We have sent a new version with some testcase fixes for 32-bit targets: https://gcc.gnu.org/pipermail/gcc-patches/2026-March/711112.html.
Thanks, Konstantinos On Thu, Mar 19, 2026 at 12:13 PM Konstantinos Eleftheriou < [email protected]> wrote: > > This patch series teaches forwprop to recognize longhand 64x64->128 > wide-multiplication idioms and replace them with native multiply > instructions (MULT_HIGHPART_EXPR / widening multiply for the high part, > 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: Flatten carry-diamond patterns to straight-line code > > A carry diamond implements unsigned overflow detection with > conditional carry propagation: > > sum = a + b; > if (addend > sum) result = base + C; > > This patch converts such conditional branches into branchless > straight-line code, exposing the carry as a comparison > expression. This is a prerequisite for the long-multiply > pattern matching, which expects carries in the form > (lshift (convert? (gt ...)) INTEGER_CST). > > 2/2 forwprop: Match and fold long-multiply patterns [PR107090] > > Adds match.pd patterns that recognize six decomposed variants > of the longhand multiplication: > > - 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 > > The actual folding is performed in forwprop after verifying > target support for umul_highpart or widening multiply. > > On SPEC2026's 750.sealcrypto_r: > > - AArch64 Neoverse-N1: 25% improvement > - x86-64 Zen4: 59% improvement > > > Konstantinos Eleftheriou (2): > forwprop: Flatten carry-diamond patterns to straight-line code > forwprop: Match and fold long-multiply patterns [PR107090] > > gcc/match.pd | 221 +++ > gcc/testsuite/gcc.dg/tree-ssa/forwprop-44.c | 21 + > gcc/testsuite/gcc.dg/tree-ssa/forwprop-45.c | 44 + > .../gcc.dg/tree-ssa/long-mul-boundary-64.c | 274 ++++ > .../gcc.dg/tree-ssa/long-mul-boundary.c | 270 ++++ > .../gcc.dg/tree-ssa/long-mul-carry.c | 310 ++++ > .../gcc.dg/tree-ssa/long-mul-ladder.c | 328 ++++ > .../gcc.dg/tree-ssa/long-mul-low-plus.c | 54 + > .../gcc.dg/tree-ssa/long-mul-partial.c | 119 ++ > .../gcc.dg/tree-ssa/long-mul-two-carry.c | 109 ++ > gcc/testsuite/gcc.target/aarch64/long_mul.c | 100 ++ > gcc/testsuite/gcc.target/i386/long_mul.c | 100 ++ > gcc/tree-ssa-forwprop.cc | 1367 ++++++++++++++++- > 13 files changed, 3312 insertions(+), 5 deletions(-) > create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/forwprop-44.c > create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/forwprop-45.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-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 > >
