[Support] Make raw_string_ostream unbuffered

Summary:
In D58580 i have noted that `llvm::to_string()` is a memory hog.
It uses `raw_string_ostream`, and since it was buffered,
every `raw_string_ostream` had a cost of `BUFSIZ` bytes
(which is `8192` at least here). So every `llvm::to_string()`
call, even to just print an `int`, costed `8192` bytes.

In D58580, getting rid of that buffering //had// significant
performance and memory consumption improvements for `llvm-xray convert`.

Similarly, in D58580 @rnk pointed out that the `raw_svector_ostream`
is already unbuffered, and `write_unsigned_impl` and friends
do internal buffering. So it should be ok performance-wise to just
make the `raw_string_ostream` itself unbuffered.

Here, i don't have any perf measurements.
Another letdown is that i'm leaving a loose-end - not deleting the
`flush()` method. I  don't expect that cleanup to be anything more
than just fixing every new compiler error, but i'm presently unable
to do that. Will look into that later.

Reviewers: rnk, zturner

Reviewed By: rnk

Subscribers: kristina, jdoerfert, llvm-commits, rnk

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D58643

llvm-svn: 354819

llvm/include/llvm/Support/raw_ostream.h

@@ -479,6 +479,9 @@ raw_ostream &nulls();
 
 /// A raw_ostream that writes to an std::string.  This is a simple adaptor
 /// class. This class does not encounter output errors.
+/// raw_string_ostream operates without a buffer, delegating all memory
+/// management to the std::string. Thus the std::string is always up-to-date,
+/// may be used directly and there is no need to call flush().
 class raw_string_ostream : public raw_ostream {
   std::string &OS;
 
@@ -490,13 +493,16 @@ class raw_string_ostream : public raw_ostream {
   uint64_t current_pos() const override { return OS.size(); }
 
 public:
-  explicit raw_string_ostream(std::string &O) : OS(O) {}
+  explicit raw_string_ostream(std::string &O)
+      : raw_ostream(/*Unbuffered=*/true), OS(O) {}
+
   ~raw_string_ostream() override;
 
-  /// Flushes the stream contents to the target string and returns  the string's
-  /// reference.
+  // FIXME: uncomment and deal with the fallout.
+  // void flush() = delete;
+
+  /// Returns the string's reference.
   std::string& str() {
-    flush();
     return OS;
   }
 };