[Bug tree-optimization/126906] back threader does not thread paths into unreachable edges but DOM does

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126906

--- Comment #3 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Aldy Hernandez <[email protected]>:

https://gcc.gnu.org/g:454e3ba345c2263563562b489bdb81fcc8e71105

commit r17-3348-g454e3ba345c2263563562b489bdb81fcc8e71105
Author: Aldy Hernandez <[email protected]>
Date:   Sun Aug 16 18:50:54 2026 +0000

    Report failed block copies in the backward threader [PR126906]

    When back_jt_path_registry::duplicate_thread_path fails, the
    registered path is dropped silently: the dump shows "Registering jump
    thread" with no matching outcome.  Every other way a thread dies is
    reported, the profitability FAILs, and the registry's other drops via
    cancel_thread.  Report why the copy failed through cancel_thread like
    everywhere else.

    gcc/ChangeLog:
            PR tree-optimization/126906

            * tree-ssa-threadupdate.h
            (back_jt_path_registry::duplicate_thread_path): Add failure_reason
            argument.
            * tree-ssa-threadupdate.cc
            (back_jt_path_registry::duplicate_thread_path): Set
            FAILURE_REASON on the two failing exits.
            (back_jt_path_registry::update_cfg): Cancel the path with the
            reason when duplicate_thread_path fails.