Re: [PATCH v8 0/7] Recognize and fold longhand wide-multiplication idioms [PR107090]

On Mon, Aug 3, 2026 at 5:57=E2=80=AFPM Konstantinos Eleftheriou
<[email protected]> wrote:
>
>
> This patch series teaches GCC to recognize longhand 64x64->128
> wide-multiplication idioms and replace them with native multiply
> instructions: a widening multiply followed by a right shift for the
> high part, and a plain MULT_EXPR for the low part.
>
> Portable C/C++ code that needs a 128-bit product on a 64-bit target
> often resorts to a longhand decomposition: split operands into 32-bit
> halves, compute four partial products, and propagate carries manually.
> This pattern appears in a number of real-world codebases, including
> SPEC2026's 750.sealcrypto_r (seal/util/uintarith.h) and several
> examples from Hacker's Delight. Targets like AArch64 (mul/umulh) and
> x86-64 can compute the full 128-bit product in one or two instructions,
> but GCC does not currently fold the longhand sequence back to these.
>
> The recognizer emits the canonical widening shape
>
>   (N)(((2N) a * (2N) b) >> N)
>
> for the high part and a plain MULT_EXPR for the low part. The high
> part is emitted as a shift of the wide product, not as a bare
> (2N) a * (2N) b: the `>> N' form is the MULT_HIGHPART idiom, so
> pass_optimize_widening_mul rewrites it to the target's native high-part
> multiply (e.g. umulh) without ever forming the full 2N product. A bare
> widening product would also compute the unwanted low half, and where 2N
> has no native multiply -- the 128x128 case, 2N =3D OImode, which the mode
> table carries but the target cannot expand -- there is no way to form
> it at all. For that case the final patch resynthesizes the longhand at
> narrow precision from (N/2)-wide partial products, so the fold never
> depends on a 2N multiply the target lacks.
>
> The series is split into seven patches:
>
>   1/7  forwprop: Match and fold the long-multiply carry form
>        [PR107090]
>
>        Adds the match.pd atom patterns and the forwprop framework:
>        linearize the outer add/ior chain, classify each summand, and
>        match the multiset against a table of decomposed variants.
>        Carries the base carry form, a single overflow comparison on
>        the cross-sum. Matching starts only at the end of a chain, and
>        leaves that are not long-multiply summands are preserved and
>        re-applied on top of the fold, so a chain that mixes the idiom
>        with unrelated addends still folds.
>
>   2/7 - 5/7  Add the remaining recognized variants: carry-low-sum,
>              two-carry, ladder, and low-plus.
>
>   6/7  match.pd, forwprop: Recognize long-multiply carries written as
>        2-arg PHI
>
>        Hand-written code often writes the carry as a 2-arg PHI
>        (`if (overflow) result +=3D pow2;`) with no top-level + at the
>        result. Adds cond_carry_add / cond_carry_add_neg recognizers
>        for that shape and a match_long_mul_phi entry that synthesizes
>        the carry summand from the PHI bindings and reuses the table
>        walk and emit path.
>
>   7/7  widening_mul: Lower long-multiply chains to inline longhand
>
>        Lowers the high-part chain to a longhand high-part at narrow
>        precision when the target has no expansion path for the 2N
>        form, using (N/2)-by-(N/2)->N widening multiplies where the
>        optab exists and plain N-bit multiplies otherwise. Operands
>        that are themselves wider than N are split into N-bit halves
>        rather than truncated, and a 2N product left with only low-half
>        uses is narrowed to an N-bit multiply rather than reaching
>        expansion. When an operand holds a product's high N bits, those
>        come from the product's own operands instead of the 2N shift.
>        Paired with pass_optimize_widening_mul so the chain is emitted
>        only when the pass will run to rescue an unsupported 2N shape.
>
> On SPEC2026's 750.sealcrypto_r:
>
>   - AArch64 Neoverse-N1: 25% improvement
>   - x86-64 Zen4:         59% improvement
>
> Compile-time impact is negligible: recompiling gcc/*.cc
> (checking=3Dyes,extra) with the series compiler versus its base adds
> about 0.1% overall, confined to forwprop, where the recognizer runs
> and grows the pass by a few percent.
>
> Bootstrapped/regtested on AArch64, x86-64, ARM and PowerPC.

This is OK.  Please squash the series before pushing, there seems to
be unused code in earlier parts of the series and possibly failing
testcases.

Thanks,
Richard.

