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Documentation: Add BuildProfilingInstructions.md

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After discussing on Discord about how to speed up the build time, I
received lots of helpful advice on measuring the build time, but none of
it was easily accessible. I thought other people might find it useful,
so I've written it down! :^)

Thanks goes to @bgianfo and @nico.
This commit is contained in:
Sam Atkins 2021-07-29 19:36:31 +01:00 committed by Gunnar Beutner
parent 4ca14d6e67
commit 59a4e87a7e
Notes: sideshowbarker 2024-07-18 07:44:22 +09:00 
Author: https://github.com/AtkinsSJ
Commit: https://github.com/SerenityOS/serenity/commit/59a4e87a7eb
Pull-request: https://github.com/SerenityOS/serenity/pull/9091
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# Build profiling instructions
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.
```console
ninja clean
ccache --clear
```
Then, execute ninja as normal:
```console
ninja
```
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:
<details>
<summary>GCC -ftime-report output</summary>
```console
Time variable usr sys wall GGC
phase setup : 0.00 ( 0%) 0.00 ( 0%) 0.01 ( 0%) 1326k ( 2%)
phase parsing : 0.57 ( 61%) 0.19 ( 83%) 1.63 ( 65%) 59M ( 74%)
phase lang. deferred : 0.10 ( 11%) 0.03 ( 13%) 0.30 ( 12%) 8761k ( 11%)
phase opt and generate : 0.23 ( 25%) 0.01 ( 4%) 0.48 ( 19%) 10M ( 13%)
phase last asm : 0.03 ( 3%) 0.00 ( 0%) 0.08 ( 3%) 539k ( 1%)
|name lookup : 0.11 ( 12%) 0.01 ( 4%) 0.25 ( 10%) 2004k ( 2%)
|overload resolution : 0.08 ( 9%) 0.00 ( 0%) 0.26 ( 10%) 7900k ( 10%)
dump files : 0.02 ( 2%) 0.00 ( 0%) 0.00 ( 0%) 0 ( 0%)
callgraph construction : 0.04 ( 4%) 0.00 ( 0%) 0.06 ( 2%) 2677k ( 3%)
callgraph optimization : 0.00 ( 0%) 0.00 ( 0%) 0.01 ( 0%) 4752 ( 0%)
callgraph functions expansion : 0.15 ( 16%) 0.00 ( 0%) 0.32 ( 13%) 6267k ( 8%)
callgraph ipa passes : 0.03 ( 3%) 0.01 ( 4%) 0.08 ( 3%) 1413k ( 2%)
ipa inheritance graph : 0.00 ( 0%) 0.00 ( 0%) 0.02 ( 1%) 3760 ( 0%)
ipa profile : 0.00 ( 0%) 0.00 ( 0%) 0.01 ( 0%) 0 ( 0%)
trivially dead code : 0.01 ( 1%) 0.00 ( 0%) 0.00 ( 0%) 224 ( 0%)
df reg dead/unused notes : 0.01 ( 1%) 0.00 ( 0%) 0.00 ( 0%) 104k ( 0%)
alias analysis : 0.01 ( 1%) 0.00 ( 0%) 0.00 ( 0%) 70k ( 0%)
preprocessing : 0.06 ( 6%) 0.03 ( 13%) 0.16 ( 6%) 1365k ( 2%)
parser (global) : 0.04 ( 4%) 0.04 ( 17%) 0.13 ( 5%) 6894k ( 8%)
parser struct body : 0.09 ( 10%) 0.03 ( 13%) 0.20 ( 8%) 9020k ( 11%)
parser function body : 0.00 ( 0%) 0.00 ( 0%) 0.01 ( 0%) 83k ( 0%)
parser inl. func. body : 0.03 ( 3%) 0.01 ( 4%) 0.05 ( 2%) 1567k ( 2%)
parser inl. meth. body : 0.11 ( 12%) 0.03 ( 13%) 0.30 ( 12%) 10M ( 13%)
template instantiation : 0.27 ( 29%) 0.08 ( 35%) 0.89 ( 36%) 25M ( 32%)
constant expression evaluation : 0.01 ( 1%) 0.00 ( 0%) 0.01 ( 0%) 356k ( 0%)
constraint satisfaction : 0.01 ( 1%) 0.00 ( 0%) 0.02 ( 1%) 130k ( 0%)
early inlining heuristics : 0.00 ( 0%) 0.00 ( 0%) 0.02 ( 1%) 21k ( 0%)
inline parameters : 0.01 ( 1%) 0.00 ( 0%) 0.03 ( 1%) 146k ( 0%)
integration : 0.00 ( 0%) 0.00 ( 0%) 0.01 ( 0%) 564k ( 1%)
tree CFG cleanup : 0.00 ( 0%) 0.00 ( 0%) 0.01 ( 0%) 5768 ( 0%)
tree SSA other : 0.00 ( 0%) 0.00 ( 0%) 0.01 ( 0%) 13k ( 0%)
tree operand scan : 0.01 ( 1%) 0.00 ( 0%) 0.00 ( 0%) 429k ( 1%)
tree CCP : 0.01 ( 1%) 0.00 ( 0%) 0.00 ( 0%) 18k ( 0%)
expand : 0.01 ( 1%) 0.00 ( 0%) 0.03 ( 1%) 1303k ( 2%)
varconst : 0.00 ( 0%) 0.00 ( 0%) 0.04 ( 2%) 6744 ( 0%)
forward prop : 0.01 ( 1%) 0.00 ( 0%) 0.02 ( 1%) 3520 ( 0%)
CSE : 0.01 ( 1%) 0.00 ( 0%) 0.00 ( 0%) 1144 ( 0%)
loop fini : 0.00 ( 0%) 0.00 ( 0%) 0.01 ( 0%) 0 ( 0%)
branch prediction : 0.00 ( 0%) 0.01 ( 4%) 0.00 ( 0%) 20k ( 0%)
combiner : 0.01 ( 1%) 0.00 ( 0%) 0.02 ( 1%) 138k ( 0%)
integrated RA : 0.01 ( 1%) 0.00 ( 0%) 0.01 ( 0%) 1112k ( 1%)
LRA reload inheritance : 0.00 ( 0%) 0.00 ( 0%) 0.02 ( 1%) 13k ( 0%)
LRA create live ranges : 0.00 ( 0%) 0.00 ( 0%) 0.01 ( 0%) 8568 ( 0%)
reload CSE regs : 0.01 ( 1%) 0.00 ( 0%) 0.02 ( 1%) 100k ( 0%)
final : 0.00 ( 0%) 0.00 ( 0%) 0.02 ( 1%) 323k ( 0%)
variable output : 0.01 ( 1%) 0.00 ( 0%) 0.00 ( 0%) 15k ( 0%)
symout : 0.09 ( 10%) 0.00 ( 0%) 0.20 ( 8%) 13M ( 17%)
variable tracking : 0.00 ( 0%) 0.00 ( 0%) 0.04 ( 2%) 471k ( 1%)
var-tracking dataflow : 0.02 ( 2%) 0.00 ( 0%) 0.03 ( 1%) 18k ( 0%)
var-tracking emit : 0.00 ( 0%) 0.00 ( 0%) 0.04 ( 2%) 117k ( 0%)
rest of compilation : 0.01 ( 1%) 0.00 ( 0%) 0.03 ( 1%) 1258k ( 2%)
TOTAL : 0.93 0.23 2.50 79M
[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.