[flang][driver] Add support for -emit-obj in the frontend driver

This patch implements the `EmitObjAction` frontend action. The
`CodeGenAction` is extended to contain an instance of `llvm::LLVMCtx`
and `llvm::Module`. These variables are unlikely to be needed in other,
non-code-gen actions (or in `CompilerInstance` or `CompilerInvocation`),
so these should be the right location. Also, a member method for
generating LLVM IR is added (see `GenerateLLVMIR`).

The target triple is hard-coded as `native`, i.e. no cross-compilation
is supported at this stage.

`tripleName` in FirContext.cpp is updated from `fir.triple` to
`llvm.target_triple`. The former was effectively ignored. The latter is
used when lowering from the LLVM dialect in MLIR to LLVM IR (i.e. it's
embedded in the generated LLVM IR module). The driver can then re-use
that when configuring the backend. With this change, the LLVM IR files
generated by e.g. `tco` will from now on contain the correct target triple.

The code-gen.f90 test is replaced with code-gen-x86.f90 and
code-gen-aarch64.f90. As these tests are arch specific (they check the
generated assembly), support for `!REQUIRES: <arch>-registered-target`
was added in LIT. The current approach is a bit fragile and will cause
problems if multiple targets are available (e.g. X86 and AArch64).
That's because `!REQUIRES: X86-registered-target` will be satisfied even
on AArch64 targets if the X86 backend is enabled (through e.g.
`LLVM_ENABLE_TARGETS`). However, the test could fail as currently
`flang-new -fc1 -emit-obj` always generates code for the host
architecture. This can be fixed by adding support for `-target`.

Author: banach-space

flang/include/flang/Frontend/CompilerInstance.h

@@ -74,7 +74,6 @@ class CompilerInstance {
   std::unique_ptr<llvm::raw_pwrite_stream> outputStream_;
 
   std::unique_ptr<mlir::MLIRContext> mlirCtx_;
-
   std::unique_ptr<mlir::ModuleOp> mlirModule_;
 
 public:

flang/include/flang/Frontend/FrontendActions.h

@@ -11,6 +11,7 @@
 
 #include "flang/Frontend/FrontendAction.h"
 #include "flang/Semantics/semantics.h"
+#include "llvm/IR/Module.h"
 #include <memory>
 
 namespace Fortran::frontend {
@@ -34,10 +35,6 @@ class InputOutputTestAction : public FrontendAction {
   void ExecuteAction() override;
 };
 
-class EmitObjAction : public FrontendAction {
-  void ExecuteAction() override;
-};
-
 class InitOnlyAction : public FrontendAction {
   void ExecuteAction() override;
 };
@@ -153,6 +150,14 @@ class CodeGenAction : public FrontendAction {
 
   void ExecuteAction() override = 0;
   bool BeginSourceFileAction() override;
+
+protected:
+  /// @name LLVM IR
+  std::unique_ptr<llvm::LLVMContext> llvmCtx_;
+  std::unique_ptr<llvm::Module> llvmModule_;
+  /// }
+
+  void GenerateLLVMIR();
 };
 
 class EmitMLIRAction : public CodeGenAction {
@@ -163,6 +168,10 @@ class EmitLLVMAction : public CodeGenAction {
   void ExecuteAction() override;
 };
 
+class EmitObjAction : public CodeGenAction {
+  void ExecuteAction() override;
+};
+
 } // namespace Fortran::frontend
 
 #endif // LLVM_FLANG_FRONTEND_FRONTENDACTIONS_H

flang/lib/Frontend/CMakeLists.txt

@@ -25,6 +25,7 @@ add_flang_library(flangFrontend
   FortranLower
   clangBasic
   clangDriver
+  LLVMAnalysis
   FIRDialect
   FIRAnalysis
   FIRSupport

flang/lib/Frontend/FrontendActions.cpp

@@ -20,10 +20,10 @@
 #include "flang/Lower/Support/Verifier.h"
 #include "flang/Optimizer/CodeGen/CodeGen.h"
 #include "flang/Optimizer/Dialect/FIRType.h"
-#include "flang/Optimizer/Support//Utils.h"
 #include "flang/Optimizer/Support/FIRContext.h"
 #include "flang/Optimizer/Support/InitFIR.h"
 #include "flang/Optimizer/Support/KindMapping.h"
+#include "flang/Optimizer/Support/Utils.h"
 #include "flang/Optimizer/Transforms/Passes.h"
 #include "flang/Parser/dump-parse-tree.h"
 #include "flang/Parser/parsing.h"
@@ -34,11 +34,18 @@
 #include "flang/Semantics/semantics.h"
 #include "flang/Semantics/unparse-with-symbols.h"
 #include "llvm/ADT/StringRef.h"
+#include "llvm/Analysis/TargetLibraryInfo.h"
+#include "llvm/Analysis/TargetTransformInfo.h"
+#include "llvm/IR/LegacyPassManager.h"
+#include "llvm/Support/CodeGen.h"
 #include "llvm/Support/ErrorHandling.h"
+#include "llvm/MC/TargetRegistry.h"
 #include "llvm/Support/raw_ostream.h"
+#include "llvm/Target/TargetMachine.h"
 #include <clang/Basic/Diagnostic.h>
 
 using namespace Fortran::frontend;
+using namespace llvm;
 
