[flang][tco] Add -emit-final-mlir flag (#146533)

Add a flag to tco for emitting the final MLIR, prior to lowering to LLVM
IR. This is intended to produce output that can be passed directly to
mlir-translate.

---------

Signed-off-by: Kajetan Puchalski <kajetan.puchalski@arm.com>

flang/test/Driver/tco-emit-final-mlir.fir

@@ -0,0 +1,19 @@
+// Test tco's -emit-final-mlir option.
+
+// RUN: tco -emit-final-mlir %s 2>&1 | FileCheck %s
+
+// Check that the FIR file is translated into the LLVM IR
+// MLIR dialect, but not into LLVM IR itself.
+
+// CHECK-NOT: func.func
+// CHECK-LABEL: llvm.func @_QPfoo()
+// CHECK-NOT: fir.alloca
+// CHECK: %[[VAL_0:.*]] = llvm.mlir.constant(1 : i64) : i64
+// CHECK: %[[VAL_1:.*]] = llvm.alloca %[[VAL_0]] x i32 : (i64) -> !llvm.ptr
+// CHECK: llvm.return
+// CHECK-NOT: func.func
+
+func.func @_QPfoo() {
+  %1 = fir.alloca i32
+  return
+}

flang/tools/tco/tco.cpp

@@ -70,6 +70,11 @@ static cl::opt<bool> codeGenLLVM(
     cl::desc("Run only CodeGen passes and translate FIR to LLVM IR"),
     cl::init(false));
 
+static cl::opt<bool> emitFinalMLIR(
+    "emit-final-mlir",
+    cl::desc("Only translate FIR to MLIR, do not lower to LLVM IR"),
+    cl::init(false));
+
 #include "flang/Optimizer/Passes/CommandLineOpts.h"
 #include "flang/Optimizer/Passes/Pipelines.h"
 
@@ -149,13 +154,15 @@ compileFIR(const mlir::PassPipelineCLParser &passPipeline) {
       fir::registerDefaultInlinerPass(config);
       fir::createMLIRToLLVMPassPipeline(pm, config);
     }
-    fir::addLLVMDialectToLLVMPass(pm, out.os());
+    if (!emitFinalMLIR)
+      fir::addLLVMDialectToLLVMPass(pm, out.os());
   }
 
   // run the pass manager
   if (mlir::succeeded(pm.run(*owningRef))) {
     // passes ran successfully, so keep the output
-    if ((emitFir || passPipeline.hasAnyOccurrences()) && !codeGenLLVM)
+    if ((emitFir || passPipeline.hasAnyOccurrences() || emitFinalMLIR) &&
+        !codeGenLLVM)
       printModule(*owningRef, out.os());
     out.keep();
     return mlir::success();