When printing MIR, output to errs() rather than outs().

Summary:
Without this, this command

  $ llvm-run llc -stop-after machine-cp -o - <( echo '' )

outputs an error, because we close stdout twice -- once when closing the
file opened for "-o", and again when closing outs().

Also clarify in the outs() definition that you can't ever call it if you
want to open your own raw_fd_ostream on stdout.

Reviewers: jroelofs, tstellarAMD

Subscribers: jholewinski, qcolombet, dsanders, llvm-commits

Differential Revision: http://reviews.llvm.org/D17422

llvm-svn: 261286

1 files changed, 4 insertions, 3 deletions

diff --git a/llvm/lib/Support/raw_ostream.cpp b/llvm/lib/Support/raw_ostream.cpp
index f5a3162e2ba..07497fcd731 100644
--- a/llvm/lib/Support/raw_ostream.cpp
+++ b/llvm/lib/Support/raw_ostream.cpp
@@ -715,9 +715,10 @@ bool raw_fd_ostream::has_colors() const {
 /// outs() - This returns a reference to a raw_ostream for standard output.
 /// Use it like: outs() << "foo" << "bar";
 raw_ostream &llvm::outs() {
-  // Set buffer settings to model stdout behavior.
-  // Delete the file descriptor when the program exits, forcing error
-  // detection. If you don't want this behavior, don't use outs().
+  // Set buffer settings to model stdout behavior.  Delete the file descriptor
+  // when the program exits, forcing error detection.  This means that if you
+  // ever call outs(), you can't open another raw_fd_ostream on stdout, as we'll
+  // close stdout twice and print an error the second time.
   std::error_code EC;
   static raw_fd_ostream S("-", EC, sys::fs::F_None);
   assert(!EC);