carbon-lang/toolchain/driver/compile_driver.cpp

// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
// Exceptions. See /LICENSE for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception

#include "toolchain/driver/compile_driver.h"

#include <functional>
#include <memory>
#include <optional>
#include <string>
#include <system_error>
#include <utility>

#include "common/error.h"
#include "common/ostream.h"
#include "common/pretty_stack_trace_function.h"
#include "common/vlog.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/ScopeExit.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/StringRef.h"
#include "llvm/MC/TargetRegistry.h"
#include "llvm/Passes/OptimizationLevel.h"
#include "llvm/Passes/PassBuilder.h"
#include "llvm/Passes/StandardInstrumentations.h"
#include "llvm/Support/SaveAndRestore.h"
#include "toolchain/base/clang_invocation.h"
#include "toolchain/base/timings.h"
#include "toolchain/check/check.h"
#include "toolchain/codegen/codegen.h"
#include "toolchain/diagnostics/emitter.h"
#include "toolchain/diagnostics/format_providers.h"
#include "toolchain/lex/lex.h"
#include "toolchain/lower/lower.h"
#include "toolchain/lower/options.h"
#include "toolchain/parse/parse.h"
#include "toolchain/parse/tree_and_subtrees.h"
#include "toolchain/sem_ir/ids.h"
#include "toolchain/source/source_buffer.h"

namespace Carbon {

CompilationUnit::CompilationUnit(SemIR::CheckIRId check_ir_id,
                                 int total_ir_count, DriverEnv* driver_env,
                                 const CompileOptions* options,
                                 Diagnostics::Consumer* consumer,
                                 llvm::StringRef input_filename,
                                 std::string output_filename,
                                 const llvm::Target* target)
    : check_ir_id_(check_ir_id),
      total_ir_count_(total_ir_count),
      driver_env_(driver_env),
      options_(options),
      target_(target),
      input_filename_(input_filename),
      output_filename_(std::move(output_filename)),
      vlog_stream_(driver_env_->vlog_stream) {
  if (vlog_stream_ != nullptr || options_->stream_errors) {
    consumer_ = consumer;
  } else {
    sorting_consumer_ = Diagnostics::SortingConsumer(*consumer);
    consumer_ = &*sorting_consumer_;
  }
}

auto CompilationUnit::IncludeInDumps() -> bool {
  return cache_->include_in_dumps().Get(check_ir_id_);
}

auto CompilationUnit::SetMultiUnitCache(MultiUnitCache* cache) -> void {
  CARBON_CHECK(!cache_, "Called SetMultiUnitCache twice");
  cache_ = cache;

  // Collect memory usage if this unit dumps it, or if the caller provided a
  // `MemUsage` to merge it into (see `PostCompile`).
  if ((options_->dump_mem_usage || driver_env_->mem_usage) &&
      IncludeInDumps()) {
    CARBON_CHECK(!mem_usage_);
    mem_usage_ = MemUsage();
  }
  if (options_->dump_timings && IncludeInDumps()) {
    CARBON_CHECK(!timings_);
    timings_ = Timings();
  }
}

auto CompilationUnit::RunLex() -> void {
  CARBON_CHECK(cache_, "Must call SetMultiUnitCache first");
  CARBON_CHECK(!tokens_, "Called RunLex twice");

  LogCall("SourceBuffer::MakeFromFileOrStdin", "source", [&] {
    source_ = SourceBuffer::MakeFromFileOrStdin(*driver_env_->fs,
                                                input_filename_, *consumer_);
  });

  if (!source_) {
    success_ = false;
    return;
  }

  if (mem_usage_) {
    mem_usage_->Add("source_", source_->text().size(), source_->text().size());
  }

  CARBON_VLOG("*** SourceBuffer ***\n```\n{0}\n```\n", source_->text());

