There are three ways you can get information about compile times:
1. Just using `time ninja install` instead of `ninja install` to time it overall.
2. Reading ninja's log files to get a per-file compile time.
3. Enabling GCC or Clang flags to get a more detailed per-file breakdown of times for compiler passes within a file.
## Ninja log files
Ninja produces a handy log file that includes a per-cpp-file compilation time. This is useful for discovering which files take the most time, and so are good targets for speeding up the overall build time. A python3 script named `ninjatracing` converts the ninja log file into a JSON format that's readable by several profile viewers.
### Prerequisites
`ninjatracing` is available on [GitHub](https://github.com/nico/ninjatracing/blob/master/ninjatracing)
It requires `python3`, available at `python`. You can either create the symlink yourself, or just modify the ninjatracing file to say `python3` instead of `python`.
You also need to make sure `ninjatracing` is marked as executable, and available from your PATH (or somewhere convenient where you can manually call it from).
### Usage
First, we need to clean the build (and `ccache` if present) to make sure every file gets compiled and gives a meaningful time reading.
The log will be written to `.ninja_log` in the current directory by default. To convert it to the more useful JSON file, call `ninjatracing` from the directory you called ninja from:
```console
ninjatracing .ninja_log > trace.json
```
You can then drag-and-drop the file onto [Speedscope](https://www.speedscope.app/) or any other compatible flamegraph visualizer.
## GCC/Clang
Adding the `-ftime-report` flag to GCC will cause it to output a breakdown for each file it compiles, which looks like this:
[24/3139] Building CXX object Kernel/CMakeFiles/Kernel.dir/CommandLine.cpp.o
```
</details>
Depending on whether you understand the internals of the compiler, this may or may not be helpful to you! Generally, this is not recommended.
Clang also supports `-ftime-report`, but I have not tested the output for it.
To add the flag, edit the `CMakeLists.txt` in the serenity root directory, and add `add_compile_options(-ftime-report)` to the list of compile options that start around line 220.
Additionally, you can add `-ftime-report-details` too, which again I have not tested.
