Add macOS ThinLTO arguments#748
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Mach-O LLD exposes distributed ThinLTO through direct --thinlto options rather than the GNU plugin options used by ELF linkers. Select the Mach-O LLD options for macOS indexing actions, including --lto-obj-path for the declared regular LTO object output. Add a macOS golden test for the complete thin_lto feature and extend the legacy feature test helper to compare cc_feature targets.
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This adds distributed ThinLTO for macOS targets after #615. The existing `windows-thin-lto.patch` remains unchanged, and the macOS rules_cc changes are isolated in `macos-thin-lto.patch`, which is applied after `windows-thin-lto.patch`. The macOS patch corresponds to bazelbuild/rules_cc#748 and only adds Mach-O LLD's direct distributed ThinLTO arguments. The LLVM source patch adds the exact-file `--lto-obj-path` contract proposed upstream in llvm/llvm-project#203365. The exact-file contract lets the indexing action produce rules_cc's declared `thinlto_merged_object_file` without reproducing Mach-O LLD's `0.<arch>.lto.o` directory naming. `is_windows_capable_of_thinlto_feature` and `is_macos_capable_of_thinlto_feature` default to false and independently control whether the toolchain exposes the rules_cc `thin_lto` feature on Windows and macOS. `windows_capable_of_thinlto_feature` and `macos_capable_of_thinlto_feature` combine the corresponding build setting with the target OS constraint. `--config=prebuilt` enables both build settings. `is_windows_capable_of_thinlto_feature` can be removed after a rules_cc release contains Windows distributed ThinLTO support. `is_macos_capable_of_thinlto_feature` can be removed after rules_cc and LLVM releases contain macOS distributed ThinLTO support. The previous macOS `-flto=thin` fallback is removed because rules_cc now creates separate indexing and backend actions. Validation: - Applied `windows-thin-lto.patch` followed by `macos-thin-lto.patch` to a clean rules_cc checkout and compared the resulting ThinLTO feature with the rules_cc source. - `bazel test //tests/rule_based_toolchain/legacy_features_as_args:thin_lto_macos_test --test_output=errors` in rules_cc. - `bazel build --config=prebuilt --config=remote --config=bootstrap //prebuilt/llvm:for_windows_amd64 //prebuilt/llvm:for_windows_arm64 //prebuilt/llvm:for_macos_amd64 //prebuilt/llvm:for_macos_arm64`. - BuildBuddy invocation: https://app.buildbuddy.io/invocation/ff484673-abb8-41c9-ba4c-a19410241625
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The
thin_ltofeature currently sends GNU plugin options to every non-Windows linker. Mach-O LLD implements distributed ThinLTO with direct--thinlto-*options instead, so macOS indexing actions fail when they inherit the ELF spellings. This selects Mach-O LLD's direct indexing, imports, prefix replacement, suffix replacement, and merged-object options on macOS while preserving the existing arguments on other platforms.The merged regular LTO object uses
--lto-obj-path, which is proposed in llvm/llvm-project#203365. Mach-O LLD's existing-object_path_ltotreats a ThinLTO path as a directory and generates0.<arch>.lto.o; rules_cc requiresthinlto_merged_object_fileto be an exact declared output consumed by the final link.This PR is based directly on
main. It overlaps thecc_featuregolden-test helper and the ThinLTOselectstructure in #747; whichever PR lands second will need a mechanical rebase.Validation:
bazel test //tests/rule_based_toolchain/legacy_features_as_args:thin_lto_macos_test --test_output=errors.