From 8617b3bb3e7b95c27f3ffa7220a2b7754d918adb Mon Sep 17 00:00:00 2001 From: Rui Ueyama Date: Fri, 6 Mar 2015 06:07:32 +0000 Subject: [PATCH] Support: Improve performance of FileOutputBuffer on Windows We extend an underlying file before mmap'ing it, but it's not needed on Windows. Extending file is slow on Windows, so we should avoid doing that. The difference gets larger as the size of an output file gets larger. It shove off 2 seconds out of 25 seconds when linking chrome.dll with LLD, for example. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@231452 91177308-0d34-0410-b5e6-96231b3b80d8 --- lib/Support/FileOutputBuffer.cpp | 7 +++++++ 1 file changed, 7 insertions(+) diff --git a/lib/Support/FileOutputBuffer.cpp b/lib/Support/FileOutputBuffer.cpp index b176a8b45ab..a35e160b2a1 100644 --- a/lib/Support/FileOutputBuffer.cpp +++ b/lib/Support/FileOutputBuffer.cpp @@ -77,9 +77,16 @@ FileOutputBuffer::create(StringRef FilePath, size_t Size, if (EC) return EC; +#ifndef LLVM_ON_WIN32 + // On Windows, CreateFileMapping (the mmap function on Windows) + // automatically extends the underlying file. We don't need to + // extend the file beforehand. _chsize (ftruncate on Windows) is + // pretty slow just like it writes specified amount of bytes, + // so we should avoid calling that. EC = sys::fs::resize_file(FD, Size); if (EC) return EC; +#endif auto MappedFile = llvm::make_unique( FD, mapped_file_region::readwrite, Size, 0, EC);