> Changes in v8:
> - 1/7 starts matching only at chain ends and sets aside leaves that are
>   not long-multiply summands, re-applying them on top of the fold. The
>   two go together. Either alone regresses a foldable chain. Now folds
>   shapes such as `acc +=3D mulh (x, y)`.
> - 1/7 factors out build_mul_high_seq, long_mul_classify_match and
>   long_mul_classify_chain for the PHI entry in 6/7 to reuse, and takes
>   a gassign * in the matcher and the emitters.
> - The long_mul_high_chain atom binds each mult operand through
>   `(convert? @X)`, so a PRE-hoisted cast into a PHI still matches. An
>   operand wider than N is now split into N-bit halves rather than
>   rejected, which previously left the 2N multiply for expansion.
> - optimize_widening_mul_active_p returns false for optimize_debug: -Og
>   runs no widening_mul pass to lower the emitted chain, so the
>   unexpandable multiply reached expand.
> - Add narrow_long_mul_low_half: when a 2N `res =3D a * b` has uses only i=
n
>   its low N bits and the target cannot expand 2N, rewrite it to
>   `res =3D (2N) ((N)a * (N)b)`. The split-based lowering covers a chained
>   2N operand by recursing into it, but not a shared 2N product left with
>   only low-half uses, which is what ICEs libgo's p521_fiat64.go on ARM32.
> - long_mul_split_operand resolves a 2N value shifted down by N to the
>   high half of what was shifted, instead of truncating the shift. The
>   truncation read the shift and so kept a chained product live past its
>   own lowering, aborting expand_mult on a target without a 2N multiply.
>   Reachable from Go on ARM32 as bits.Mul64 (bits.Mul64 (x, y), z), where
>   the unexpandable mode is TImode, and on aarch64 and x86-64 from a
>   chained __int128 longhand, where it is OImode.
> - New coverage: near misses of the idiom, checked at runtime against
>   their literal meaning, a signed narrow-cast operand on Thumb-1, and
>   chained longhands through both halves on ARM32.
>
> Changes in v7:
> - Split the single long-multiply fold (was 1/2) into five patches: a
>   base patch carrying the framework and the carry form, then one
>   patch each for the carry-low-sum, two-carry, ladder and low-plus
>   variants.  Easier to review and to bisect a variant in isolation.
>   The PHI-form recognition follows as 6/7, unchanged from v6's 2/2.
> - New 7/7: lower the emitted high-part chain to inline longhand at
>   narrow precision when the target has no expansion path for the 2N
>   form.  v6 only emitted a HIGH_PART when the 2N scalar mode existed
>   and skipped it otherwise; v7 emits it and pairs the recognizer with
>   lower_long_mul_high_chain via optimize_widening_mul_active_p, so a
>   128x128 high part on a target whose mode table has OImode but no
>   scalar OImode support is now built from (N/2)-wide partial products
>   instead of a 2N multiply the target cannot expand.
> - Refuse the HIGH_PART emit and the chain lowering for BITINT_TYPE,
>   keeping the recognizer and the lowering gate symmetric.
> - Add per-variant tree-ssa tests, 128-bit torture and runtime tests,
>   arm thumb1 / umull inline tests, and a check_effective_target
>   _oi_mode helper.
>
> Changes in v6:
> - Reorder so the long-multiply fold (was 2/2) is now 1/2 and a new
>   PHI-form recognition pass is 2/2.  Reverting 2/2 leaves a working
>   long-multiply fold for the flat-shifted-compare carry form.
> - Drop v5's standalone flatten_cond_carry_add driver.  The same
>   cond_carry_add / cond_carry_add_neg match.pd recognizers now feed
>   a match_long_mul_phi entry inside the long-multiply fold, so a
>   PHI-shaped carry folds straight to the wide-multiply output.
> - Factor long_mul_classify_chain, long_mul_classify_match and
>   build_mul_high_seq for sharing between match_long_mul and the new
>   match_long_mul_phi.
> - cond_carry_add_neg uses le / ge instead of gt / lt to encode the
>   carry condition strictly.  v5 inverted the compare via
>   invert_tree_comparison in the flatten driver; v6 synthesises the
>   carry summand directly inside match_long_mul_phi and so requires
>   the recogniser to encode the strict form.
> - Delete forwprop-44/45/46.c; add PHI-form coverage in
>   long-mul-carry.c, long-mul-two-carry.c, long-mul-boundary.c
>   and long-mul-boundary-64.c.
> - Add PHI-form near-miss tests in long-mul-partial.c and
>   operand-swap polarity coverage in long-mul-boundary{,-64}.c.
