test-zig-cc/README.md

bazel-zig-cc and llvm
---------------------

bazel-zig-cc has a performance issue when compiling many files. This repository
reproduces that.

The test
--------

Compiles 64 small binaries with [bazel-zig-cc][1] and [llvm14][2]. The tests
were run on an x86_64 8-core machine running Ubuntu 22.04.

Steps to reproduce
------------------

Baseline is llvm14: 12-13 seconds:

```
bazel --batch clean; bazel --batch build --extra_toolchains=@llvm_toolchain//:cc-toolchain-x86_64-linux ...
INFO: Elapsed time: 12.454s, Critical Path: 1.11s
```

zig cc without the sandbox. Which means different invocations of `zig cc` will
see that all files in `zig_lib_dir` are the same file. 17 seconds:

```
bazel --batch clean; bazel --batch build --spawn_strategy=local  --platforms=@zig_sdk//libc_aware/platform:linux_amd64_gnu.2.28 ...
INFO: Elapsed time: 17.021s, Critical Path: 1.67s
```

zig cc plain: 61 seconds. Sandbox is on a real disk, which means it will take
even longer to re-hash all its dependencies:

```
bazel --batch clean; bazel --batch build --platforms=@zig_sdk//libc_aware/platform:linux_amd64_gnu.2.28 ...
INFO: Elapsed time: 61.128s, Critical Path: 8.35s
```

zig cc with hardlinks: `--config=hermetic-sandbox`. This uses hardlinks to
zig_sdk instead of symlinks:

```
bazel --batch clean; bazel --batch build --config=hermetic-sandbox --platforms=@zig_sdk//libc_aware/platform:linux_amd64_gnu.2.28 ...
INFO: Elapsed time: 45.886s, Critical Path: 9.56s
```

llvm with sysroot (this sysroot is about half the size of zig):

```
bazel --batch clean; bazel --batch build --config=hermetic-sandbox --extra_toolchains=@llvm_toolchain_with_sysroot//:cc-toolchain-x86_64-linux ...
INFO: Elapsed time: 25.644s, Critical Path: 3.03s
```

Flame graphs and discussion
---------------------------

Flame graphs are in results/. As of 2022-12-11 most of the overhead comes from
creating and deleting the sandboxes. Time for a new sandboxfs!


[1]: https://sr.ht/~motiejus/bazel-zig-cc
[2]: https://github.com/grailbio/bazel-toolchain