M2compiler-V3/docs/comparison-PIM4-gm2-V3.md

PIM4 vs GNU Modula-2 vs M2R10 vs TopSpeed V3 vs V3 — feature comparison

Date: 2026-10-23 · V3 state: v3-smallfix (gates run 2026-10-22: suite 259/259, self-hosting fixpoint OK 4,028,447 bytes, corpus 511/612, 0 crashes).

Scope and sources.

• PIM4 — Wirth's Programming in Modula-2, 4th edition (the language specification, not a compiler).
• GNU Modula-2 (gm2) — the GCC front end: PIM and ISO modes plus the extensions common in its testsuite.
• M2R10 — the Kowarsch/Sutcliffe Modula-2 R10 revision (2012). It is a deliberate redesign, not source-compatible with classic Modula-2. Cells follow the published R10 grammar and the document “Differences between Modula-2 R10 and classic Modula-2”. Some features are library-level or outside the base grammar; those are marked as such. A cell saying “not in the R10 grammar” means the classic keyword/operator is not in the base grammar, not necessarily that no library equivalent exists.
• TopSpeed V3 — the JPI/TopSpeed (Clarion) V3 dialect as reconstructed in compiler/src/TopSpeed-V3-M2.atg and docs/OOP.txt. This is a historical compiler dialect, not a standard; cells describe that grammar/dialect where it is documented.
• V3 — this compiler: a PIM/ISO subset plus the gm2 compatibility it deliberately chose.

Legend: ✅ supported / standard · 🔄 partial or caveated · ❌ absent · ⚠️ non-standard extension · n/a not applicable.

The tables describe common/default behaviour; exact flags and modes can change individual cells.

Standards & dialect

Feature	PIM4	GNU Modula-2	M2R10	TopSpeed V3	V3
PIM base language	✅ (definition)	✅ (PIM 2/3/4 modes)	🔄 modern revision, not source-compatible	✅ PIM-based dialect	✅ (PIM/ISO subset)
ISO 10514-1 base	❌ (predates ISO)	✅	❌ (independent R10 spec)	❌	🔄 mostly; see missing ISO rows
ISO 10514-2 generics	❌	✅	❌ (external template engine, not ISO generics)	❌ (not in the V3 dialect grammar)	✅ (refinement form)
ISO 10514-3 OO	❌	❌	❌	❌ (Clarion classes, not ISO OO)	❌ (CLASS is Clarion-form)
Declaration before use	✅ strict	🔄 multi-pass, accepts forward refs	✅ strict	✅ strict (single-pass)	✅ strict for data; 🔄 forward procedure calls
FORWARD declarations	✅	✅	❌ (not in the R10 grammar)	✅	✅
EXPORT QUALIFIED / UNQUALIFIED	❌ (PIM export list only)	✅	❌ (EXPORT removed; import qualifiers/re-export)	✅ (EXPORT [QUALIFIED])	✅
Foreign modules (DEFINITION MODULE FOR "C")	❌	✅	⚠️ FFI via pragma, not FOR "C"	❌ (not in the reconstructed grammar)	✅

Generics (explicit)

Dialect	Language-level generics
PIM4	❌ no generic modules
GNU Modula-2	✅ ISO 10514-2 generic modules (type/const/procedure parameters)
M2R10	❌ not in the language; R10 uses an external template engine (GENLIB/BLUEPRINT, ##…## placeholders) expanded before compilation
TopSpeed V3	❌ no GENERIC construct in the reconstructed V3 grammar
V3	✅ ISO 10514-2 refinement modules: type/value/procedure parameters, constraints, nested generics, generic classes (docs/analysis_generics.md)

Lexical

Feature	PIM4	GNU Modula-2	M2R10	TopSpeed V3	V3
Identifiers with _	❌ (classic names)	✅	✅	✅ (_ and @)	✅
Numeric bases	✅ octal B, hex H	✅ B/H (and more)	✅ C-style 0b/0x, U char-code suffix (octal removed)	✅ H/C/B suffixes + 0x	❌ (0x/0X only)
Nested (* *) comments	✅	✅	✅ (max 10 levels)	✅	✅
'…' and "…" strings	✅	✅	✅ + C-style escapes	✅	✅
Unicode	❌	❌	✅ UNICHAR	❌	✅ UCHAR/UString (extension)

