[Support] Use outs() in ToolOutputFile

Summary:
If the output filename was specified as "-", the ToolOutputFile class
would create a brand new raw_ostream object referring to the stdout.
This patch changes it to reuse the llvm::outs() singleton.

At the moment, this change should be "NFC", but it does enable other
enhancements, like the automatic stdout/stderr synchronization as
discussed on D80803.

I've checked the history, and I did not find any indication that this
class *has* to use a brand new stream object instead of outs() --
indeed, it is special-casing "-" in a number of places already, so this
change fits the pattern pretty well. I suspect the main reason for the
current state of affairs is that the class was originally introduced
(r111595, in 2010) as a raw_fd_ostream subclass, which made any other
solution impossible.

Another potential benefit of this patch is that it makes it possible to
move the raw_ostream class out of the business of special-casing "-" for
stdout handling. That state of affairs does not seem appropriate because
"-" is a valid filename (albeit hard to access with a lot of command
line tools) on most systems. Handling "-" in ToolOutputFile seems more
appropriate.

To make this possible, this patch changes the return type of
llvm::outs() and errs() to raw_fd_ostream&. Previously the functions
were constructing objects of that type, but returning a generic
raw_ostream reference. This makes it possible for new ToolOutputFile and
other code to use raw_fd_ostream methods like error() on the outs()
object. This does not seem like a bad thing (since stdout is a file
descriptor which can be redirected to anywhere, it makes sense to ask it
whether the writing was successful or if it supports seeking), and
indeed a lot of code was already depending on this fact via the
ToolOutputFile "back door".

Reviewers: dblaikie, JDevlieghere, MaskRay, jhenderson

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D81078

llvm/include/llvm/Support/ToolOutputFile.h

@@ -13,6 +13,7 @@
 #ifndef LLVM_SUPPORT_TOOLOUTPUTFILE_H
 #define LLVM_SUPPORT_TOOLOUTPUTFILE_H
 
+#include "llvm/ADT/Optional.h"
 #include "llvm/Support/raw_ostream.h"
 
 namespace llvm {
@@ -38,8 +39,12 @@ class ToolOutputFile {
     ~CleanupInstaller();
   } Installer;
 
-  /// The contained stream. This is intentionally declared after Installer.
-  raw_fd_ostream OS;
+  /// Storage for the stream, if we're owning our own stream. This is
+  /// intentionally declared after Installer.
+  Optional<raw_fd_ostream> OSHolder;
+
+  /// The actual stream to use.
+  raw_fd_ostream *OS;
 
 public:
   /// This constructor's arguments are passed to raw_fd_ostream's
@@ -50,7 +55,7 @@ public:
   ToolOutputFile(StringRef Filename, int FD);
 
   /// Return the contained raw_fd_ostream.
-  raw_fd_ostream &os() { return OS; }
+  raw_fd_ostream &os() { return *OS; }
 
   /// Indicate that the tool's job wrt this output file has been successful and
   /// the file should not be deleted.

llvm/include/llvm/Support/raw_ostream.h

@@ -496,13 +496,13 @@ public:
   void clear_error() { EC = std::error_code(); }
 };
 
-/// This returns a reference to a raw_ostream for standard output. Use it like:
-/// outs() << "foo" << "bar";
-raw_ostream &outs();
+/// This returns a reference to a raw_fd_ostream for standard output. Use it
+/// like: outs() << "foo" << "bar";
+raw_fd_ostream &outs();
 
-/// This returns a reference to a raw_ostream for standard error. Use it like:
-/// errs() << "foo" << "bar";
-raw_ostream &errs();
+/// This returns a reference to a raw_fd_ostream for standard error. Use it
+/// like: errs() << "foo" << "bar";
+raw_fd_ostream &errs();
 
 /// This returns a reference to a raw_ostream which simply discards output.
 raw_ostream &nulls();

llvm/lib/Support/ToolOutputFile.cpp

@@ -15,31 +15,45 @@
 #include "llvm/Support/Signals.h"
 using namespace llvm;
 