 //===----------------------------------------------------------------------===//
 // Custom BeginSourceFileAction
@@ -436,15 +443,16 @@ void EmitMLIRAction::ExecuteAction() {
 
 #include "flang/Tools/CLOptions.inc"
 
-void EmitLLVMAction::ExecuteAction() {
+// Lower the previously generated MLIR module into an LLVM IR module
+void CodeGenAction::GenerateLLVMIR() {
   CompilerInstance &ci = this->instance();
   auto mlirMod = ci.mlirModule();
 
   auto &ctx = ci.mlirCtx();
   fir::support::loadDialects(ctx);
   fir::support::registerLLVMTranslation(ctx);
 
-  // Set-up the pass manager
+  // Set-up the MLIR pass manager
   fir::setTargetTriple(mlirMod, "native");
   auto &defKinds = ci.invocation().semanticsContext().defaultKinds();
   fir::KindMapping kindMap(&ci.mlirCtx(),
@@ -468,16 +476,21 @@ void EmitLLVMAction::ExecuteAction() {
 
   // Translate to LLVM IR
   auto optName = mlirMod.getName();
-  llvm::LLVMContext llvmCtx;
-  auto llvmModule = (mlir::translateModuleToLLVMIR(
-      mlirMod, llvmCtx, optName ? *optName : "FIRModule"));
+  llvmCtx_ = std::make_unique<llvm::LLVMContext>();
+  llvmModule_ = mlir::translateModuleToLLVMIR(
+      mlirMod, *llvmCtx_, optName ? *optName : "FIRModule");
 
-  if (!llvmModule) {
+  if (!llvmModule_) {
     unsigned diagID = ci.diagnostics().getCustomDiagID(
         clang::DiagnosticsEngine::Error, "failed to create the LLVM module");
     ci.diagnostics().Report(diagID);
     return;
   }
+}
+
+void EmitLLVMAction::ExecuteAction() {
+  CompilerInstance &ci = this->instance();
+  GenerateLLVMIR();
 
   // Print the generated LLVM IR. If there is no pre-defined output stream to
   // print to, create an output file.
@@ -495,20 +508,69 @@ void EmitLLVMAction::ExecuteAction() {
   }
 
   if (!ci.IsOutputStreamNull()) {
-    llvmModule->print(
+    llvmModule_->print(
         ci.GetOutputStream(), /*AssemblyAnnotationWriter=*/nullptr);
   } else {
-    llvmModule->print(*os, /*AssemblyAnnotationWriter=*/nullptr);
+    llvmModule_->print(*os, /*AssemblyAnnotationWriter=*/nullptr);
   }
 
