[flang] Add -O flag to tco (#151869)

At the moment, there is no established way to emit LLVM IR before
optimization in LLVM. tco can partially does, but the optimization level
is fixed to 2. This patch adds -O flag to tco.

Note that this is not completely equivalent to the frontend option. tco
does not accept -O flag without numbers, and the default optimization
level is not 0 but 2.

Author: parabola94

flang/tools/tco/tco.cpp

@@ -51,6 +51,12 @@ static cl::opt<bool> emitFir("emit-fir",
                              cl::desc("Parse and pretty-print the input"),
                              cl::init(false));
 
+static cl::opt<unsigned>
+    OptLevel("O",
+             cl::desc("Optimization level. [-O0, -O1, -O2, or -O3] "
+                      "(default = '-O2')"),
+             cl::Prefix, cl::init(2));
+
 static cl::opt<std::string> targetTriple("target",
                                          cl::desc("specify a target triple"),
                                          cl::init("native"));
@@ -96,6 +102,22 @@ static void printModule(mlir::ModuleOp mod, raw_ostream &output) {
   output << mod << '\n';
 }
 
+static std::optional<llvm::OptimizationLevel>
+getOptimizationLevel(unsigned level) {
+  switch (level) {
+  default:
+    return std::nullopt;
+  case 0:
+    return llvm::OptimizationLevel::O0;
+  case 1:
+    return llvm::OptimizationLevel::O1;
+  case 2:
+    return llvm::OptimizationLevel::O2;
+  case 3:
+    return llvm::OptimizationLevel::O3;
+  }
+}
+
 // compile a .fir file
 static llvm::LogicalResult
 compileFIR(const mlir::PassPipelineCLParser &passPipeline) {
@@ -157,9 +179,17 @@ compileFIR(const mlir::PassPipelineCLParser &passPipeline) {
     if (mlir::failed(passPipeline.addToPipeline(pm, errorHandler)))
       return mlir::failure();
   } else {
-    MLIRToLLVMPassPipelineConfig config(llvm::OptimizationLevel::O2);
+    std::optional<llvm::OptimizationLevel> level =
+        getOptimizationLevel(OptLevel);
+    if (!level) {
+      errs() << "Error invalid optimization level\n";
+      return mlir::failure();
+    }
+    MLIRToLLVMPassPipelineConfig config(*level);
+    // TODO: config.StackArrays should be set here?
     config.EnableOpenMP = true;  // assume the input contains OpenMP
     config.AliasAnalysis = enableAliasAnalysis && !testGeneratorMode;
+    config.LoopVersioning = OptLevel > 2;
     if (codeGenLLVM) {
       // Run only CodeGen passes.
       fir::createDefaultFIRCodeGenPassPipeline(pm, config);