https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107090
--- Comment #23 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:91cca3ac45e7f286e770536030da982d65615d4f commit r17-3440-g91cca3ac45e7f286e770536030da982d65615d4f Author: Konstantinos Eleftheriou <[email protected]> Date: Wed Jul 8 02:40:33 2026 -0700 forwprop: Add long-multiply ladder variants Extend the long-multiply fold to the ladder forms, which propagate the cross-product carries via mask / shift / add steps on intermediate sums rather than via an explicit overflow compare: ladder_sum1 = (hilo & mask) + hilo' + (xl*yl >> N) ladder_part_sum = (xl*yl >> N) + hilo ladder_sum2 = (ladder_part_sum & mask) + hilo' ladder_sum3 = (hilo & mask) + (hilo' & mask) + (xl*yl >> N) The high-part chain is xh*yh + (hilo >> N) + (ladder_sumN >> N) [+ ...]; the corresponding LOW_PART recovers (xl*yl & mask) | (ladder_sumN << N). PR tree-optimization/107090 gcc/ChangeLog: * match.pd: Add mul_ladder_sum1, mul_ladder_sum2, mul_ladder_sum3 and mul_ladder_part_sum atom recognizers. * tree-ssa-forwprop.cc (gimple_mul_ladder_sum1): Declare. (gimple_mul_ladder_sum2): Likewise. (gimple_mul_ladder_sum3): Likewise. (gimple_mul_ladder_part_sum): Likewise. (enum long_mul_kind): Add LMK_LADDER_SUM1 / LMK_LADDER_SUM2 / LMK_LADDER_SUM3 / LMK_LADDER_PART_SUM, with the long_mul_table HIGH_PART and LOW_PART rows for each ladder form. (long_mul_set_summand): Handle the new kinds. (long_mul_classify_carry): Update the specificity note above long_mul_classify_plus_kinds. (long_mul_classify_plus_kinds): Try mul_ladder_sum3 and mul_ladder_sum1 before mul_low_sum (more-constrained first); add mul_ladder_sum2. (long_mul_classify_hi_extract): Add mul_ladder_part_sum branch. (long_mul_classify_lo_extract): Adjust. gcc/testsuite/ChangeLog: * gcc.target/aarch64/long_mul.c: Add mulh_ladder and mulh_ladder_32 functions; bump expected umulh / umull scan counts from 1 to 2. * gcc.target/i386/long_mul.c: Same for mulq / imulq. * gcc.dg/tree-ssa/long-mul-ladder.c: New test.
