From: Andrew Ng <andrew.ng@sony.com>
Date: Fri, 31 Mar 2023 16:50:22 +0000 (+0100)
Subject: [Support] Improve Windows performance of buffered raw_ostream
X-Git-Tag: upstream/17.0.6~12630
X-Git-Url: http://review.tizen.org/git/?a=commitdiff_plain;h=a3aa916d019c8deb10c750acecdef650b3365f22;p=platform%2Fupstream%2Fllvm.git

[Support] Improve Windows performance of buffered raw_ostream

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.

Differential Revision: https://reviews.llvm.org/D147340
---

diff --git a/llvm/lib/Support/raw_ostream.cpp b/llvm/lib/Support/raw_ostream.cpp
index 7b9b8b2..fae4a43 100644
--- a/llvm/lib/Support/raw_ostream.cpp
+++ b/llvm/lib/Support/raw_ostream.cpp
@@ -84,8 +84,15 @@ raw_ostream::~raw_ostream() {
 }
 
 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() {