   return;
 }
 
 void EmitObjAction::ExecuteAction() {
   CompilerInstance &ci = this->instance();
-  unsigned DiagID = ci.diagnostics().getCustomDiagID(
-      clang::DiagnosticsEngine::Error, "code-generation is not available yet");
-  ci.diagnostics().Report(DiagID);
+  GenerateLLVMIR();
+
+  // Create `Target`
+  std::string error;
+  std::string theTriple = llvmModule_->getTargetTriple();
+  const llvm::Target *theTarget =
+      TargetRegistry::lookupTarget(theTriple, error);
+  assert(theTarget && "Failed to create Target");
+
+  // Create `TargetMachine`
+  std::unique_ptr<TargetMachine> TM;
+  TM.reset(theTarget->createTargetMachine(
+      theTriple, /*CPU=*/"", /*Features=*/"", llvm::TargetOptions(), None));
+  llvmModule_->setDataLayout(TM->createDataLayout());
+  assert(TM && "Failed to create TargetMachine");
+
+  // Create an LLVM code-gen pass pipeline. Currently only the legacy pass
+  // manager is supported.
+  // TODO: Switch to the new PM once it's available in the backend.
+  legacy::PassManager CodeGenPasses;
+  CodeGenPasses.add(createTargetTransformInfoWrapperPass(TargetIRAnalysis()));
+  Triple triple(llvmModule_->getTargetTriple());
+  std::unique_ptr<llvm::TargetLibraryInfoImpl> TLII =
+      std::make_unique<llvm::TargetLibraryInfoImpl>(triple);
+  CodeGenPasses.add(new TargetLibraryInfoWrapperPass(*TLII));
+
+  // Get the output buffer/file
+  std::unique_ptr<llvm::raw_pwrite_stream> os{ci.CreateDefaultOutputFile(
+      /*Binary=*/true, /*InFile=*/GetCurrentFileOrBufferName(), "o")};
+  if (!os) {
+    unsigned diagID = ci.diagnostics().getCustomDiagID(
+        clang::DiagnosticsEngine::Error, "failed to create the output file");
+    ci.diagnostics().Report(diagID);
+    return;
+  }
+
+  if (TM->addPassesToEmitFile(CodeGenPasses, *os, nullptr,
+          /*CodeGenFileType*/ llvm::CodeGenFileType::CGFT_ObjectFile))
+    assert(false && "Something went wrong");
+  // The output stream, `os`, is a smart pointer that will be destroyed at the
+  // end of this method. However, it won't be written to until `CodeGenPasses`
+  // is destroyed. Hence, we need to release `as` here as otherwise it will be
+  // destroyed before it is written to. This pointer is later wrapped into a
+  // smart pointer inside `LLVMTargetMachine::createMCStreamer`, so no
+  // resources are leaked.
+  // TODO: Implement a safer way to pass `os` around.
+  os.release();
+
+  // Run the code-gen passes
+  CodeGenPasses.run(*llvmModule_);
+
+  return;
 }
 
 void InitOnlyAction::ExecuteAction() {

flang/lib/Optimizer/Support/FIRContext.cpp

@@ -16,7 +16,7 @@
 #include "mlir/IR/BuiltinOps.h"
 #include "llvm/Support/Host.h"
 
-static constexpr const char *tripleName = "fir.triple";
+static constexpr const char *tripleName = "llvm.target_triple";
 
 void fir::setTargetTriple(mlir::ModuleOp mod, llvm::StringRef triple) {
   auto target = fir::determineTargetTriple(triple);

flang/test/Driver/code-gen-aarch64.f90

@@ -0,0 +1,15 @@
+! TODO Add `-target <arch>` once that's available
+
+! RUN: rm -f %t.o
+! RUN: %flang_fc1 -emit-obj %s -o %t.o
+! RUN: llvm-objdump --disassemble-all %t.o | FileCheck %s
+! RUN: rm -f %t.o
+! RUN: %flang -c %s -o %t.o
+! RUN: llvm-objdump --disassemble-all %t.o | FileCheck %s
+
+! REQUIRES: aarch64-registered-target
+
+! CHECK-LABEL: <_QQmain>:
+! CHECK-NEXT:  	ret
+
+end program

flang/test/Driver/code-gen-x86.f90

@@ -0,0 +1,15 @@
+! TODO Add `-target <arch>` once that's available
+
+! RUN: rm -f %t.o
+! RUN: %flang_fc1 -emit-obj %s -o %t.o
+! RUN: llvm-objdump --disassemble-all %t.o | FileCheck %s
+! RUN: rm -f %t.o
+! RUN: %flang -c %s -o %t.o
+! RUN: llvm-objdump --disassemble-all %t.o | FileCheck %s
+
+! REQUIRES: x86-registered-target
+
+! CHECK-LABEL: <_QQmain>:
+! CHECK-NEXT:  	retq
+
+end program

flang/test/Driver/code-gen.f90

@@ -7,16 +7,13 @@
 ! RUN LINES
 !-----------
 ! RUN: not %flang -fsyntax-only %s 2>&1 | FileCheck %s --check-prefix=FSYNTAX_ONLY
-! RUN: not %flang  %s 2>&1 | FileCheck %s --check-prefix=NO_FSYNTAX_ONLY
 
 !-----------------
 ! EXPECTED OUTPUT
 !-----------------
 ! FSYNTAX_ONLY: IF statement is not allowed in IF statement
 ! FSYNTAX_ONLY_NEXT: Semantic errors in {{.*}}syntax-only.f90
 
-! NO_FSYNTAX_ONLY: error: code-generation is not available yet
-
 !-------
 ! INPUT
 !-------

flang/test/Driver/syntax-only.f90

@@ -4,6 +4,8 @@
 // RUN: %CC %t print_complex.o
 // RUN: ./a.out | FileCheck %s --check-prefix=EXECHECK
 
+// REQUIRES: x86-registered-target
+
 // EXECHECK: <0.935893, 2.252526>
 
 // LLVMIR-LABEL: define <2 x float> @foo(<2 x float> %