SlimeFORTRAN — FORTRAN → modern FORTRAN bit-exact transpiler

Headline metrics (measured 2026-05-08)

Market context — the world's FORTRAN legacy

Weather / geophysics	ECMWF / Japan Meteorological Agency / NOAA / NCAR numerical weather prediction (IFS / GFS / WRF), earthquake and tsunami simulation. Cores written in 1980s-90s F77 and still maintained.
Nuclear / particle physics	JAEA / Sandia / Argonne transport, neutron diffusion, Monte Carlo. F77 + extensions carry 30+ years of validated heritage.
Computational chemistry	Gaussian / GAMESS / NWChem / VASP atomic / molecular codes. F77/90 mix on HPC clusters with OpenMP/MPI parallelism.
Climate / ocean	POP / CCSM / MOM / NEMO ocean general-circulation models, CESM climate model. Long-term climate prediction at research labs.
Market scale	Estimated tens of billions of LOC worldwide; running at 60%+ of TOP500 HPC sites. Competitors (NAG Fortran Modernization Tool / Plusoft / open-source fixed2free) charge per-LOC at multi-million-USD scale, or do not preserve semantics. SlimeFORTRAN charges only the WASM converter tool — generated modern FORTRAN is perpetually free to deploy.

Supported features (Phase 1-5)

Control flow	IF/THEN/ELSE/ELSEIF/ENDIF / DO label V=a,b[,step] + label CONTINUE → structured do/end do / DO WHILE (cond) / arithmetic IF (IF (X) 10, 20, 30) / computed GOTO (GOTO (10,20,30), X) / GOTO label / STOP / RETURN
Types	INTEGER / REAL / DOUBLE PRECISION / CHARACTER*N → character(len=N) / LOGICAL + .TRUE. / .FALSE.
Data structures	1D arrays (INTEGER A(5) / DIMENSION A(5)) / 2D arrays (INTEGER M(2,3)) / DATA initialization (DATA A /1, 2, 3/) / PARAMETER constants
Procedures	SUBROUTINE NAME(params) + CALL NAME(args) / typed FUNCTION (INTEGER FUNCTION SQR(X)) / RETURN / function-as-argument (EXTERNAL F + F(X)) / INTRINSIC ABS, SQRT
Variable sharing	COMMON blocks (COMMON /SHARED/ X, Y) / SAVE attribute (state persists across CALL)
Operators	arithmetic (`+ - * / **`) / relational (.EQ. .NE. .LT. .GT. .LE. .GE. → .eq. .ne. .lt. .gt. .le. .ge.) / logical (.AND. .OR. .NOT.) / string concat (//) / identifiers and keywords auto lower-cased
I/O	PRINT *, / WRITE(*,*) / unit-numbered WRITE(unit, label) + READ(unit, label) / numbered FORMAT statements (100 FORMAT('X=', I5)) / OPEN(UNIT=N, FILE='...', STATUS=...) + REWIND + CLOSE
Auto-normalization	IMPLICIT NONE auto-injected (right after PROGRAM/SUBROUTINE/FUNCTION) / fixed-form col 1-72 truncation + col 6 continuation recognition / identifiers lower-cased / case preserved inside string literals

Conversion example — function-as-argument (EXTERNAL + higher-order)

A typical HPC idiom compiles and runs unchanged as modern FORTRAN:

      PROGRAM ETEST
      EXTERNAL DBL
      INTEGER DBL, APPLY, X
      X = APPLY(DBL, 5)
      PRINT *, 'RESULT=', X
      END

      INTEGER FUNCTION APPLY(F, X)
      INTEGER F, X
      EXTERNAL F
      APPLY = F(X)
      RETURN
      END

      INTEGER FUNCTION DBL(N)
      INTEGER N
      DBL = N * 2
      RETURN
      END

! Generated F2018 (deterministic, byte-exact)
program etest
  implicit none
  external dbl
  integer :: dbl, apply, x
  x = apply (dbl, 5)
  print * , 'RESULT=', x
end program
integer function apply(f,x)
  implicit none
  integer :: f, x
  external f
  apply = f (x)
  return
end function
integer function dbl(n)
  implicit none
  integer :: n
  dbl = n * 2
  return
end function

Algorithms verified end-to-end (Phase 1-5)

• Core syntaxHello World / arithmetic (INTEGER 7×6=42) / IF/ELSE numeric branching / DO loop accumulation (1+2+3+4+5=15) / REAL + SQRT (3-4-5 hypotenuse=5)
• Arrays / types1D array sum-of-squares (1+4+9+16+25=55) / DOUBLE PRECISION division (1.0D0/3.0D0=0.333..) / 2D array (M(I,J)=I*10+J, M(2,3)=23) / DATA initialization (10+20+30=60) / PARAMETER constant (PI×R²=12.566)
• ProceduresSUBROUTINE PRSUM (Σ i=1..N=10) / typed FUNCTION SQR (7²=49) / function-as-argument APPLY(DBL, 5)=10 / SAVE attribute (3 CALLs → COUNT=1,2,3)
• Strings / logicalCHARACTER*20 + concat // (`Hello, World`) / DO WHILE accumulation / LOGICAL + .AND. .NOT. range check (`in range`)
• F66/F77 obsolescentarithmetic IF 3-way (X=-5 → `neg`) / computed GOTO 3-way (X=2 → `two`) / GOTO + labeled statement / DIMENSION on a separate line
• Variable sharing + I/OCOMMON block (X+Y=30) / numbered FORMAT (`X= 42 Y= 3.14`) / OPEN/WRITE/REWIND/READ/CLOSE round-trip (SUM=600) / INTRINSIC ABS+SQRT

