tree 9665a2e73012de6397a9cdc46ff6b58dda680cfc
parent ed1abc107815210dc66ec439542bee2f6cbabc00
author David 'Digit' Turner <digit@google.com> 1737579203 +0100
committer GN LUCI <gn-scoped@luci-project-accounts.iam.gserviceaccount.com> 1738003320 -0800

Speed-up GN with custom OutputStream interface.

For legacy reasons, std::ostream is a very inefficient
interface for appending text to an output stream, something
that GN does a lot.

This CL speeds GN by:

- Introducing a new OutputStream abstract class to replace
  std::ostream entirely. It provides the same API subset needed
  by the rest of the code to minimize changes.
  See gn/output_stream.h.

- Adding StringOutputStream, a concrete OutputStream to store
  the result into an std::stream. This replaces
  std::ostringstream. See gn/output_stream.h.

- Adding FileOutputStream, a concrete OutputStream to store
  the result into a file. This replaces std::ofstream.
  See gn/output_stream.h.

- Making StringOutputBuffer an OutputStream derived class
  as well, simplify its use as an output destination
  for many calls.

- Adjusting all call sites appropriately.

- Replace `out << std::endl` statements with
  `out << "\n"` as the type of `std::endl` is unspecified
  by the standard and hard to guess at compile time for
  implementing an OutputStream::operator<< overload.

- Remove obsolete `#include <sstream>` statements
  from the sources. Same for `<fstream>`.

Benchmarking shows that on Linux, this saves about 6% of
`gn gen` time for a small Fuchsia build graph
(6.27s -> 5.90s), and that the stripped LTO-optimized
executable, is reduced by about 50 kiB.

Change-Id: I00c7e1db67c59ab57c64756382683159df0662a6
Reviewed-on: https://gn-review.googlesource.com/c/gn/+/18140
Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: David Turner <digit@google.com>