  LogCall("Lex::Lex", "lex", [&] {
    Lex::LexOptions options;
    options.consumer = consumer_;
    options.vlog_stream = vlog_stream_;
    if (options_->dump_tokens && IncludeInDumps()) {
      options.dump_stream = driver_env_->output_stream;
      options.omit_file_boundary_tokens = options_->omit_file_boundary_tokens;
    }
    tokens_ = Lex::Lex(value_stores_, *source_, options);
  });
  if (mem_usage_) {
    mem_usage_->Collect("tokens_", *tokens_);
  }
  if (tokens_->has_errors()) {
    success_ = false;
  }
}

auto CompilationUnit::RunParse() -> void {
  LogCall("Parse::Parse", "parse", [&] {
    Parse::ParseOptions options;
    options.consumer = consumer_;
    options.vlog_stream = vlog_stream_;
    if (options_->dump_parse_tree && IncludeInDumps()) {
      options.dump_stream = driver_env_->output_stream;
      options.dump_preorder_parse_tree = options_->preorder_parse_tree;
    }
    parse_tree_ = Parse::Parse(*tokens_, options);
  });
  if (mem_usage_) {
    mem_usage_->Collect("parse_tree_", *parse_tree_);
  }
  if (parse_tree_->has_errors()) {
    success_ = false;
  }
}

auto CompilationUnit::GetCheckUnit() -> Check::Unit {
  CARBON_CHECK(parse_tree_, "Must call RunParse first");
  CARBON_CHECK(!sem_ir_, "Called GetCheckUnit twice");

  tree_and_subtrees_getter_ = [this]() -> const Parse::TreeAndSubtrees& {
    return this->GetParseTreeAndSubtrees();
  };
  sem_ir_.emplace(&*parse_tree_, check_ir_id_, parse_tree_->packaging_decl(),
                  value_stores_, input_filename_);
  if (!llvm_context_) {
    llvm_context_ = std::make_unique<llvm::LLVMContext>();
  }
  return {.consumer = consumer_,
          .value_stores = &value_stores_,
          .timings = timings_ ? &*timings_ : nullptr,
          .sem_ir = &*sem_ir_,
          .llvm_context = llvm_context_.get(),
          .total_ir_count = total_ir_count_};
}

auto CompilationUnit::PostCheck() -> void {
  CARBON_CHECK(sem_ir_, "Must call GetCheckUnit first");

  // We've finished all steps that can produce diagnostics. Emit the
  // diagnostics now, so that the developer sees them sooner and doesn't need
  // to wait for code generation.
  consumer_->Flush();

  if (mem_usage_) {
    mem_usage_->Collect("sem_ir_", *sem_ir_);
  }

  if (sem_ir_->has_errors()) {
    success_ = false;
  }
}

auto CompilationUnit::RunLower() -> void {
  LogCall("Lower::LowerToLLVM", "lower", [&] {
    if (!llvm_context_) {
      llvm_context_ = std::make_unique<llvm::LLVMContext>();
    }
    Lower::LowerToLLVMOptions options;
    options.llvm_verifier_stream =
        options_->run_llvm_verifier ? driver_env_->error_stream : nullptr;
    options.want_debug_info = options_->include_debug_info;
    options.vlog_stream = vlog_stream_;
    options.opt_level = options_->opt_level;
    options.mangle_string_fingerprint = options_->mangle_string_fingerprint;
    module_ = Lower::LowerToLLVM(*llvm_context_, driver_env_->fs,
                                 cache_->tree_and_subtrees_getters(), *sem_ir_,
                                 total_ir_count_, options);
  });
}

auto CompilationUnit::MakeTargetMachine(
    const clang::CompilerInvocation& clang_invocation) -> void {
  CARBON_CHECK(module_, "Must call RunLower first");
  CARBON_CHECK(!target_machine_, "Should not call this multiple times");

  // Set the target on the module.
  // TODO: We should do this earlier. Lower should be passed the target triple
  // so it can create the module with this already set.
  llvm::Triple target_triple(options_->codegen_options->target);
  module_->setTargetTriple(target_triple);

