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[llvm-objcopy][NFC] replace class Buffer/MemBuffer/FileBuffer with streams.

Authored by avl on Oct 24 2020, 7:35 AM.

Description

[llvm-objcopy][NFC] replace class Buffer/MemBuffer/FileBuffer with streams.

During D88827 it was requested to remove the local implementation
of Memory/File Buffers:

TODO: refactor the buffer classes in LLVM to enable us to use them here
directly.

This patch uses raw_ostream instead of Buffers. Generally, using streams
could allow us to reduce memory usages. No need to load all data into the
memory - the data could be streamed through a smaller buffer.
Thus, this patch uses raw_ostream as an interface for output data:

Error executeObjcopyOnBinary(CopyConfig &Config,

object::Binary &In,
raw_ostream &Out);

Note 1. This patch does not change the implementation of Writers
so that data would be directly stored into raw_ostream.
This is assumed to be done later.

Note 2. It would be better if Writers would be implemented in a such way
that data could be streamed without seeking/updating. If that would be
inconvenient then raw_ostream could be replaced with raw_pwrite_stream
to have a possibility to seek back and update file headers.
This is assumed to be done later if necessary.

Note 3. Current FileOutputBuffer allows using a memory-mapped file.
The raw_fd_ostream (which could be used if data should be stored in the file)
does not allow us to use a memory-mapped file. Memory map functionality
could be implemented for raw_fd_ostream:

It is possible to add resize() method into raw_ostream.

class raw_ostream {

void resize(uint64_t size);

}

That method, implemented for raw_fd_ostream, could create a memory-mapped file.
The streamed data would be written into that memory file then.
Thus we would be able to use memory-mapped files with raw_fd_ostream.
This is assumed to be done later if necessary.

Differential Revision: https://reviews.llvm.org/D91028

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