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 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 series is split into two patches:
1/2 forwprop: Match and fold long-multiply patterns [PR107090]
Adds match.pd atom patterns plus a forwprop framework that
linearises the outer add/ior chain, classifies summands, and
matches the multiset against a table of six decomposed
variants:
- carry: single overflow comparison on the cross-sum
- carry-long: cross-carry with separate high/low accumulation
- two-carry: both cross-carry and low-carry as separate
comparisons
- ladder: sequential accumulation without explicit carry
comparison
- ladder-long: ladder with separate high/low accumulation
- low-plus: low part as a direct sum of partial products
Emits the canonical widening shape
(N)(((2N) a * (2N) b) >> N)
for high parts. pass_optimize_widening_mul later lowers this
to WIDEN_MULT_EXPR / MULT_HIGHPART_EXPR on supporting targets.
Skipped when the target lacks the 2N scalar mode.
2/2 match.pd, forwprop: Recognise long-multiply carries written as
2-arg PHI
The 1/2 fold's entry walks a top-level + statement. Hand-
written long-multiply code often writes the carry as a 2-arg
PHI (`if (overflow) result += pow2;`) instead, with no
top-level + at the result. 2/2 adds cond_carry_add and
cond_carry_add_neg match.pd recognizers for that PHI shape
and a match_long_mul_phi entry that linearises the no-carry
arm, synthesises the carry summand from the PHI bindings,
and reuses 1/2's table walk and emit path.
On SPEC2026's 750.sealcrypto_r:
- AArch64 Neoverse-N1: 25% improvement
- x86-64 Zen4: 59% improvement
Compile-time impact: single-threaded recompile of gcc/*.cc with each
branch's stage3 cc1plus (checking=yes,extra) takes 87:05 on master vs
87:55 on this series -- ~1% overhead.
Bootstrapped/regtested on AArch64, x86-64 and PowerPC.
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 (2):
forwprop: Match and fold long-multiply patterns [PR107090]
match.pd, forwprop: Recognise long-multiply carries written as 2-arg
PHI
gcc/match.pd | 177 +++
.../gcc.dg/tree-ssa/long-mul-boundary-64.c | 398 ++++++
.../gcc.dg/tree-ssa/long-mul-boundary.c | 394 ++++++
.../gcc.dg/tree-ssa/long-mul-carry.c | 373 ++++++
.../gcc.dg/tree-ssa/long-mul-ladder.c | 329 +++++
.../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 | 142 +++
gcc/testsuite/gcc.target/aarch64/long_mul.c | 100 ++
gcc/testsuite/gcc.target/i386/long_mul.c | 100 ++
gcc/tree-ssa-forwprop.cc | 1066 ++++++++++++++++-
11 files changed, 3318 insertions(+), 8 deletions(-)
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-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/i386/long_mul.c
--
2.52.0