> - Refresh stale long-mul comment references (check_hilo_and_ops,
>   fold_mul_low_plus) and reword mul_carry_low's :c-on-gt note to
>   the correct LT form (a + b < a).
>
> Changes in v5:
> - 1/2:
>   - Replace the match.pd simplify on COND_EXPR with cond_carry_add
>     / cond_carry_add_neg match recognizers (cond^), split by gcond
>     polarity, plus a flatten_cond_carry_add driver in
>     tree-ssa-forwprop.cc.  The driver inverts the gcond's
>     comparison for the _neg form.  Modelled on match_saturation_add.
>   - Remove fold_cond_carry_add_profitable_p and the tm_p.h /
>     predict.h includes from gimple-match-head.cc.  The width >
>     MAX_FIXED_MODE_SIZE and width % 2 !=3D 0 guards were
>     prerequisites for the can_mult_highpart_p fallback path, not
>     soundness checks.  type_has_mode_precision_p subsumes them.
>   - Retarget the test scans from phiopt2 to forwprop1.  Add
>     forwprop-46.c covering all four arm/comparison polarities.
>   - forwprop-45.c uses __UINT64_TYPE__ instead of unsigned long
>     and drops the lp64 restriction, covering the type > word_mode
>     regime on 32-bit targets.
> - 2/2:
>   - Lower the high-part as (N)(((2N) op1 * (2N) op2) >> N).
>     pass_optimize_widening_mul rewrites this to WIDEN_MULT_EXPR /
>     MULT_HIGHPART_EXPR on supporting targets.  Removes
>     can_mult_highpart_p queries from forwprop.
>   - Replace the can_mult_highpart_p prefilter in match_long_mul
>     with a targetm.scalar_mode_supported_p check on the 2N mode.
>     Test scans select on int128, mirroring the gate, instead of
>     lp64.
>   - Drop the m_long_mul_fold_p pass parameter and its passes.def
>     arguments.  The long-mul fold runs in every forwprop instance.
>     Test scans retargeted from forwprop2 to forwprop1.
>   - Stop restricting forwprop-44.c to lp64.  With the
>     can_mult_highpart_p gating gone, the fold is target-independent
>     and the test passes on ilp32 targets too.
>
> Changes in v4:
> - 1/2:
>   - Rebuild the guard with per-conjunct reasoning: require both
>     operands to be SSA names (drops degenerate one-side-constant
>     cases that fold trivially elsewhere), require the type to
>     have_mode_precision_p (excludes BITINT_TYPE precision !=3D mode
>     and similar oddities), drop the explicit MAX_FIXED_MODE_SIZE
>     width cap (subsumed by have_mode_precision_p), and gate on the
>     flat optab via can_mult_highpart_p of the 2N mode.
>   - Retain BRANCH_COST >=3D 2: keep the flatten conditional on a
>     target where the branchless form is generally cheaper.
>   - Rewrite the cover letter to describe the gate as the
>     composition of these conjuncts and to clarify that the
>     transformation now only ever introduces a (mul_hi-like)
>     can_mult_highpart_p shape, not a libgcc multi-precision call.
> - 2/2:
>   - Convert per-variant fold_mul_* functions into a
>     table-driven long_mul fold framework.
>   - Migrate each variant into a row in long_mul_table (six
>     HIGH_PART, six LOW_PART rows) keyed by (kind, extract).
>   - Add cross-summand consistency checks
>     (long_mul_check_consistency, long_mul_check_two_carries,
>     long_mul_check_low_plus_defer) shared across rows.
>   - Drop emission to a libgcc multi-precision call from RTL
>     expansion; defer to pass_optimize_widening_mul / RTL
>     expansion to pick native umul_highpart, a widening multiply, or
>     a synthesised sequence.  Emission is gated on
>     can_mult_highpart_p.
>   - Structural redesign: per-variant fold_mul_* functions
>     consolidated into a single linearise + classify + table-lookup
>     framework (long_mul_table, match_long_mul,
>     long_mul_classify_summand, long_mul_check_consistency).  Each
>     variant is now a row in long_mul_table; consistency checks are
>     shared across rows.
>   - Fast-fail prefilters in match_long_mul: LHS-type prefilter at
>     entry (no legitimate long-mul leaf has a signed / pointer /
>     float / odd-width type) and a can_mult_highpart_p probe before
>     the row loop to skip HIGH_PART rows on unsupported targets.
>   - Bound long_mul_linearize_chain mid-walk by LONG_MUL_MAX_SUMMANDS
>     so an overlong addition / BIT_IOR chain bails immediately rather
>     than after a full traversal.