  // TODO: Provide flags to control these.
  constexpr llvm::StringLiteral CPU = "generic";
  constexpr llvm::StringLiteral Features = "";

  const auto& codegen_opts = clang_invocation.getCodeGenOpts();

  // TODO: Make the code in Clang's BackendUtil.cpp externally accessible and
  // call it from here. This is doing a subset of the same work to translate
  // Clang code generation options into target options.
  llvm::TargetOptions target_opts;
  target_opts.UseInitArray = codegen_opts.UseInitArray;
  target_opts.FunctionSections = codegen_opts.FunctionSections;
  target_opts.DataSections = codegen_opts.DataSections;
  target_opts.UniqueSectionNames = codegen_opts.UniqueSectionNames;
  target_machine_.reset(target_->createTargetMachine(
      target_triple, CPU, Features, target_opts, llvm::Reloc::PIC_));
}

auto CompilationUnit::RunOptimize(
    const clang::CompilerInvocation& clang_invocation) -> void {
  CARBON_CHECK(module_, "Must call RunLower first");

  // TODO: A lot of the work done here duplicates work done by Clang setting up
  // its pass manager. Moreover, we probably want to pick up Clang's
  // customizations and make use of its flags for controlling LLVM passes. We
  // should consider whether we would be better off running Clang's pass
  // pipeline rather than building one of our own, or factoring out enough of
  // Clang's pipeline builder that we can reuse and further customize it.

  MakeTargetMachine(clang_invocation);

  // TODO: There's no way to set these automatically from an
  // llvm::OptimizationLevel. Add such a mechanism to LLVM and use it from
  // here. For now we reconstruct what Clang does by default.
  llvm::PipelineTuningOptions pto;
  bool opt_for_speed = options_->opt_level == Lower::OptimizationLevel::Speed;
  bool opt_for_size_or_speed =
      opt_for_speed || options_->opt_level == Lower::OptimizationLevel::Size;
  // Loop unrolling is enabled by `--optimize=size` but isn't actually performed
  // because we add `optsize` attributes to the function definitions we emit.
  pto.LoopUnrolling = opt_for_size_or_speed;
  pto.LoopInterleaving = opt_for_size_or_speed;
  pto.LoopVectorization = opt_for_speed;
  pto.SLPVectorization = opt_for_size_or_speed;

  llvm::LoopAnalysisManager lam;
  llvm::FunctionAnalysisManager fam;
  llvm::CGSCCAnalysisManager cgam;
  llvm::ModuleAnalysisManager mam;

  llvm::PassInstrumentationCallbacks pic;

  // Register standard pass instrumentations. This adds support for things like
  // `-print-after-all`.
  llvm::StandardInstrumentations si(module_->getContext(),
                                    /*DebugLogging=*/false);
  si.registerCallbacks(pic);

  llvm::PassBuilder builder(target_machine_.get(), pto,
                            /*PGOOpt=*/std::nullopt, &pic);

  // TODO: Add an AssignmentTrackingPass for at least `--optimize=debug`.

  // Set up target library information and add an analysis pass to supply it.
  std::unique_ptr<llvm::TargetLibraryInfoImpl> tlii(llvm::driver::createTLII(
      module_->getTargetTriple(), llvm::driver::VectorLibrary::NoLibrary));
  fam.registerPass([&] { return llvm::TargetLibraryAnalysis(*tlii); });

  builder.registerModuleAnalyses(mam);
  builder.registerCGSCCAnalyses(cgam);
  builder.registerFunctionAnalyses(fam);
  builder.registerLoopAnalyses(lam);
  builder.crossRegisterProxies(lam, fam, cgam, mam);

  llvm::ModulePassManager pass_manager = builder.buildPerModuleDefaultPipeline(
      CompileOptions::GetLLVMOptimizationLevel(options_->opt_level));

  if (vlog_stream_) {
    CARBON_VLOG("*** Running pass pipeline: ");
    pass_manager.printPipeline(
        *vlog_stream_, [&pic](llvm::StringRef class_name) {
          auto pass_name = pic.getPassNameForClassName(class_name);
          return pass_name.empty() ? class_name : pass_name;
        });
    CARBON_VLOG(" ***\n");
  }

