Reverted. On Tue, 4 Aug 2026 at 18:58, Andrea Pinski <[email protected]> wrote:
> On Mon, Aug 3, 2026 at 8:57 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: > > Two things, this breaks bootstrap on x86_64-linux-gnu. > I filed https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126642 . > Can we revert this until that is fixed? > > Second please don't squash the patches into one commit next time. If > you do please send a full updated patch to the list of what you > committed. This has been GCC's policy. > > Thanks, > Andrea > > > > > > 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. > > 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, ARM and PowerPC. > > > > 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 | 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-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.c > > create mode 100644 gcc/testsuite/gcc.target/i386/widen_mult_high_chain.c > > > > -- > > 2.55.0 >
