Support: Extract fs::resize_file_before_mapping_readwrite from FileOutputBuffer

Add a variant of `fs::resize_file` for use immediately before opening a
file with `mapped_file_region::readwrite`. On Windows, `_chsize`
(`ftruncate`) is slow, but `CreateFileMapping` (`mmap`) automatically
extends the file so the call to `fs::resize_file` can be skipped.

This optimization was added to `FileOutputBuffer` in
da9bc2e56d5a5c6332a9def1a0065eb399182b93; this commit just extracts the
logic out and adds a unit test.

Differential Revision: https://reviews.llvm.org/D95490

1 files changed, 1 insertions, 8 deletions

diff --git a/llvm/lib/Support/FileOutputBuffer.cpp b/llvm/lib/Support/FileOutputBuffer.cpp
index 3342682..6d21258 100644
--- a/llvm/lib/Support/FileOutputBuffer.cpp
+++ b/llvm/lib/Support/FileOutputBuffer.cpp
@@ -132,17 +132,10 @@ createOnDiskBuffer(StringRef Path, size_t Size, unsigned Mode) {
     return FileOrErr.takeError();
   fs::TempFile File = std::move(*FileOrErr);
 
-#ifndef _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 function.
-  if (auto EC = fs::resize_file(File.FD, Size)) {
+  if (auto EC = fs::resize_file_before_mapping_readwrite(File.FD, Size)) {
     consumeError(File.discard());
     return errorCodeToError(EC);
   }
-#endif
 
   // Mmap it.
   std::error_code EC;