  LogCall("ModulePassManager::run", "optimize",
          [&] { pass_manager.run(*module_, mam); });

  if (vlog_stream_) {
    CARBON_VLOG("*** Optimized llvm::Module ***\n");
    module_->print(*vlog_stream_, /*AAW=*/nullptr,
                   /*ShouldPreserveUseListOrder=*/false,
                   /*IsForDebug=*/true);
  }
}

auto CompilationUnit::PostLower() -> void {
  CARBON_CHECK(module_, "Must call RunLower first");
  if (options_->dump_llvm_ir && IncludeInDumps()) {
    *driver_env_->output_stream << "; ---\n";
    module_->print(*driver_env_->output_stream, /*AAW=*/nullptr,
                   /*ShouldPreserveUseListOrder=*/true);
    *driver_env_->output_stream << "\n";
  }
}

auto CompilationUnit::RunCodeGen() -> void {
  CARBON_CHECK(module_, "Must call RunLower first");
  LogCall("CodeGen", "codegen", [&] { success_ = RunCodeGenHelper(); });
}

auto CompilationUnit::PostCompile() -> void {
  if (options_->dump_shared_values && IncludeInDumps()) {
    Yaml::Print(*driver_env_->output_stream,
                value_stores_.OutputYaml(input_filename_));
  }
  if (mem_usage_) {
    mem_usage_->Collect("value_stores_", value_stores_);
    if (options_->dump_mem_usage && IncludeInDumps()) {
      Yaml::Print(*driver_env_->output_stream,
                  mem_usage_->OutputYaml(input_filename_));
    }
    // Merge this file's usage into the caller-provided sink, if any, so it can
    // be queried programmatically.
    if (driver_env_->mem_usage) {
      driver_env_->mem_usage->Add(*mem_usage_);
    }
  }
  if (timings_) {
    Yaml::Print(*driver_env_->output_stream,
                timings_->OutputYaml(input_filename_));
  }

  // The diagnostics consumer must be flushed before compilation artifacts are
  // destructed, because diagnostics can refer to their state.
  consumer_->Flush();
}

auto CompilationUnit::RunCodeGenHelper() -> bool {
  CARBON_CHECK(module_, "Must call RunLower first");
  CARBON_CHECK(target_machine_, "Must call MakeTargetMachine first");

  CodeGen codegen(module_.get(), target_machine_.get(), consumer_);
  if (vlog_stream_) {
    CARBON_VLOG("*** Assembly ***\n");
    codegen.EmitAssembly(*vlog_stream_);
  }

  if (output_filename_ == "-") {
    // TODO: The output file name, forcing object output, and requesting
    // textual assembly output are all somewhat linked flags. We should add
    // some validation that they are used correctly.
    if (options_->force_obj_output) {
      if (!codegen.EmitObject(*driver_env_->output_stream)) {
        return false;
      }
    } else {
      if (!codegen.EmitAssembly(*driver_env_->output_stream)) {
        return false;
      }
    }
  } else {
    llvm::SmallString<256> output_filename = llvm::StringRef(output_filename_);
    if (output_filename.empty()) {
      if (!source_->is_regular_file()) {
        // Don't invent file names like `-.o` or `/dev/stdin.o`.
        // TODO: Consider rephrasing the diagnostic to use the file as the
        // `Emit` location.
        CARBON_DIAGNOSTIC(CompileInputNotRegularFile, Error,
                          "output file name must be specified for input `{0}` "
                          "that is not a regular file",
                          std::string);
        driver_env_->emitter.Emit(CompileInputNotRegularFile, input_filename_);
        return false;
      }
      output_filename = input_filename_;
      llvm::sys::path::replace_extension(output_filename,
                                         options_->asm_output ? ".s" : ".o");
    }
    CARBON_VLOG("Writing output to: {0}\n", output_filename);

