llvm-project/flang/documentation/Calls.md

Procedure reference implementation protocol

Fortran function and subroutine references are complicated. This document attempts to collect the requirements imposed by the 2018 standard (and legacy extensions) on programs and implementations, work through the implications of the various features, and propose both a runtime model and a compiler design.

All section, requirement, and constraint numbers herein pertain to the Fortran 2018 standard.

This note does not consider calls to intrinsic procedures, statement functions, or calls to internal runtime support library routines.

Quick review of terminology

• A dummy argument is a function or subroutine parameter. It is associated with an effective argument at each call to the procedure.
• The shape of an array is a vector containing its extent (size) on each dimension; the rank of an array is the number of its dimensions (i.e., the shape of its shape). The absolute values of the lower and upper bounds of the dimensions of an array are not part of its shape, just their difference (plus 1).
• An explicit-shape array has all of its bounds specified; lower bounds default to 1. These can be passed by with a single address and their contents are contiguous.
• An assumed-size array is an explicit-shape array with * as its final dimension, which is the most-significant one in Fortran and whose value does not affect indexed address calculations.
• A deferred-shape array (DIMENSION::A(:)) is a POINTER or ALLOCATABLE. POINTER target data might not be contiguous.
• An assumed-shape (not size!) array (DIMENSION::A(:)) is a dummy argument that is neither POINTER nor ALLOCATABLE; its lower bounds can be set by the procedure that receives them (defaulting to 1), and its upper bounds are functions of the lower bounds and the extents of dimensions in the shape of the effective argument.
• An assumed-length CHARACTER(*) dummy argument takes its length from the effective argument.
• An assumed-length CHARACTER(*) result of an external function (C721) has its length determined by its eventual declaration in a calling scope.
• An assumed-rank DIMENSION::A(..) dummy argument array has an unknown number of dimensions.
• A polymorphic CLASS(t) dummy argument, ALLOCATABLE, or POINTER has a specific derived type or some extension of that type. An unlimited polymorphic CLASS(*) object can have any intrinsic or derived type.
• Interoperable BIND(C) procedures are written in C or callable from C.

Interfaces

Referenced procedures may or may not have declared interfaces available to their call sites.

Procedures with some post-Fortran '77 features require an explicit interface to be called (15.4.2.2) or even passed (4.3.4(5)):

• use of argument keywords in a call
• procedures that are ELEMENTAL or BIND(C)
• procedures that are required to be PURE due to the context of the call (specification expression, DO CONCURRENT, FORALL)
• dummy arguments with these attributes: ALLOCATABLE, POINTER, VALUE, TARGET, OPTIONAL, ASYNCHRONOUS, VOLATILE, and, as a consequence of limitations on its use, CONTIGUOUS; INTENT(), however, does not require an explicit interface
• dummy arguments that are coarrays
• dummy arguments that are assumed-shape or assumed-rank arrays
• dummy arguments with parameterized derived types
• dummy arguments that are polymorphic
• function result that is an array
• function result that is ALLOCATABLE or POINTER
• CHARACTER function result whose length is neither constant nor assumed
• derived type function result with LEN type parameter value that is not constant (note that result derived type parameters cannot be assumed (C795))

Module procedures, internal procedures, procedure pointers, type-bound procedures, and recursive references by a procedure to itself always have explicit interfaces. (Consequently, they cannot be assumed-length CHARACTER(*) functions; conveniently, assumed-length CHARACTER(*) functions are prohibited from recursion (15.6.2.1(3))).

Other uses of procedures besides calls may also require explicit interfaces, such as procedure pointer assignment, type-bound procedure bindings, &c.

Note that non-parameterized monomorphic derived type arguments do not by themselves require the use of an explicit interface. However, dummy arguments with any derived type parameters do require an explicit interface, even if they are all KIND type parameters.

