Pass fission flags to ThinLTO backend actions - #837
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With --fission enabled, a .dwo output is declared for every LTO backend action, but the default Unix toolchain config never passes -gsplit-dwarf to that action, so the output can never be written and --fission combined with --features=thin_lto fails at execution time with output '<target>.lto/.../foo.dwo' was not created This test documents the current behaviour: it asserts that the .dwo is declared and that -gsplit-dwarf is absent. The following commit fixes the toolchain configs and flips the assertion. The existing ThinLTO fission coverage in tests/cc/common/cc_binary_thin_lto_tests.bzl cannot catch this, because the test toolchain in tests/cc/testutil/toolchains defines its own fission_flags_for_lto_backend feature covering ACTION_NAMES.lto_backend. Toolchains built on unix_cc_toolchain_config.bzl have no such feature, so the gap can only be seen by testing against that config -- hence a new linux toolchain registration in tests/default_unix_toolchain, which so far registered only a macOS toolchain. The "dwp" tool_paths entry is required: without it, cc_binary analysis under --fission fails with a NoneType error out of create_debug_packager_actions before any flag can be inspected.
cc/private/link/lto_backends.bzl declares <object>.dwo as an output of
each LTO backend action and binds both per_object_debug_info_file and
is_using_fission for it, but neither production toolchain config puts
-gsplit-dwarf on ACTION_NAMES.lto_backend. The declared output is
therefore never written and the action always fails:
ERROR: output 'prog.lto/.../a.dwo' was not created
ERROR: LTO Backend Compile prog.lto/.../a.o failed:
not all outputs were created or valid
so --fission cannot be combined with --features=thin_lto at all.
Add ACTION_NAMES.lto_backend to the per_object_debug_info flag_set in
unix_cc_toolchain_config.bzl and legacy_features.bzl, and flip the
assertion added by the previous commit. The build variable the
flag_group gates on is already bound for that action, so no further
plumbing is needed.
Verified end to end outside the test suite with bazel 8.7.0 and clang
22.1.8 on a two-file cc_binary, --features=thin_lto --fission=yes
--features=per_object_debug_info and no --ltobackendopt: before, the
build fails as above; after, it succeeds and writes non-empty .dwo
files that llvm-dwp packs into a valid .dwp.
akonradi
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Ha, it was the Claude co-author attribution. Removed and force-pushed, same contents. |
lilygorsheneva
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Aug 3, 2026
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Fix interaction between
--fissionand--features=thin_lto. Without this, every LTO backend action fails:Fixed by adding
ACTION_NAMES.lto_backendto theper_object_debug_infoflag_set incc/private/toolchain/unix_cc_toolchain_config.bzlandcc/private/toolchain_config/legacy_features.bzl.Adds a test that uses the real Linux
cc_toolchain_configsince that wasn't covered by the existing test, and was broken.