    std::error_code ec;
    llvm::raw_fd_ostream output_file(output_filename, ec,
                                     llvm::sys::fs::OF_None);
    if (ec) {
      // TODO: Consider rephrasing the diagnostic to use the file as the `Emit`
      // location.
      CARBON_DIAGNOSTIC(CompileOutputFileOpenError, Error,
                        "could not open output file `{0}`: {1}", std::string,
                        std::string);
      driver_env_->emitter.Emit(CompileOutputFileOpenError,
                                output_filename.str().str(), ec.message());
      return false;
    }
    if (options_->asm_output) {
      if (!codegen.EmitAssembly(output_file)) {
        return false;
      }
    } else {
      if (!codegen.EmitObject(output_file)) {
        return false;
      }
    }
  }
  return true;
}

auto CompilationUnit::GetParseTreeAndSubtrees() const
    -> const Parse::TreeAndSubtrees& {
  if (!parse_tree_and_subtrees_) {
    parse_tree_and_subtrees_ = Parse::TreeAndSubtrees(*tokens_, *parse_tree_);
    if (mem_usage_) {
      mem_usage_->Collect("parse_tree_and_subtrees_",
                          *parse_tree_and_subtrees_);
    }
  }
  return *parse_tree_and_subtrees_;
}

auto CompilationUnit::LogCall(llvm::StringLiteral logging_label,
                              llvm::StringLiteral timing_label,
                              llvm::function_ref<auto()->void> fn) -> void {
  PrettyStackTraceFunction trace_file([&](llvm::raw_ostream& out) {
    out << "Filename: " << input_filename_ << "\n";
  });
  CARBON_VLOG("*** {0}: {1} ***\n", logging_label, input_filename_);
  Timings::ScopedTiming timing(timings_ ? &*timings_ : nullptr, timing_label);
  fn();
  CARBON_VLOG("*** {0} done ***\n", logging_label);
}

CompileDriver::CompileDriver(CompileOptions* options) : options_(options) {}

auto CompileDriver::Initialize(
    DriverEnv& driver_env,
    llvm::function_ref<auto(llvm::StringRef)->std::string> map_input) -> bool {
  if (!options_->ValidatePhase(driver_env.emitter)) {
    return false;
  }

  // Validate the target before passing it to Clang.
  const llvm::Target* target;
  if (auto t = options_->ValidateTarget(driver_env.emitter); !t.ok()) {
    return false;
  } else {
    target = *t;
  }

  if (auto i = options_->BuildClangInvocation(driver_env); !i.ok()) {
    return false;
  } else {
    clang_invocation_ = std::move(*i);
  }

  // Find the files comprising the prelude if we are importing it.
  // TODO: Replace this with a search for library api files in a
  // package-specific search path based on the library name.
  llvm::SmallVector<std::string> prelude;
  if (options_->prelude_import && !options_->custom_core &&
      options_->phase >= CompileOptions::Phase::Check) {
    if (auto find = driver_env.installation->ReadPreludeManifest();
        !find.ok()) {
      // TODO: Change ReadPreludeManifest to produce diagnostics.
      CARBON_DIAGNOSTIC(CompilePreludeManifestError, Error, "{0}", std::string);
      driver_env.emitter.Emit(CompilePreludeManifestError,
                              PrintToString(find.error()));
      return false;
    } else {
      prelude = std::move(*find);
    }
  }

