[llvm] a3aa916 - [Support] Improve Windows performance of buffered raw_ostream

[Andrew Ng via llvm-commits]

Author: Andrew Ng
Date: 2023-04-05T11:03:12+01:00
New Revision: a3aa916d019c8deb10c750acecdef650b3365f22

URL: https://github.com/llvm/llvm-project/commit/a3aa916d019c8deb10c750acecdef650b3365f22
DIFF: https://github.com/llvm/llvm-project/commit/a3aa916d019c8deb10c750acecdef650b3365f22.diff

LOG: [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

Added: 
    

Modified: 
    llvm/lib/Support/raw_ostream.cpp

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Support/raw_ostream.cpp b/llvm/lib/Support/raw_ostream.cpp
index 7b9b8b2f53fba..fae4a4308fd7f 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() {