Types

Feature	PIM4	GNU Modula-2	M2R10	TopSpeed V3	V3
Basic integer/real/bool/char types	✅	✅	✅ (+ OCTET, LONGCARD)	✅ PIM types	✅
Subranges	✅	✅	✅ ([lo..hi] OF Base)	✅	✅ (typed CHAR/enum subranges)
Enumerations	✅	✅	✅ extensible	✅	✅
Sets (SET OF)	✅ any finite ordinal (impl. limit)	✅	🔄 SET OF <enumType> only	✅ range/base/enum bases	🔄 bool/char/byte/16-bit/enum/small-int; span ≤ 65536; no SET OF CARDINAL
PACKEDSET	❌ (ISO)	✅	❌	❌	❌
Fixed / multi-dimensional arrays	✅ [a], [b] OF	✅	✅ ARRAY count, count OF (zero-based)	✅	✅ both [a], [b] and [a, b]
Open arrays (1-D)	✅	✅	✅ (BARE ARRAY OF)	✅	✅
Multi-dimensional open arrays	❌ (ISO addition)	✅	🔄 one-dimensional BARE arrays	❌	❌ (descriptor-per-level representation)
Records	✅	✅	✅ extensible	✅	✅
Variant records	✅	✅	❌ removed (extensible records replace them)	✅	✅ (plus empty arms/ELSE)
Pointers, NIL, opaque types	✅	✅	✅ (OPAQUE keyword)	✅	✅
Procedure types	✅ (PROCEDURE)	✅ (PROCEDURE, PROC)	✅ (comma-separated formals)	✅	✅ (PROCEDURE, PROC; comma/unnamed forms)
Complex types (COMPLEX, LONGCOMPLEX)	❌	✅ (ISO)	❌ (not in the base R10 grammar)	❌	❌ (COMPLEX64 is a REAL placeholder)
SYSTEM sized types (WORD32, CARDINAL8, BITSET32, …)	❌	✅	🔄 OCTET (8-bit), LONGCARD, LONGBITSET	🔄 SHORTINT/LONGINT/SHORTCARD/LONGCARD family	✅ 8/16/32 names; WORD/WORD32 32-bit
BYTE granularity	✅ 8-bit	✅ 8-bit	✅ OCTET 8-bit	✅ (PIM BYTE)	✅ BYTE/SBYTE/WORD8/CARDINAL8/INTEGER8 are 8-bit; ARRAY OF BYTE is a real buffer
SHORTREAL (32-bit)	❌	✅	❌ (REAL/LONGREAL)	⚠️ dialect-dependent	❌ (all reals are 8 bytes)
ADDRESS arithmetic (ADDADR/SUBADR/DIFADR)	❌	✅	❌	❌	❌
Absolute variables VAR [addr]: T	❌	✅ (extension)	✅ (pragma ADDR)	❌	❌
Set ordering (<, <=, >, >=)	❌ (=/#/IN only)	✅ (extension)	❌	❌	❌

Statements & procedures

Feature	PIM4	GNU Modula-2	M2R10	TopSpeed V3	V3
IF/CASE/WHILE/REPEAT/LOOP/EXIT/WITH, RETURN, HALT	✅	✅	🔄 no WITH/GOTO; LOOP/EXIT, FOR … IN	✅ + IN/IS, <</>>	✅
FOR over any ordinal (CHAR/enum)	✅	✅	🔄 FOR … IN over ranges/iterables (loop variable declared in the header)	✅ classic FOR … TO	✅ (since 2026-10-22)
GOTO / labels	✅	✅	❌ (not in the R10 grammar)	✅	✅
INC/DEC on enumerations	❌ (integer)	✅	❌ replaced by SUCC/PRED	🔄 dialect-dependent	✅ (any ordinal)
Empty string ""/'' as CHAR	❌/unclear (ISO rule)	✅	🔄 (char literals compatible with ARRAY OF CHAR; empty-to-CHAR not explicit)	✅	✅ (ISO rule)
LENGTH	❌ (ISO adds it)	✅	🔄 library/pervasive, not in the base grammar	❌	✅
HIGH	✅	✅	❌ (not in the R10 grammar)	✅	✅ (including string CONST)
TSIZE/SIZE/ADR	✅	✅	🔄 SYSTEM/library forms	✅	✅
SHIFT/ROTATE	⚠️ implementation SYSTEM extension	✅	❌	✅ (<</>> operators)	✅
TBITSIZE	❌	✅	❌	❌	❌
ISO exceptions (EXCEPT, RETRY, EXCEPTIONS)	❌	✅	❌ (not in the base R10 grammar)	❌	❌
Module finalization (FINALLY)	❌	✅	❌	❌	❌
__FILE__ / __LINE__ / __FUNCTION__	❌	✅	🔄 COMPILER pseudo-module introspection	❌	✅ / ✅ / ✅
__BUILTIN__ / __ATTRIBUTE__ pragmas	❌	✅	✅ language pragmas <* … *>	⚠️ compiler directives	✅ (parsed, ignored)
Inline ASM	❌	✅	⚠️ optional ASSEMBLER pseudo-module	❌ (not in the reconstructed grammar)	❌

