[Support] Improve Windows performance of buffered raw_ostream

[andrewng]

Summary

The "preferred" buffer size for raw_ostream is set to BUFSIZ which on
Windows is only 512. This results in more calls to write and this
overhead can have a significant negative impact on performance,
especially when Anti-Virus is also involved.

Therefore increase the "preferred" buffer size to 16KB for Windows.

One example of where this helps is the LLD --Map option which dumps out
the symbol map for a link. In a link of UE4, this change has been seen
to improve the performance of the symbol map writing by more than a
factor of 6.

[mstorsjo]

LGTM, I think this is fine (but wait a little while if someone else wants to comment too).

[aganea]

Sounds good!
I'm just curious, have you tried experimenting with different sizes? Would it make sense to set a higher value for the mapfile using .SetBufferSize()?

[andrewng]

In D147340#4240456, @aganea wrote:

Sounds good!
I'm just curious, have you tried experimenting with different sizes? Would it make sense to set a higher value for the mapfile using .SetBufferSize()?

Yes, I did experiment with different sizes on a variety of PCs and the 16KB size was the "sweet" spot for these systems. On one, there were very minor gains from going larger but I decided it was not worthwhile, at least for the "preferred" size. A larger buffer could be set for LLD symbol map generation but the best part of the gains are already realized with the 16KB buffer size for my test systems.

llvm/lib/Support/raw_ostream.cpp

raw_ostream::~raw_ostream() {
   assert(OutBufCur == OutBufStart &&
          "raw_ostream destructor called with non-empty buffer!");
 
   if (BufferMode == BufferKind::InternalBuffer)
     delete [] OutBufStart;
 }
 
 size_t raw_ostream::preferred_buffer_size() const {
+#ifdef _WIN32
+  // On Windows BUFSIZ is only 512 which results in more calls to write. This
+  // overhead can cause significant performance degradation. Therefore use a
+  // better default.
+  return (16 * 1024);
+#else
   // BUFSIZ is intended to be a reasonable default.
   return BUFSIZ;
+#endif
 }
 
 void raw_ostream::SetBuffered() {
   // Ask the subclass to determine an appropriate buffer size.
   if (size_t Size = preferred_buffer_size())
     SetBufferSize(Size);
   else
     // It may return 0, meaning this stream should be unbuffered.