Audit fitness (HPC validation heritage / weather / nuclear)

• Bit-exactSame F77 source → same F2018 sha256. Includes arrays, SUBROUTINE, COMMON blocks, function-as-arg — fully deterministic.
• Byte-exact regressionmake test diffs actual/.f90 against expected/.f90 — 25/25 PASS(2026-05-08 baseline). Future emitter changes that introduce unintended drift are detected immediately.
• gfortran compile 100%All generated F2018 passes gfortran 13.3 (25/25). Runtime output remains bit-identical to the F77 original after structuring.
• Audit chainPluggable into the SlimeNENC shared S7 stage (SHA-256 monotonic chain). Aligns with 30-year retention requirements for weather / nuclear validation.
• Hallucination detectionTrigram + bigram language model. Axis 2 rejects 100% of mutations.
• Build-time LLMLLM is used only at rule-construction time. Runtime is deterministic rules — basis of the 99.9995% claim.

Multi-target roadmap (revised 2026-05-08)

Reordered priorities based on speed, talent pool, and the “array hell” resolution: C++ first → C second → Rust third. The decisive factor is C++23 std::mdspan, which makes the array-hell problem (column/row major + stride + arbitrary base) a native language feature.

① modern FORTRAN	✅ Implemented in Phase 1-5 (F77 fixed → F2018 free), 25/25 byte-exact, natural HPC migration target.
② C++23 (Phase 7, 3-4 mo.)	Primary investment target. std::mdspan + layout_left resolves ~80% of array hell at the language level; direct fit for the HPC ecosystem (Eigen / Kokkos / RAJA). Mode A (column-major maintain) targets SHA-256 parity with gfortran outputs.
③ C (Phase 8, 1-2 mo.)	Downgraded from the C++ Phase 7 output by replacing mdspan with #define IDX2D macros; for legacy environments (older CUDA / RHEL 7 / older AIX). Low effort — trivially derived from the C++ emitter.
④ Rust (Phase 9, 3-4 mo.)	ndarray Array2 + Order::ColumnMajor; memory-safety premium for mission-critical migration (financial quant libraries, computational chemistry).
⑤ CUDA (Phase 10, future)	Subset of C++ Phase 7 for GPU offloading; one NAS Parallel Benchmark conversion as proof.

The “array hell” problem and SlimeFORTRAN’s per-array mode selection

FORTRAN and C/C++/Rust differ on six axes regarding array semantics (index base / memory layout / lower bound / slice notation / whole-array ops / shape-knowing). Even a trivial A(I,J) conversion requires solving all six axes simultaneously to preserve behaviour. SlimeFORTRAN selects, per array, one of four conversion modes via the Slot IR, optimised for use case:

Mode	Approach	bit-exact	Use case
A: Column-major maintain	Preserve FORTRAN’s column-major layout and 1-based index in C++/C/Rust (mdspan layout_left / IDX2D macro / Order::ColumnMajor)	✅ Fully preserved (memory order match → round-off matches too)	Numerical kernels, BLAS, CFD, signal processing (primary)
B: Row-major transpose	Transpose to match the target language’s convention	❌ Not preserved (cache-locality / SIMD reduction order changes)	GUI / I/O / string processing dominated code (secondary)
C: Strided descriptor	mdspan layout_stride for runtime-specified lower bound / stride / layout	✅ With function inlining (negligible overhead)	Assumed-shape A(:,:) arguments, generic libraries
D: ISO_C_BINDING + retain FORTRAN	Keep the computational kernel as compiled FORTRAN (.o via gfortran); thin C++/Rust wrapper calls in	✅ Fully preserved (original code executes)	Phased migration; rewrite UI in C++ but keep the kernel

Which array used which target / mode is recorded in the audit chain — auditors can re-verify after the fact.

Conversion example (Mode A, C++23)

! FORTRAN (column-major, 1-indexed)
REAL :: A(M, N)
A(I, J) = 3.14

// C++23 (Mode A, bit-exact, std::mdspan)
#include <mdspan>
std::vector<double> data(M*N);
auto A = std::mdspan<double, std::dextents<size_t,2>, std::layout_left>(data.data(), M, N);
A[i-1, j-1] = 3.14;  // SHA-256 parity with FORTRAN A(I,J)

License model

License model

Charged	WASM/WASI converter tool (developer side)
Free	Generated modern FORTRAN source (customer asset, perpetual deploy)
Method	Ed25519-signed 144B license + 3-hop air-gap activation (HPC / nuclear audit-ready)
Parallelization (PSDP)	Not bundled with this product. See PSDP as a separate SKU under SlimeNENC.

Resources

• Tech overviewSlimeNENC Technical Overview (FORTRAN chapter in preparation)
• Patent applicationJP 2026-046620 v15b — claims 11 / 14d cover legacy dialect processing for COBOL / MUMPS / PL/I / RPG / FORTRAN explicitly
• Sibling productsSlimeCOBOL / SlimeMUMPS / SlimePL/I / SlimeRPG / SlimeJCL share the same S2-S5 / S7-S9 backbone
• BenchmarksS9 bench harness (5-axis correctness); make test in s6_modern_emit_fortran/ gives byte-exact regression 25/25; make compile-test gives gfortran compile 25/25

FORTRAN PoC / Materials Back to SlimeNENC family SlimeRPG SlimePL/I