Modules, compilation, libraries

Feature	PIM4	GNU Modula-2	M2R10	TopSpeed V3	V3
Definition / implementation / program modules	✅	✅	✅ (+ blueprints)	✅	✅
Local modules	✅	✅	❌ removed	✅	✅
Separate compilation	✅	✅ (object files/archives)	✅ compilation units	✅	🔄 one invocation compiles all units into one image
Generics	❌	✅ ISO 10514-2	❌ external templates (GENLIB/BLUEPRINT)	❌	✅ ISO 10514-2 refinement
Object orientation	❌	❌	❌ (extensible records only)	✅ Clarion-form CLASS, VIRTUAL, inheritance	🔄 Clarion-form CLASS (single inheritance, virtual dispatch), not ISO 10514-3
PIM coroutines (PROCESS, NEWPROCESS, TRANSFER, IOTRANSFER)	✅	✅	🔄 COROUTINE type/YIELD (R10 phase II)	✅ PIM-style	✅ (IOTRANSFER degrades to a transfer)
ISO COROUTINES module	❌	✅	❌	❌	❌ (PIM interface only)
Standard library	❌ (none defined)	✅ PIM + ISO libraries	✅ redesigned R10 library	✅ TopSpeed libraries (historical)	🔄 subset: ISO/PIM I/O, strings, storage, SysClock; no ISO exceptions/termination libs
Self-hosting	n/a	✅ (part of GCC)	n/a (reference compiler under development)	n/a (historical commercial compiler)	✅ (byte-identical 3-stage fixpoint)
Backend / target	n/a	GCC (many targets)	reference compiler	x86/DOS/Windows (historical)	QBE → C (amd64 SysV today)

The practical differences

• PIM4 is the clean specification: strict declaration order, no ISO exceptions/complex numbers/PACKEDSET/generics, no standard library, no sized SYSTEM types or address arithmetic.
• GNU Modula-2 is the superset in practice: PIM and ISO modes, generics, exceptions, complex numbers, PACKEDSET, multi-dimensional open arrays, sized SYSTEM types, address arithmetic, absolute variables, ASM, pragmas, and many extensions — but deliberately permissive (multi-pass resolution accepts forward references).
• M2R10 is a redesign, not an extension: no variant records, no local modules, no WITH, no EXPORT, no GOTO, no FORWARD; it adds OCTET, UNICHAR, LONGBITSET, ALIAS OF, OPAQUE, extensible records/enumerations, FOR … IN, SUCC/PRED, CAST, structured literals, pragmas and a redesigned library. Its “generics” are an external template engine, not ISO generic modules.
• TopSpeed V3 is a PIM-based dialect with OO: Clarion-form CLASS/VIRTUAL and inheritance are the main additions; it has its own lexical forms (_/@, H/C/B literals, <</>>, WITH/IN/IS). It has no language-level generics or ISO exceptions.
• V3 is a PIM/ISO compiler with the opposite bias to gm2: it rejects gm2 leniencies (declaration order), implements the common ISO/PIM constructs (ISO 10514-2 generics, coroutines, foreign modules, ISO strings/SYSTEM/procedure types, empty string as CHAR), and documents its chosen gm2 compatibility (__FILE__/__LINE__/__FUNCTION__, scalar→ARRAY OF BYTE, same-size VAR, bare proc values, variant extensions, set CASE labels, libc prototypes). Its hard gaps are the large ISO features (exceptions, complex arithmetic, PACKEDSET, finalization/protection, multi-dimensional open arrays, ISO OO) and parts of the word model (SHORTREAL, REAL width), plus a 65536-bit set span limit.

V3-specific deviations and limits

• Strict data declaration order (PIM-conforming); forward procedure calls are allowed where strict PIM would require FORWARD.
• Word model: BYTE/SBYTE/WORD8/CARDINAL8/INTEGER8 are true 8-bit types (SIZE 1), WORD16/CARDINAL16/INTEGER16 are true 16-bit types (SIZE 2); INTEGER/CARDINAL/WORD/WORD32 are 32-bit; LONGINT/LONGCARD are 64-bit; every REAL/SHORTREAL is 8 bytes; COMPLEX* is a placeholder.
• Sets: bases limited to bool/char/byte/16-bit/enum/small subranges; total span ≤ 65536; no SET OF CARDINAL.
• No runtime range checks on subrange assignment (V3 is already lenient for the integer family).
• Multi-dimensional open arrays are not supported: V3 represents an array level as its own descriptor, whereas gm2 passes a flat block — accepting the corpus files would compile but miscompile.
• Single-invocation compilation: definition, implementation and program units are compiled together into one image; there is no incremental object/archive format.
• OO is the Clarion-form CLASS dialect (single inheritance, virtual dispatch), not ISO 10514-3.

Sources

• V3: docs/STATUS.md, docs/features.md, docs/gaps/README.md, docs/analysis_gaps.md, docs/summary_session-2026-10-22.md; the gates in this document were run at v3-smallfix.
• gm2: the GCC Modula-2 testsuite and gm2-libs/gm2-libs-iso definition modules in the sibling gcc-git tree.
• M2R10: the published R10 grammar (m2r10-grammar.gll) and “Differences between Modula-2 R10 and classic Modula-2” (Kowarsch/Sutcliffe, 2012).
• TopSpeed V3: compiler/src/TopSpeed-V3-M2.atg (reconstructed V3 grammar) and docs/OOP.txt (Clarion class model).
• PIM4: N. Wirth, Programming in Modula-2, 4th edition.