[mlir][cmake] Respect MLIR_TABLEGEN_EXE overrides in MLIRConfig.cmake

[Acture]

Fixes #150986. Supersedes #150987.

Problem

MLIRConfig.cmake.in unconditionally overwrites MLIR_TABLEGEN_EXE (line 12), discarding user-provided -DMLIR_TABLEGEN_EXE=... overrides. For the install tree, the value becomes the bare name mlir-tblgen (not an absolute path). When MLIRTargets.cmake fails to define the corresponding imported target — as happens in Nix sandboxed builds where the binary is in a separate store path — TableGen.cmake passes the bare string into add_custom_command(DEPENDS ...), where CMake treats it as a relative file path:

ninja: error: 'include/flang/Optimizer/Dialect/CUF/Attributes/mlir-tblgen',
needed by '...CUFEnumAttr.h.inc', missing and no known rule to make it

Additionally, MLIR_TABLEGEN_TARGET is never exported by MLIRConfig.cmake, so TableGen.cmake has no target to track for dependency rebuilds.

Fix

All changes are in mlir/cmake/modules/MLIRConfig.cmake.in:

1. Make MLIR_TABLEGEN_EXE (and siblings) conditional — respect user overrides
2. Resolve bare names to full paths via find_program with HINTS "${LLVM_TOOLS_BINARY_DIR}"
3. Create an imported mlir-tblgen target as fallback when MLIRTargets.cmake doesn't provide one
4. Set MLIR_TABLEGEN_TARGET so TableGen.cmake has a proper target for DEPENDS

This replaces the previous workaround in flang/CMakeLists.txt (#150987) with a fix at the source — any downstream project using find_package(MLIR) benefits.

Reproduction

No Nix required. Configure LLVM+MLIR (no build needed), assemble a fake install tree, then run a minimal test project with -DMLIR_TABLEGEN_EXE=/some/path:

-- >>> Before find_package(MLIR):
-- >>>   MLIR_TABLEGEN_EXE = '/nix/store/fake/bin/mlir-tblgen'

-- >>> After find_package(MLIR):
-- >>>   MLIR_TABLEGEN_EXE    = 'mlir-tblgen'        ← override lost!
-- >>>   MLIR_TABLEGEN_TARGET = ''                    ← never set!

CMake Error: BUG REPRODUCED!
  MLIR_TABLEGEN_EXE = 'mlir-tblgen' — bare name, not absolute, not a target.
  The user-provided override was silently discarded by MLIRConfig.cmake.

Full reproduction steps in #150986.

[llvmbot]

@llvm/pr-subscribers-mlir

Author: acture (Acture)

Changes

Fixes #150986. Supersedes #150987.

Problem

MLIRConfig.cmake.in unconditionally overwrites MLIR_TABLEGEN_EXE (line 12), discarding user-provided -DMLIR_TABLEGEN_EXE=... overrides. For the install tree, the value becomes the bare name mlir-tblgen (not an absolute path). When MLIRTargets.cmake fails to define the corresponding imported target — as happens in Nix sandboxed builds where the binary is in a separate store path — TableGen.cmake passes the bare string into add_custom_command(DEPENDS ...), where CMake treats it as a relative file path:

ninja: error: 'include/flang/Optimizer/Dialect/CUF/Attributes/mlir-tblgen',
needed by '...CUFEnumAttr.h.inc', missing and no known rule to make it

Additionally, MLIR_TABLEGEN_TARGET is never exported by MLIRConfig.cmake, so TableGen.cmake has no target to track for dependency rebuilds.

Fix

All changes are in mlir/cmake/modules/MLIRConfig.cmake.in:

1. Make MLIR_TABLEGEN_EXE (and siblings) conditional — respect user overrides
2. Resolve bare names to full paths via find_program with HINTS "${LLVM_TOOLS_BINARY_DIR}"
3. Create an imported mlir-tblgen target as fallback when MLIRTargets.cmake doesn't provide one
4. Set MLIR_TABLEGEN_TARGET so TableGen.cmake has a proper target for DEPENDS

This replaces the previous workaround in flang/CMakeLists.txt (#150987) with a fix at the source — any downstream project using find_package(MLIR) benefits.

Reproduction

No Nix required. Configure LLVM+MLIR (no build needed), assemble a fake install tree, then run a minimal test project with -DMLIR_TABLEGEN_EXE=/some/path:

-- >>> Before find_package(MLIR):
-- >>>   MLIR_TABLEGEN_EXE = '/nix/store/fake/bin/mlir-tblgen'

-- >>> After find_package(MLIR):
-- >>>   MLIR_TABLEGEN_EXE    = 'mlir-tblgen'        ← override lost!
-- >>>   MLIR_TABLEGEN_TARGET = ''                    ← never set!

CMake Error: BUG REPRODUCED!
  MLIR_TABLEGEN_EXE = 'mlir-tblgen' — bare name, not absolute, not a target.
  The user-provided override was silently discarded by MLIRConfig.cmake.

Full reproduction steps in #150986.

---

Full diff: https://github.com/llvm/llvm-project/pull/189918.diff

1 Files Affected:

• (modified) mlir/cmake/modules/MLIRConfig.cmake.in (+33-3)

diff --git a/mlir/cmake/modules/MLIRConfig.cmake.in b/mlir/cmake/modules/MLIRConfig.cmake.in
index 71f3e028b1e88..b30b6cdc0de16 100644
--- a/mlir/cmake/modules/MLIRConfig.cmake.in
+++ b/mlir/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.