llvm/mlir: fix MLIRConfig.cmake to support external tablegen overrides

MLIRConfig.cmake unconditionally overwrites MLIR_TABLEGEN_EXE and does
not create an imported target, breaking standalone builds that provide
their own mlir-tblgen binary (e.g. Nix sandboxed builds).

The patch adds guards to respect caller-set MLIR_TABLEGEN_EXE,
MLIR_PDLL_TABLEGEN_EXE, and MLIR_SRC_SHARDER_TABLEGEN_EXE values, and
auto-creates an imported mlir-tblgen target for downstream consumers.
This replaces the previous dummy-target workaround in flang's
CMakeLists.txt.

Upstream issue: https://github.com/llvm/llvm-project/issues/150986

Co-authored-by: Alexander Bantyev <balsoft@balsoft.ru>

pkgs/development/compilers/llvm/common/flang/default.nix

@@ -40,9 +40,7 @@ stdenv.mkDerivation (finalAttrs: {
         chmod -R +w $out/llvm
       '';
 
-  patches = [
-    ./dummy_target_19+.patch
-  ];
+  patches = [ ];
   patchFlags = [ "-p1" ];
 
   sourceRoot = "${finalAttrs.src.name}/flang";
@@ -73,7 +71,6 @@ stdenv.mkDerivation (finalAttrs: {
     (lib.cmakeFeature "CLANG_DIR" "${libclang.dev}/lib/cmake/clang")
     (lib.cmakeFeature "MLIR_DIR" "${mlir.dev}/lib/cmake/mlir")
     (lib.cmakeFeature "MLIR_TABLEGEN_EXE" "${buildLlvmPackages.tblgen}/bin/mlir-tblgen")
-    (lib.cmakeFeature "MLIR_TABLEGEN_TARGET" "MLIR-TBLGen")
     (lib.cmakeBool "MLIR_LINK_MLIR_DYLIB" (!stdenv.hostPlatform.isStatic))
     (lib.cmakeFeature "LLVM_LIT_ARGS" "-v")
     (lib.cmakeBool "LLVM_ENABLE_PLUGINS" false)

pkgs/development/compilers/llvm/common/flang/dummy_target_19+.patch

@@ -1,27 +0,0 @@
-diff --git a/CMakeLists.txt b/CMakeLists.txt
-index 070c39eb6e9a..168c97524943 100644
---- a/CMakeLists.txt
-+++ b/CMakeLists.txt
-@@ -1,6 +1,22 @@
- cmake_minimum_required(VERSION 3.20.0)
- set(LLVM_SUBPROJECT_TITLE "Flang")
- 
-+# Patch: define dummy mlir-tblgen target for TableGen.cmake
-+if(DEFINED MLIR_TABLEGEN_EXE AND NOT TARGET mlir-tblgen)
-+  add_executable(mlir-tblgen IMPORTED GLOBAL)
-+  set_target_properties(mlir-tblgen PROPERTIES
-+    IMPORTED_LOCATION "${MLIR_TABLEGEN_EXE}"
-+  )
-+endif()
-+
-+if(DEFINED MLIR_TABLEGEN_EXE AND NOT TARGET MLIR-TBLGen)
-+  add_executable(MLIR-TBLGen IMPORTED GLOBAL)
-+  set_target_properties(MLIR-TBLGen PROPERTIES
-+    IMPORTED_LOCATION "${MLIR_TABLEGEN_EXE}"
-+  )
-+endif()
-+
-+
- if(NOT DEFINED LLVM_COMMON_CMAKE_UTILS)
-   set(LLVM_COMMON_CMAKE_UTILS ${CMAKE_CURRENT_SOURCE_DIR}/../cmake)
- endif()

pkgs/development/compilers/llvm/common/mlir/default.nix

@@ -39,6 +39,14 @@ stdenv.mkDerivation (finalAttrs: {
 
   patches = [
     ./gnu-install-dirs.patch
+    # MLIRConfig.cmake unconditionally overwrites MLIR_TABLEGEN_EXE, breaking standalone
+    # builds that provide their own pre-built mlir-tblgen (e.g. in Nix sandboxed builds).
+    # The patch adds guards to respect caller-set values and auto-creates an imported
+    # mlir-tblgen target for downstream consumers. This replaces the previous dummy target
+    # workaround in flang's CMakeLists.txt.
+
+    # Upstream issue: https://github.com/llvm/llvm-project/issues/150986
+    ./mlir-tablegen-imported-target.patch
   ]
   ++ lib.optional (lib.versionOlder release_version "20") [
     # Fix build with gcc15

