Plan for supporting MSVC in rules-based toolchain?

[calebzulawski]

I'm using a rules-based toolchain after using the old style for a while and I'm very happy with it. I've recently tried to expand support of my toolchain to Windows and noticed there is currently no support for MSVC-style flags.

Is there a plan for how to pass the compiler type such that flags can be properly selected? I'd be interested in implementing this and basic MSVC support with some guidance. Thanks!

[armandomontanez]

The tentative idea I had for how to support this was via tool capabilities. The idea was to have a signal like compiler_uses_msvc_style_flags, and then in the argument expansions use requires_none or requires_not_none to properly expand to the right set of arguments.

A select is more intuitive, but in order to use select we'd need to invert dependencies somehow since cc_args would be depending on other properties of a toolchain.

[calebzulawski]

That makes sense!

I figured I'd try to get a GCC/clang style compiler working with the Windows linker first--a few tweaks and I have it generally working. One thing that isn't working however is implementation libraries. I added this:

cc_feature(
    name = "build_interface_libraries_feature",
    args = [":build_interface_libraries"],
    overrides = "//cc/toolchains/features/legacy:build_interface_libraries",
)

cc_args(
    name = "build_interface_libraries",
    actions = ["//cc/toolchains/actions:link_actions"],
    args = ["-Wl,/IMPLIB:{interface_library_output_path}"],
    format = {"interface_library_output_path": "//cc/toolchains/variables:interface_library_output_path"},
    requires_not_none = "//cc/toolchains/variables:interface_library_output_path",
)

inspired by:

rules_cc/cc/private/toolchain/windows_cc_toolchain_config.bzl

Lines 472 to 483 in bdd93e8

	flag_set(
	actions = [
	ACTION_NAMES.cpp_link_dynamic_library,
	ACTION_NAMES.cpp_link_nodeps_dynamic_library,
	],
	flag_groups = [
	flag_group(
	flags = ["/IMPLIB:%{interface_library_output_path}"],
	expand_if_available = "interface_library_output_path",
	),
	],
	),

For some reason, the flag that gets added is -Wl,/IMPLIB:ignored. When I disable my custom toolchain I get a path as expected. Any thoughts why the path is "ignored"?

[fmeum]

That most likely comes from here: https://github.com/bazelbuild/bazel/blob/4c939b02e5818387527c0e4e2694a1d1398f3a22/src/main/starlark/builtins_bzl/common/cc/link/link_build_variables.bzl#L277.

I noticed a supports_interface_shared_libraries feature in the legacy Windows toolchain, did you port that over?

[calebzulawski]

I added it as a capability (along with supports_dynamic_linker, after fixing a bug related to capability validation), but adding it as a capability seems to change the linker to external\rules_cc+\cc\private\toolchain\link_dynamic_library.sh from my specified linker (clang++). I did some debugging and the tool map seems to look right at the point it's converted to a legacy toolchain. I'm not sure how the linker is getting unset, but that might explain the interface library being disabled:
https://github.com/bazelbuild/bazel/blob/4c939b02e5818387527c0e4e2694a1d1398f3a22/src/main/starlark/builtins_bzl/common/cc/link/link_build_variables.bzl#L320

Removing the capabilities and manually adding /IMPLIB via linkopts switches the linker back to clang++ and everything works! So it's close, just not sure what's happening here.

[armandomontanez]

Boy do I love magic.

Suggest adding a has_configured_linker_path nop cc_feature.

[calebzulawski]

Thanks, that was it! I also needed to add targets_windows, and now everything works :)

I opened #436 to get halfway there--no support for MSVC as a compiler, but support for linking on Windows.

[calebzulawski]

To handle different compilers, what about adding a variable that returns compiler? That would allow adding /clang: flags when using clang-cl instead of MSVC. It would also allow identifying mingw-gcc, or even just having gcc or clang specific flags on Linux/mac.

[ScottHo]

I'm also working on a msvc rules-based toolchain, and linking was difficult.

Thanks for the /IMPLIB:ignored fixes!