[Bug tree-optimization/127471] New: vectorizer scalar loop costing misses non-root pattern covered stmts

[rguenth at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=127471

            Bug ID: 127471
           Summary: vectorizer scalar loop costing misses non-root pattern
                    covered stmts
           Product: gcc
           Version: 17.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rguenth at gcc dot gnu.org
            Blocks: 53947
  Target Milestone: ---

vect_compute_single_scalar_iteration_cost does

          /* Skip stmts that are not vectorized inside the loop.  */
          stmt_vec_info vstmt_info = vect_stmt_to_vectorize (stmt_info);
          if (!STMT_VINFO_RELEVANT_P (vstmt_info)

but if there's a sequence of stmts matched to a pattern stmt, like for
widening multiplication or highpart multiplication, only the root scalar
stmt is costed, not the other stmts part of the detected pattern stmt.

This under-estimates scalar cost.


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53947
[Bug 53947] [meta-bug] vectorizer missed-optimizations