What are the best practices of the platform and cc_toolchain?

[刘宾]

I'm moving a huge project which constains 100+ modules(cmake/autoconf/GNU make/java/go) from conan to bazel.

I have built the bazel 6.1.0.

I found there are two ways to build with different building flavour

1. build --cpu.. --crosstool_top...

2. build --platforms ...

I search in google for help, but the documents are 2020/2019 ...

I think they may be too old

What are the best practices of the platform and cc_toolchain under bazel 6.1.0?

[Tobias Werth]

+John Cater 

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[John Cater]

+Ivo Ristovski List, who has the most updated planning.

As of the Bazel 6.0 series, C++ rules do not use toolchain resolution by default. That means that the canonical flags to control cross-compilation are `--cpu` and `--crosstool_top`.

However, the platform-based system is very mature as long as you are not building for Apple (Objective-C and/or iOS) or Android.

If you want to experiment with the new system you can specify `--incompatible_enable_cc_toolchain_resolution` and use the `--platforms` flag, after defining your platform as discussed in https://bazel.build/extending/platforms.

My understanding is that +Ivo Ristovski List plans to make this flag the default for all C++ builds in the next Bazel LTS release (but he can confirm that).

The canonical source of information for which rules support platforms (and to what extent) is https://bazel.build/concepts/platforms.

I hope this helps clarify the situation.

John Cater

[Fredrik Medley]

I copy my post from a different thread, it might be helpful: https://groups.google.com/g/bazel-discuss/c/ysTeeYsszL0/m/EFmLw8E9AQAJ

The way I think about using the platforms is that the number of (remote) execution platforms are finite and you should be able to define all of them explicitly in Bazel. It is then up to the toolchain resolution to choose a specific execution platform, so `some_rule` needs to define a dependency on at least one toolchain (https://bazel.build/extending/toolchains#writing-rules-toolchains).

I highly recommend watching https://www.youtube.com/watch?v=5Y2WiUafVs0&list=PLxNYxgaZ8RsdH4GCIZ69dzxQCOPyuNlpF&index=18 from BazelCon 2022 where Susan Steinman and John Cater walks through the whole toolchain, platform and exec_group handling. For example, I didn't know that `--toolchain_resolution_debug` can take a target label as argument. Another feature I recently picked up is the `target_settings` attribute of the toolchain rule (https://bazel.build/reference/be/platform#toolchain).

/Fredrik

[刘宾]

Thanks a lot, it's very helpfull