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Support: Improve performance of FileOutputBuffer on Windows

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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
This commit is contained in:
Rui Ueyama 2015-03-06 06:07:32 +00:00
parent 517e68fe7b
commit 8617b3bb3e
1 changed files with 7 additions and 0 deletions
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7
lib/Support/FileOutputBuffer.cpp
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@ -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<mapped_file_region>(
FD, mapped_file_region::readwrite, Size, 0, EC);