}"
-  EXPORT Clang
-  ASTTableGen.cpp
-  CIRLoweringEmitter.cpp
-  ClangASTNodesEmitter.cpp
-  ClangASTPropertiesEmitter.cpp
-  ClangAttrEmitter.cpp
-  ClangBuiltinsEmitter.cpp
-  ClangBuiltinTemplatesEmitter.cpp
-  ClangCommentCommandInfoEmitter.cpp
-  ClangCommentHTMLNamedCharacterReferenceEmitter.cpp
-  ClangCommentHTMLTagsEmitter.cpp
-  ClangDataCollectorsEmitter.cpp
-  ClangDiagnosticsEmitter.cpp
-  ClangOpcodesEmitter.cpp
-  ClangOpenCLBuiltinEmitter.cpp
-  ClangOptionDocEmitter.cpp
-  ClangSACheckersEmitter.cpp
-  ClangSyntaxEmitter.cpp
-  ClangTypeNodesEmitter.cpp
-  MveEmitter.cpp
-  NeonEmitter.cpp
-  RISCVVEmitter.cpp
-  SveEmitter.cpp
-  TableGen.cpp
-  )
+  add_tablegen(clang-tblgen CLANG
+    DESTINATION "${CLANG_TOOLS_INSTALL_DIR}"
+    EXPORT Clang
+    ASTTableGen.cpp
+    CIRLoweringEmitter.cpp
+    ClangASTNodesEmitter.cpp
+    ClangASTPropertiesEmitter.cpp
+    ClangAttrEmitter.cpp
+    ClangBuiltinsEmitter.cpp
+    ClangBuiltinTemplatesEmitter.cpp
+    ClangCommentCommandInfoEmitter.cpp
+    ClangCommentHTMLNamedCharacterReferenceEmitter.cpp
+    ClangCommentHTMLTagsEmitter.cpp
+    ClangDataCollectorsEmitter.cpp
+    ClangDiagnosticsEmitter.cpp
+    ClangOpcodesEmitter.cpp
+    ClangOpenCLBuiltinEmitter.cpp
+    ClangOptionDocEmitter.cpp
+    ClangSACheckersEmitter.cpp
+    ClangSyntaxEmitter.cpp
+    ClangTypeNodesEmitter.cpp
+    MveEmitter.cpp
+    NeonEmitter.cpp
+    RISCVVEmitter.cpp
+    SveEmitter.cpp
+    TableGen.cpp
+    )
 
-target_link_libraries(clang-tblgen PRIVATE clangSupport_tablegen)
+  target_link_libraries(clang-tblgen PRIVATE clangSupport_tablegen)
+endif()