On Tue, Aug 18, 2026 at 3:12 PM Konstantinos Eleftheriou <[email protected]> wrote: > > Hi Richard, > > Do we need to squash the series before merging?
If the series steps build OK, no. Andrew seems to prefer not squashing. Richard. > Thanks, > Konstantinos > > On Mon, Aug 10, 2026 at 2:19 PM Richard Biener <[email protected]> > wrote: >> >> On Fri, Aug 7, 2026 at 8:52 AM 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 = 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 += 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. >> > An operand PHI is split through its arguments. A closing sweep >> > rewrites any 2N product that no lowering claimed into the N-bit >> > halves its uses read. 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=yes,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, i686, ARM and PowerPC. >> >> Stil LGTM. >> >> Thanks, >> Richard. >> >> > Changes in v9: >> > - New narrow_long_mul_halves: a sweep after the widening_mul walk that >> > rewrites a 2N multiply no lowering claimed into the N-bit halves its >> > uses read. Fixes the ICE building libbid on i686 (PR126642). >> > - New long_mul_split_phi: split an operand PHI through its arguments. >> > A PHI merging a zero-extended with a sign-extended value fits N bits >> > on neither arm, so the split refused it and the multiply reached >> > expand again. Fixes the rest of PR126642, bid128_sqrt.c on i686. >> > - Add testcases for the two ICE fixes. >> > - Drop long_mul_summand's mask field from 1/7. The mul_lo atom only >> > matches the half-width mask, so nothing ever read it back. Its >> > parameter and the tree_to_uhwi that fed it go with it. >> > - Fix the formatting of the `with` blocks in the match.pd >> > long-multiply atoms. >> > >> > 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 += 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 = a * b` has uses only in >> > its low N bits and the target cannot expand 2N, rewrite it to >> > `res = (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 != 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 != 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 >= 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 | 193 +++ >> > 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.target/i386/long-mul-phi-split.c | 50 + >> > .../gcc.target/i386/long-mul-sweep.c | 30 + >> > 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 | 1193 ++++++++++++++++- >> > gcc/tree-ssa-math-opts.cc | 714 +++++++++- >> > gcc/tree-ssa-math-opts.h | 2 + >> > 25 files changed, 4983 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-128.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-phi-split.c >> > create mode 100644 gcc/testsuite/gcc.target/i386/long-mul-sweep.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
