[LLVMdev] [RFC] llvm/include/Support/OutputBuffer.h

Nick Kledzik
Mon, 07 May 2012 19:56:42 -0000

For the reasons listed in my 03-May-2012 email, I am proposing a new
llvm/Support class for using in writing binary files:

/// OutputBuffer - This interface provides simple way to create an in-memory
/// buffer which when done will be written to a file. During the lifetime of
/// these  objects, the content or existence of the specified file is undefined.
/// That is, creating an OutputBuffer for a file may immediately remove the
/// file.
/// If the OutputBuffer is committed, the target file's content will become
/// the buffer content at the time of the commit.  If the OutputBuffer is not
/// committed, the file will be deleted in the OutputBuffer buffer destructor.
class OutputBuffer {
public:
  enum Flags {
    F_executable = 1, /// set the 'x' bit on the resulting file
  };

  /// Factory method to create an OutputBuffer object which manages a read/write
  /// buffer of the specified size. When committed, the buffer will be written
  /// to the file at the specified path.
  static error_code createFile(StringRef filePath, Flags flags, size_t size,
                               OwningPtr<OutputBuffer> &result);


  /// Returns a pointer to the start of the buffer.
  uint8_t *bufferStart();

  /// Returns a pointer to the end of the buffer.
  uint8_t *bufferEnd();

  /// Returns size of the buffer.
  size_t size();

  /// Flushes the content of the buffer to its file and deallocates the
  /// buffer.  If commit() is not called before this object's destructor
  /// is called, the file is deleted in the destructor. The optional parameter
  /// is used if it turns out you want the file size to be smaller than
  /// initially requested.
  void commit(int64_t newSmallerSize = -1);
};


The Flags will probable need to be extended over time to handle other clients
needs.

For Unix/Darwin, my plan is to implement this by:
1) delete the file
2) create a new file with a random name in same directory
3) truncate the file to the new size
4) mmap() in the file r/w
5) On commit, unmap the file, rename() to final name
6) In destructor, if not committed, unmap, delete the randomly named file

I'll leave the windows implementation empty and let someone with windows
experience do the implementation.

Comments? Suggestions?

-Nick


> On May 3, 2012, at 6:10 PM, Nick Kledzik wrote:
> Existing llvm code tends to use raw_ostream for writing files.  But
raw_ostream is not a good match for a linker for a couple of reasons:
>
> 1) When the linker creates an executable, the file needs the 'x'
bit set.  Currently raw_fd_ostream has no way to set that.
>
> 2) The Unix conformance suite actually has some test cases where the linker
is run and the output file does exists but is not writable, or is not writable
but is in a writable directory, or with funky umask values.   raw_fd_ostream
interface has no way to match those semantics.
>
> 3) On darwin we have found the linker performs better if it opens the
output file, truncates it to the output size, then mmaps in the file, then
writes directly into that memory buffer.  This avoids the memory copy from the
private buffer to the OS file system buffer in the write() syscall.
>
> 4) In the model we are using for lld, a streaming output interface is not
optimal.   Currently, lld copies chunks of code from the (read-only) input
files, to a temporary buffer, then applies any fixups (relocations), then
streams out that temporary buffer.  If instead we had a big output buffer, the
linker could copy the code chunks directly to the output buffer and apply the
fixups there, avoiding an extra copy.
>
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