>   - Emit a dump-file hint pointing at the shared inner addition when
>     long-mul folding rejects a chain because of a multi-used
>     intermediate (caching the partial sum into a single-use SSA
>     name normally enables the fold).
>
> Changes in v3:
> - Moved carry-diamond flattening from forwprop to match.pd,
> replacing ~460 lines of C++ with a 17-line match.pd pattern.
> - Two-carry test scans forwprop3 (the first forwprop after phiopt2,
> since early phiopt restricts which tree codes are allowed).
> - Set location for new sequences.
> - Updated mul_carry_low pattern.
> - Added the `mul_low_plus` pattern.
> - Fixed formatting issues.
>
> Changes in v2:
> - Fixed the testcases by separating the high part's fold count for
> 32-bit and 64-bit targets.
>
> Konstantinos Eleftheriou (7):
>   forwprop: Match and fold the long-multiply carry form [PR107090]
>   forwprop: Add long-multiply carry-low-sum variant
>   forwprop: Add long-multiply two-carry variant
>   forwprop: Add long-multiply ladder variants
>   forwprop: Add long-multiply low-plus variant
>   match.pd, forwprop: Recognize long-multiply carries written as 2-arg
>     PHI
>   widening_mul: Lower long-multiply chains to inline longhand
>
>  gcc/match.pd                                  |  198 +++
>  gcc/testsuite/gcc.dg/long-mul-128-Og.c        |   26 +
>  gcc/testsuite/gcc.dg/torture/long-mul-128.c   |  121 ++
>  .../gcc.dg/torture/long-mul-64-run.c          |  180 +++
>  .../gcc.dg/tree-ssa/long-mul-boundary-64.c    |  417 ++++++
>  .../gcc.dg/tree-ssa/long-mul-boundary.c       |  394 ++++++
>  .../gcc.dg/tree-ssa/long-mul-carry.c          |  385 ++++++
>  .../gcc.dg/tree-ssa/long-mul-chain-cse-128.c  |   52 +
>  .../tree-ssa/long-mul-chain-trunc-128.c       |   80 ++
>  .../gcc.dg/tree-ssa/long-mul-extra-addend.c   |   63 +
>  .../gcc.dg/tree-ssa/long-mul-ladder.c         |  333 +++++
>  .../gcc.dg/tree-ssa/long-mul-low-plus.c       |   54 +
>  .../gcc.dg/tree-ssa/long-mul-partial.c        |  193 +++
>  .../gcc.dg/tree-ssa/long-mul-two-carry.c      |  140 ++
>  gcc/testsuite/gcc.target/aarch64/long_mul.c   |  100 ++
>  .../gcc.target/arm/long-mul-thumb1-inline.c   |   47 +
>  gcc/testsuite/gcc.target/arm/long-mul-umull.c |   73 +
>  gcc/testsuite/gcc.target/i386/long_mul.c      |  100 ++
>  .../gcc.target/i386/widen_mult_high_chain.c   |   32 +
>  gcc/testsuite/lib/target-supports.exp         |   20 +
>  gcc/tree-ssa-forwprop.cc                      | 1197 ++++++++++++++++-
>  gcc/tree-ssa-math-opts.cc                     |  492 ++++++-
>  gcc/tree-ssa-math-opts.h                      |    2 +
>  23 files changed, 4690 insertions(+), 9 deletions(-)
>  create mode 100644 gcc/testsuite/gcc.dg/long-mul-128-Og.c
>  create mode 100644 gcc/testsuite/gcc.dg/torture/long-mul-128.c
>  create mode 100644 gcc/testsuite/gcc.dg/torture/long-mul-64-run.c
>  create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/long-mul-boundary-64.c
>  create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/long-mul-boundary.c
>  create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/long-mul-carry.c
>  create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/long-mul-chain-cse-128.=
c
>  create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/long-mul-chain-trunc-12=
8.c
>  create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/long-mul-extra-addend.c
>  create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/long-mul-ladder.c
>  create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/long-mul-low-plus.c
>  create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/long-mul-partial.c
>  create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/long-mul-two-carry.c
>  create mode 100644 gcc/testsuite/gcc.target/aarch64/long_mul.c
>  create mode 100644 gcc/testsuite/gcc.target/arm/long-mul-thumb1-inline.c
>  create mode 100644 gcc/testsuite/gcc.target/arm/long-mul-umull.c
>  create mode 100644 gcc/testsuite/gcc.target/i386/long_mul.c
>  create mode 100644 gcc/testsuite/gcc.target/i386/widen_mult_high_chain.c
>
> --
> 2.55.0