https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107090
--- Comment #22 from GCC Commits <cvs-commit at gcc dot gnu.org> --- The master branch has been updated by Philipp Tomsich <[email protected]>: https://gcc.gnu.org/g:6c171ef2c1ba0b71505f1c6e93c8d3c8d90bdaca commit r17-3439-g6c171ef2c1ba0b71505f1c6e93c8d3c8d90bdaca Author: Konstantinos Eleftheriou <[email protected]> Date: Wed Jul 8 02:56:54 2026 -0700 forwprop: Add long-multiply two-carry variant Extend the long-multiply fold to the variant where the cross sum and the low-half accumulation each carry through a separate unsigned overflow compare: xh*yh + (cross_sum >> N) + carry_low + ((hilo > cross_sum) << N) cross_sum = xh*yl + xl*yh carry_low = (xl*yl + (cross_sum << N)) < (cross_sum << N) An extra check validates the carry_low operands against the multiset's canonical (op0, op1). PR tree-optimization/107090 gcc/ChangeLog: * match.pd: Add mul_carry_low atom recognizer. * tree-ssa-forwprop.cc (gimple_mul_carry_low): Declare. (enum long_mul_kind): Add LMK_CARRY_LOW. (struct long_mul_summand): Add carry_a / carry_b for the LMK_CARRY_LOW summand's two operands. (long_mul_set_summand): Handle LMK_CARRY_LOW. (long_mul_classify_carry): Try mul_carry_low after the more-constrained mul_carry_low_sum / mul_carry_cross_sum. (long_mul_find_summand): New helper, find the first summand with a given kind. (long_mul_check_two_carries): New extra check, validates the LMK_CARRY_LOW summand's operands against the canonical (op0, op1); gates the two-carry HIGH_PART row added to long_mul_table. gcc/testsuite/ChangeLog: * gcc.dg/tree-ssa/long-mul-two-carry.c: New test.
