https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107090
--- Comment #25 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:0502e6b1e7be3d3fe00727e2936d3c3a03537c4e commit r17-3442-g0502e6b1e7be3d3fe00727e2936d3c3a03537c4e Author: Konstantinos Eleftheriou <[email protected]> Date: Thu Jul 9 02:22:29 2026 -0700 match.pd, forwprop: Recognize long-multiply carries written as 2-arg PHI Hand-written long-multiply code commonly writes the carry as a 2-arg PHI if (mul_hilo0 > low_sum) result = result_partial + (1ULL << N); else result = result_partial; with no top-level + at the join for the chain walk to start from. Add a PHI-driven entry: cond_carry_add / cond_carry_add_neg match PHI<base + pow2, base> guarded by an unsigned compare on (a, sum) in either polarity; the captured sum is classified into a carry summand, base is linearized for the remaining summands, and on a match the 2N-bit multiply is emitted with the PHI result as its LHS. Only HIGH_PART rows are reachable; LOW_PART rows are BIT_IOR chains and never produce a carry PHI. PR tree-optimization/107090 gcc/ChangeLog: * match.pd: Add cond_carry_add and cond_carry_add_neg match recognizers for the 2-arg PHI form PHI<base + pow2, base>, one per gcond polarity. * tree-ssa-forwprop.cc (gimple_cond_carry_add): Declare. (gimple_cond_carry_add_neg): Likewise. (long_mul_check_low_plus_defer): Note that the PHI entry commits only to HIGH_PART rows. (long_mul_classify_match): Take the candidate statement for the per-row checks. (long_mul_classify_chain): Take an optional pre-classified extra summand, for the carry the PHI entry synthesizes. (match_long_mul): Adjust. (match_long_mul_phi): New PHI-driven entry; recognizes a cond_carry_add(_neg) PHI and folds the long-multiply when the rest of the high-part chain matches. (pass_forwprop::execute): Call match_long_mul_phi on each PHI in the degenerate-PHI walk. gcc/testsuite/ChangeLog: * gcc.dg/tree-ssa/long-mul-carry.c: Add PHI-form CARRY_LOW_SUM, CARRY_CROSS_SUM, and negated-branch compile variants; split the forwprop1 high-part fold count into 10 on int128 and 8 on !int128. * gcc.dg/tree-ssa/long-mul-two-carry.c: Add the mulh_two_carry_low_phi function and its forwprop3 high-part (carry PHI) fold scan. * gcc.dg/tree-ssa/long-mul-boundary-64.c: New test. * gcc.dg/tree-ssa/long-mul-boundary.c: New test. * gcc.dg/tree-ssa/long-mul-partial.c: New test. Co-authored-by: Philipp Tomsich <[email protected]>