+static bool isStdout(StringRef Filename) { return Filename == "-"; }
+
 ToolOutputFile::CleanupInstaller::CleanupInstaller(StringRef Filename)
     : Filename(std::string(Filename)), Keep(false) {
   // Arrange for the file to be deleted if the process is killed.
-  if (Filename != "-")
+  if (!isStdout(Filename))
     sys::RemoveFileOnSignal(Filename);
 }
 
 ToolOutputFile::CleanupInstaller::~CleanupInstaller() {
+  if (isStdout(Filename))
+    return;
+
   // Delete the file if the client hasn't told us not to.
-  if (!Keep && Filename != "-")
+  if (!Keep)
     sys::fs::remove(Filename);
 
   // Ok, the file is successfully written and closed, or deleted. There's no
   // further need to clean it up on signals.
-  if (Filename != "-")
-    sys::DontRemoveFileOnSignal(Filename);
+  sys::DontRemoveFileOnSignal(Filename);
 }
 
 ToolOutputFile::ToolOutputFile(StringRef Filename, std::error_code &EC,
                                sys::fs::OpenFlags Flags)
-    : Installer(Filename), OS(Filename, EC, Flags) {
+    : Installer(Filename) {
+  if (isStdout(Filename)) {
+    OS = &outs();
+    EC = std::error_code();
+    return;
+  }
+  OSHolder.emplace(Filename, EC, Flags);
+  OS = OSHolder.getPointer();
   // If open fails, no cleanup is needed.
   if (EC)
     Installer.Keep = true;
 }
 
 ToolOutputFile::ToolOutputFile(StringRef Filename, int FD)
-    : Installer(Filename), OS(FD, true) {}
+    : Installer(Filename) {
+  OSHolder.emplace(FD, true);
+  OS = OSHolder.getPointer();
+}

llvm/lib/Support/raw_ostream.cpp

@@ -864,9 +864,7 @@ void raw_fd_ostream::anchor() {}
 //  outs(), errs(), nulls()
 //===----------------------------------------------------------------------===//
 
-/// outs() - This returns a reference to a raw_ostream for standard output.
-/// Use it like: outs() << "foo" << "bar";
-raw_ostream &llvm::outs() {
+raw_fd_ostream &llvm::outs() {
   // Set buffer settings to model stdout behavior.
   std::error_code EC;
   static raw_fd_ostream S("-", EC, sys::fs::OF_None);
@@ -874,9 +872,7 @@ raw_ostream &llvm::outs() {
   return S;
 }
 
-/// errs() - This returns a reference to a raw_ostream for standard error.
-/// Use it like: errs() << "foo" << "bar";
-raw_ostream &llvm::errs() {
+raw_fd_ostream &llvm::errs() {
   // Set standard error to be unbuffered by default.
   static raw_fd_ostream S(STDERR_FILENO, false, true);
   return S;

llvm/unittests/Support/CMakeLists.txt

@@ -75,6 +75,7 @@ add_llvm_unittest(SupportTests
   ThreadPool.cpp
   Threading.cpp
   TimerTest.cpp
+  ToolOutputFileTest.cpp
   TypeNameTest.cpp
   TypeTraitsTest.cpp
   TrailingObjectsTest.cpp

llvm/unittests/Support/ToolOutputFileTest.cpp

@@ -0,0 +1,22 @@
+//===- ToolOutputFileTest.cpp - ToolOutputFile tests ----------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Support/ToolOutputFile.h"
+#include "llvm/Support/FileSystem.h"
+#include "gtest/gtest.h"
+
+using namespace llvm;
+
+namespace {
+
+TEST(ToolOutputFileTest, DashOpensOuts) {
+  std::error_code EC;
+  EXPECT_EQ(&ToolOutputFile("-", EC, sys::fs::OF_None).os(), &outs());
+}
+
+} // namespace

mlir/include/mlir/Pass/PassManager.h

@@ -19,7 +19,8 @@
 
 namespace llvm {
 class Any;
-raw_ostream &errs();
+class raw_fd_ostream;
+raw_fd_ostream &errs();
 } // end namespace llvm
 
 namespace mlir {