On Mon, 17 Aug 2026, Aldy Hernandez wrote:
> When back_jt_path_registry::duplicate_thread_path fails, the
> registered path was 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.
>
> Tested on ppc64le Linux.
>
> OK?
It conflicts with my pushed patch, but otherwise OK. Note I really
want some TLC here, this "late" failing in random places makes
hunting down the various checks difficult ...
> 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.
> ---
> gcc/tree-ssa-threadupdate.cc | 24 ++++++++++++++++++------
> gcc/tree-ssa-threadupdate.h | 3 ++-
> 2 files changed, 20 insertions(+), 7 deletions(-)
>
> diff --git a/gcc/tree-ssa-threadupdate.cc b/gcc/tree-ssa-threadupdate.cc
> index db3520b42fc..73a995815c0 100644
> --- a/gcc/tree-ssa-threadupdate.cc
> +++ b/gcc/tree-ssa-threadupdate.cc
> @@ -2382,14 +2382,16 @@ back_jt_path_registry::adjust_paths_after_duplication
> (unsigned curr_path_num)
> CURRENT_PATH_NO is an index into the global paths[] table
> specifying the jump-thread path.
>
> - Returns false if it is unable to copy the region, true otherwise. */
> + Returns false if it is unable to copy the region, true otherwise.
> + On failure *FAILURE_REASON says why. */
>
> bool
> back_jt_path_registry::duplicate_thread_path (edge entry,
> edge exit,
> basic_block *region,
> unsigned n_region,
> - unsigned current_path_no)
> + unsigned current_path_no,
> + const char **failure_reason)
> {
> unsigned i;
> class loop *loop = entry->dest->loop_father;
> @@ -2398,7 +2400,10 @@ back_jt_path_registry::duplicate_thread_path (edge
> entry,
> profile_count curr_count;
>
> if (!can_copy_bbs_p (region, n_region))
> - return false;
> + {
> + *failure_reason = "Cannot copy the blocks in the path";
> + return false;
> + }
>
> /* Some sanity checking. Note that we do not check for all possible
> missuses of the functions. I.e. if you ask to copy something weird,
> @@ -2409,7 +2414,10 @@ back_jt_path_registry::duplicate_thread_path (edge
> entry,
> /* We do not handle subloops, i.e. all the blocks must belong to the
> same loop. */
> if (region[i]->loop_father != loop)
> - return false;
> + {
> + *failure_reason = "Path crosses loops";
> + return false;
> + }
> }
>
> initialize_original_copy_tables ();
> @@ -2638,16 +2646,20 @@ back_jt_path_registry::update_cfg (bool
> /*peel_loop_headers*/)
> for (unsigned int j = 0; j < len - 1; j++)
> region[j] = (*path)[j]->e->dest;
>
> - if (duplicate_thread_path (entry, exit, region, len - 1, 0))
> + const char *failure_reason = NULL;
> + if (duplicate_thread_path (entry, exit, region, len - 1, 0,
> + &failure_reason))
> {
> /* We do not update dominance info. */
> free_dominance_info (CDI_DOMINATORS);
> visited_starting_edges.add (entry);
> retval = true;
> m_num_threaded_edges++;
> + path->release ();
> }
> + else
> + cancel_thread (path, failure_reason);
>
> - path->release ();
> m_paths.unordered_remove (0);
> free (region);
> }
> diff --git a/gcc/tree-ssa-threadupdate.h b/gcc/tree-ssa-threadupdate.h
> index 7faac34fb8f..624bc10a64d 100644
> --- a/gcc/tree-ssa-threadupdate.h
> +++ b/gcc/tree-ssa-threadupdate.h
> @@ -116,7 +116,8 @@ private:
> bool update_cfg (bool peel_loop_headers) override;
> void adjust_paths_after_duplication (unsigned curr_path_num);
> bool duplicate_thread_path (edge entry, edge exit, basic_block *region,
> - unsigned n_region, unsigned current_path_no);
> + unsigned n_region, unsigned current_path_no,
> + const char **failure_reason);
> bool rewire_first_differing_edge (unsigned path_num, unsigned edge_num);
> };
>
>
--
Richard Biener <[email protected]>
SUSE Software Solutions Germany GmbH,
Frankenstrasse 146, 90461 Nuernberg, Germany;
GF: Jochen Jaser, Andrew McDonald, Abhinav Puri; (HRB 36809, AG Nuernberg)
