From a3aa916d019c8deb10c750acecdef650b3365f22 Mon Sep 17 00:00:00 2001 From: Andrew Ng Date: Fri, 31 Mar 2023 17:50:22 +0100 Subject: [PATCH] [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 --- llvm/lib/Support/raw_ostream.cpp | 7 +++++++ 1 file changed, 7 insertions(+) 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() { -- 2.7.4