On Mon, Nov 3, 2025 at 7:21 AM Richard Biener <[email protected]> wrote:
>
> The following fixes forwprop using FOR_EACH_IMM_USE_STMT to iterate
> over stmts and then eventually removing the active stmt, releasing
> its defs. This can cause debug stmt insertion with a RHS referencing
> the SSA name we iterate over, adding to its immediate use list
> but also adjusting all other debug stmts refering to the released
> SSA name, updating those. And those can refer to the original
> iterated over variable.
>
> In the end the destructive behavior of update_stmt is a problem
> here, which unlinks all uses of a stmt and then links in the
> newly computed ones instead of leaving those in place that are.
>
> * tree-ssa-forwprop.cc (forward_propagate_addr_expr):
> Avoid removing stmts inside FOR_EACH_IMM_USE_STMT.
LGTM.
Note optimize_vector_load has the same issue. Maybe you did hit that
but didn't go and didn't analyze all of the fallout of the checking.
Thanks,
Andrew
> ---
> gcc/tree-ssa-forwprop.cc | 15 +++++++++------
> 1 file changed, 9 insertions(+), 6 deletions(-)
>
> diff --git a/gcc/tree-ssa-forwprop.cc b/gcc/tree-ssa-forwprop.cc
> index 9a993ab04de..4bac938cf54 100644
> --- a/gcc/tree-ssa-forwprop.cc
> +++ b/gcc/tree-ssa-forwprop.cc
> @@ -930,6 +930,7 @@ forward_propagate_addr_expr (tree name, tree rhs, bool
> parent_single_use_p)
> bool all = true;
> bool single_use_p = parent_single_use_p && has_single_use (name);
>
> + auto_vec<gimple *, 4> to_remove;
> FOR_EACH_IMM_USE_STMT (use_stmt, iter, name)
> {
> bool result;
> @@ -963,12 +964,14 @@ forward_propagate_addr_expr (tree name, tree rhs, bool
> parent_single_use_p)
> && TREE_CODE (gimple_assign_lhs (use_stmt)) == SSA_NAME
> && TREE_CODE (use_rhs) == SSA_NAME
> && has_zero_uses (gimple_assign_lhs (use_stmt)))
> - {
> - gimple_stmt_iterator gsi = gsi_for_stmt (use_stmt);
> - fwprop_invalidate_lattice (gimple_get_lhs (use_stmt));
> - release_defs (use_stmt);
> - gsi_remove (&gsi, true);
> - }
> + to_remove.safe_push (use_stmt);
> + }
> + for (gimple *use : to_remove)
> + {
> + gimple_stmt_iterator gsi = gsi_for_stmt (use);
> + fwprop_invalidate_lattice (gimple_get_lhs (use));
> + release_defs (use);
> + gsi_remove (&gsi, true);
> }
>
> return all && has_zero_uses (name);
> --
> 2.51.0