pkgs/development/compilers/llvm/common/mlir/mlir-tablegen-imported-target.patch

@@ -0,0 +1,54 @@
+diff --git a/cmake/modules/MLIRConfig.cmake.in b/cmake/modules/MLIRConfig.cmake.in
+index 71f3e028b1e8..b30b6cdc0de1 100644
+--- a/cmake/modules/MLIRConfig.cmake.in
++++ b/cmake/modules/MLIRConfig.cmake.in
+@@ -9,9 +9,16 @@ find_package(LLVM ${LLVM_VERSION} EXACT REQUIRED CONFIG
+ set(MLIR_EXPORTED_TARGETS "@MLIR_EXPORTS@")
+ set(MLIR_CMAKE_DIR "@MLIR_CONFIG_CMAKE_DIR@")
+ set(MLIR_INCLUDE_DIRS "@MLIR_CONFIG_INCLUDE_DIRS@")
+-set(MLIR_TABLEGEN_EXE "@MLIR_CONFIG_TABLEGEN_EXE@")
+-set(MLIR_PDLL_TABLEGEN_EXE "@MLIR_CONFIG_PDLL_TABLEGEN_EXE@")
+-set(MLIR_SRC_SHARDER_TABLEGEN_EXE "@MLIR_CONFIG_SRC_SHARDER_TABLEGEN_EXE@")
++# Allow users to override tablegen executables (e.g. for sandboxed builds).
++if(NOT MLIR_TABLEGEN_EXE)
++  set(MLIR_TABLEGEN_EXE "@MLIR_CONFIG_TABLEGEN_EXE@")
++endif()
++if(NOT MLIR_PDLL_TABLEGEN_EXE)
++  set(MLIR_PDLL_TABLEGEN_EXE "@MLIR_CONFIG_PDLL_TABLEGEN_EXE@")
++endif()
++if(NOT MLIR_SRC_SHARDER_TABLEGEN_EXE)
++  set(MLIR_SRC_SHARDER_TABLEGEN_EXE "@MLIR_CONFIG_SRC_SHARDER_TABLEGEN_EXE@")
++endif()
+ set(MLIR_IRDL_TO_CPP_EXE "@MLIR_CONFIG_IRDL_TO_CPP_EXE@")
+ set(MLIR_INSTALL_AGGREGATE_OBJECTS "@MLIR_INSTALL_AGGREGATE_OBJECTS@")
+ set(MLIR_ENABLE_BINDINGS_PYTHON "@MLIR_ENABLE_BINDINGS_PYTHON@")
+@@ -32,6 +39,29 @@ if(NOT TARGET MLIRSupport)
+   @MLIR_CONFIG_INCLUDE_EXPORTS@
+ endif()
+
++# Ensure MLIR_TABLEGEN_EXE is a full path and has a corresponding imported
++# target. In install trees, MLIR_TABLEGEN_EXE may be a bare name
++# ("mlir-tblgen") rather than an absolute path. Resolve it so that
++# TableGen.cmake can use it as both a COMMAND and a DEPENDS entry.
++if(MLIR_TABLEGEN_EXE AND NOT IS_ABSOLUTE "${MLIR_TABLEGEN_EXE}"
++    AND NOT TARGET "${MLIR_TABLEGEN_EXE}")
++  find_program(_mlir_tblgen_exe "${MLIR_TABLEGEN_EXE}"
++    HINTS "${LLVM_TOOLS_BINARY_DIR}")
++  if(_mlir_tblgen_exe)
++    set(MLIR_TABLEGEN_EXE "${_mlir_tblgen_exe}")
++  endif()
++  unset(_mlir_tblgen_exe)
++endif()
++
++if(NOT TARGET mlir-tblgen AND MLIR_TABLEGEN_EXE)
++  add_executable(mlir-tblgen IMPORTED GLOBAL)
++  set_target_properties(mlir-tblgen PROPERTIES
++    IMPORTED_LOCATION "${MLIR_TABLEGEN_EXE}")
++endif()
++if(NOT DEFINED MLIR_TABLEGEN_TARGET)
++  set(MLIR_TABLEGEN_TARGET mlir-tblgen)
++endif()
++
+ # By creating these targets here, subprojects that depend on MLIR's
+ # tablegen-generated headers can always depend on these targets whether building
+ # in-tree with MLIR or not.