  // Append the Core library files on request.
  llvm::SmallVector<std::string> core_library;
  if (options_->include_carbon_core && !options_->custom_core &&
      options_->phase >= CompileOptions::Phase::Check) {
    if (auto find = driver_env.installation->ReadCarbonCoreManifest();
        !find.ok()) {
      CARBON_DIAGNOSTIC(CompileCoreManifestError, Error, "{0}", std::string);
      driver_env.emitter.Emit(CompileCoreManifestError,
                              PrintToString(find.error()));
      return false;
    } else {
      // Note we also compile any .impl files, as we will need to include them
      // in the subsequent link step.
      core_library = std::move(*find);
    }
  }

  // Prepare CompilationUnits before building scope exit handlers.
  int unit_index = -1;
  int total_unit_count =
      prelude.size() + core_library.size() + options_->input_filenames.size();
  auto unit_builder = [&](llvm::StringRef filename) {
    ++unit_index;
    return std::make_unique<CompilationUnit>(
        SemIR::CheckIRId(unit_index), total_unit_count, &driver_env, options_,
        driver_env.consumer, filename, map_input(filename), target);
  };
  llvm::append_range(units_, llvm::map_range(prelude, unit_builder));
  llvm::append_range(units_, llvm::map_range(core_library, unit_builder));
  input_filenames_index_ = units_.size();
  llvm::append_range(units_,
                     llvm::map_range(options_->input_filenames, unit_builder));
  CARBON_CHECK(units_.size() == static_cast<size_t>(total_unit_count));

  // Add the cache to all units. This must be done after all units are created.
  cache_ = std::make_unique<MultiUnitCache>(options_, units_);
  for (auto& unit : units_) {
    unit->SetMultiUnitCache(cache_.get());
  }

  return true;
}

auto CompileDriver::Compile(DriverEnv& driver_env) -> DriverResult {
  auto on_exit = llvm::scope_exit([&]() {
    // Finish compilation units. This flushes their diagnostics in the order in
    // which they were specified on the command line.
    for (auto& unit : units_) {
      unit->PostCompile();
    }

    driver_env.consumer->Flush();
  });

  PrettyStackTraceFunction flush_on_crash([&](llvm::raw_ostream& out) {
    // When crashing, flush diagnostics. If sorting diagnostics, they can be
    // redirected to the crash stream; if streaming, the original stream is
    // flushed.
    // TODO: Eventually we'll want to limit the count.
    if (options_->stream_errors) {
      out << "Flushing diagnostics\n";
    } else {
      out << "Pending diagnostics:\n";
    }

    // In non-streaming mode, swap out the consumer for one that writes to the
    // given ostream before flushing the diagnostics.
    Diagnostics::StreamConsumer stack_trace_consumer(&out);
    llvm::SaveAndRestore<Diagnostics::Consumer*> restore(
        driver_env.consumer,
        options_->stream_errors ? driver_env.consumer : &stack_trace_consumer);

    for (auto& unit : units_) {
      unit->FlushForStackTrace();
    }
    driver_env.consumer->Flush();
  });

  // Returns a DriverResult object. Called whenever Compile returns.
  auto make_result = [&]() {
    DriverResult result = {.success = true};
    for (const auto& unit : units_) {
      result.success &= unit->success();
      result.per_file_success.push_back(
          {unit->input_filename().str(), unit->success()});
    }
    return result;
  };

  // Lex.
  for (auto& unit : units_) {
    unit->RunLex();
  }
  if (options_->phase == CompileOptions::Phase::Lex) {
    return make_result();
  }
  cache_->ApplyPerFileIncludeInDumps();
  // Parse and check phases examine `has_source` because they want to proceed if
  // lex failed, but not if source doesn't exist. Later steps are skipped if
  // anything failed, so don't need this.

