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README: --extra_toolchains and --platform

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Motiejus Jakštys 2022-02-02 15:11:10 +02:00
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@ -73,6 +73,27 @@ SIGILL: illegal instruction
This is by design: it encourages program authors to fix the undefined behavior.
There are [many ways][ubsan2] to find the undefined behavior.
## Specifying non-default toolchains (and not registering at all)
You may explicitly request Bazel to use a specific toolchain, even though one
is registered using `--extra_toolchains <toolchain>` flag. For example, if you
wish to compile a specific binary (or run tests) using musl on linux/amd64, you
may specify:
```
--extra_toolchains @zig_sdk//:linux_amd64_musl_toolchain
```
As an extension to this, you may not register the toolchains at all:
```
zig_register_toolchains()
```
In that case, you will need to specify the `--extra_toolchains <toolchain>`
command-line argument. Otherwise Bazel will use the default one -- the host
toolchain.
# Known Issues In bazel-zig-cc
These are the things you may stumble into when using bazel-zig-cc. I am
@ -80,8 +101,6 @@ unlikely to implement them, but patches implementing those will be accepted.
See [Questions & Contributions](#questions-amp-contributions) on how to
contribute.
## OSX: sysroot
For non-trivial programs (and for all darwin/arm64 cgo programs) MacOS SDK may
@ -105,12 +124,31 @@ Currently zig is downloaded from
[dl.jakstys.lt/zig](https://dl.jakstys.lt/zig/), which is nuts. One should
provide a way to specify alternative URLs for the zig toolchain.
## Bazel toolchain target locations
## Toolchain and platform target locations
The path to Bazel toolchains is `@zig_sdk//:<toolchain>_toolchain`. It should
be moved to `@zig_sdk//toolchain:<toolchain>` or similar; so the user-facing
targets are in their own namespace.
Currently, platform is not defined in `@zig_sdk`. As you may see in the
examples, we are currently using the one provided by cgo. E.g.
`@io_bazel_rules_go//go/toolchain:linux_amd64_cgo`. This is because we want to
define it in `@zig_sdk//:platform:<platform>`. This is waiting for mine or a
contributor's attention. You may work around this by sending a patch to
bazel-zig-cc or specifying the platform in your project:
```
platform(
name = "linux_aarch64",
constraint_values = [
"@bazel_tools//platforms:linux",
"@bazel_tools//platforms:aarch64",
]
)
```
and `--platforms="//:linux_aarch64"`.
# Known Issues In Upstream
This section lists issues that I've stumbled into when using `zig cc`, and is