  // Parse.
  for (auto& unit : units_) {
    if (unit->has_source()) {
      unit->RunParse();
    }
  }
  if (options_->phase == CompileOptions::Phase::Parse) {
    return make_result();
  }

  // Gather Check::Units.
  llvm::SmallVector<Check::Unit> check_units;
  check_units.reserve(units_.size());
  for (auto& unit : units_) {
    if (unit->has_source()) {
      check_units.push_back(unit->GetCheckUnit());
    }
  }

  // Execute the actual checking.
  CARBON_VLOG_TO(driver_env.vlog_stream, "*** Check::CheckParseTrees ***\n");
  Check::CheckParseTreesOptions options;
  options.prelude_import = options_->prelude_import;
  options.vlog_stream = driver_env.vlog_stream;
  options.fuzzing = driver_env.fuzzing;
  options.mangle_string_fingerprint = options_->mangle_string_fingerprint;
  if (options.vlog_stream || options_->dump_sem_ir || options_->dump_cpp_ast ||
      options_->dump_raw_sem_ir) {
    options.include_in_dumps = &cache_->include_in_dumps();
    if (options_->dump_sem_ir) {
      options.dump_stream = driver_env.output_stream;
    }
    if (options_->dump_cpp_ast) {
      options.dump_cpp_ast_stream = driver_env.output_stream;
    }
    if (options.vlog_stream || options_->dump_sem_ir) {
      options.dump_sem_ir_ranges = options_->dump_sem_ir_ranges;
    }
    if (options_->dump_raw_sem_ir) {
      options.raw_dump_stream = driver_env.output_stream;
      options.dump_raw_sem_ir_builtins = options_->builtin_sem_ir;
    }
    options.sem_ir_crash_dump = options_->sem_ir_crash_dump;
  }

  Check::CheckParseTrees(check_units, cache_->tree_and_subtrees_getters(),
                         driver_env.fs, options, clang_invocation_);
  CARBON_VLOG_TO(driver_env.vlog_stream,
                 "*** Check::CheckParseTrees done ***\n");
  for (auto& unit : units_) {
    if (unit->has_source()) {
      unit->PostCheck();
    }
  }
  if (options_->phase == CompileOptions::Phase::Check) {
    return make_result();
  }

  // Unlike previous steps, errors block further progress.
  if (llvm::any_of(units_,
                   [&](const auto& unit) { return !unit->success(); })) {
    CARBON_VLOG_TO(driver_env.vlog_stream,
                   "*** Stopping before lowering due to errors ***\n");
    return make_result();
  }

  // Lower and optimize.
  for (const auto& unit : units_) {
    unit->RunLower();

    if (options_->phase != CompileOptions::Phase::Lower) {
      unit->RunOptimize(*clang_invocation_);
    }

    unit->PostLower();
  }
  if (options_->phase == CompileOptions::Phase::Lower ||
      options_->phase == CompileOptions::Phase::Optimize) {
    return make_result();
  }
  CARBON_CHECK(options_->phase == CompileOptions::Phase::CodeGen,
               "CodeGen should be the last stage");

  bool output_last_input_only = options_->output_last_input_only;
  if (!output_last_input_only && units_.size() > 1 &&
      !options_->output_filename.empty() && options_->output_filename != "-") {
    // TODO: Command line structure should change to make this implicit
    // (passing non-compiling inputs differently), and the warning should be
    // removed.
    CARBON_DIAGNOSTIC(
        CompileMultipleInputsWithOutput, Warning,
        "only outputting {0} to {1}, skipping output of {2} input "
        "file{2:s}; pass `--output-last-input-only` to silence this warning",
        std::string, std::string, Diagnostics::IntAsSelect);
    driver_env.emitter.Emit(CompileMultipleInputsWithOutput,
                            units_.back()->input_filename().str(),
                            options_->output_filename.str(), units_.size() - 1);
    output_last_input_only = true;
  }

  // Codegen.
  if (output_last_input_only) {
    units_.back()->RunCodeGen();
  } else {
    for (const auto& unit : units_) {
      unit->RunCodeGen();
    }
  }
  return make_result();
}

}  // namespace Carbon