15.5.2.9(2) explicitly allows an assumed-length CHARACTER(*) function to be passed as an actual argument to an explicit-length dummy; this has implications for calls to character-valued dummy functions and function pointers. (In the scopes that reference CHARACTER functions, they must have visible definitions with explicit result lengths.)

Implicit interfaces

In the absence of any characteristic or context that requires an explicit interface (see above), an external function or subroutine (R503) or ENTRY (R1541) can be called directly or indirectly via its implicit interface. Each of the arguments can be passed as a simple address, including dummy procedures. Procedures that can be called via an implicit interface can undergo more thorough checking by semantics when an explicit interface for them exists, but they must be compiled as if all calls to them were through the implicit interface. This note will mention special handling for procedures that are exposed to the possibility of being called with an implicit interface as F77ish procedures below; this is of course not standard terminology.

Internal and module subprograms that are ever passed as arguments &/or assigned as targets of procedure pointers may be F77ish.

Every F77ish procedure can and must be distiguished at compilation time. Such procedures should respect the external naming conventions (when external) and any legacy ABI used for Fortran '77 programs on the target architecture, so that portable libraries can be compiled and used by distinct implementations (and their versions) of Fortran.

Note that F77ish functions still have known result types, possibly by means of implicit typing of their names. They can also be CHARACTER(*) assumed-length character functions.

In other words: these F77sh procedures that do not require the use of an explicit interface and that can possibly be referenced, directly or indirectly, with implicit interfaces are limited to argument lists that comprise only the addresses of effective arguments and the length of a CHARACTER function result (when there is one), and they can return only scalar values with constant type parameter values. None of their arguments or results need be (or can be) implemented with descriptors, and any internal procedures passed to them as arguments must be simple addresses of non-internal subprograms or trampolines for internal procedures.

Note that the INTENT attribute does not, by itself, require the use of explicit interface; neither does the use of a dummy procedure (implicit or explicit in their interfaces). So the analyis of calls to F77ish procedures must allow for the invisible use of INTENT(OUT).

Protocol overview

Here is a summary script of all of the actions that may need to be taken by the calling procedure and its referenced procedure to effect the call, entry, exit, and return steps of the procedure reference protocol. The order of these steps is not particularly strict, and we have some design alternatives that are explored further below.

Before the call:

1. Compute &/or copy into temporary storage the values of some effective argument expressions and designators (see below).
2. Create and populate descriptors for arguments that use them (see below).
3. Possibly allocate function result storage, when its size can be known by all callers; function results that are neither POINTER nor ALLOCATABLE must have explicit shapes (C816).
4. Create and populate a descriptor for the function result, if it needs one (deferred-shape/-length POINTER, any ALLOCATABLE, derived type with non-constant length parameters, &c.).
5. Capture the values of host-escaping local objects in memory; package them into single address (for calls to internal procedures & for calls that pass internal procedures as arguments).
6. Resolve the target procedure's polymorphic binding, if any.
7. Marshal effective argument addresses (or values for %VAL() and some discretionary VALUE arguments) into registers.
8. Marshal CHARACTER argument lengths in additional value arguments for CHARACTER effective arguments not passed via descriptors. These lengths must be 64-bit integers.
9. Marshal an extra argument for the length of a CHARACTER function result if the function is F77ish.
10. Marshal an extra argument for the function result's descriptor, if it needs one.
11. Set the "host instance" (static link) register when calling an internal procedure from its host or another internal procedure, a procedure pointer, or dummy procedure (when it has a descriptor).
12. Jump.

On entry:

1. For subprograms with alternate ENTRY points: shuffle ENTRY dummy arguments set a compiler-generated variable to identify the alternate entry point, and jump to the common entry point for common processing and a switch() to the statement after the ENTRY.
2. Capture CHARACTER argument &/or assumed-length result length values.
3. Complete VALUE copying if this step will not always be done by the caller (as I think it should be).
4. Finalize &/or re-initialize INTENT(OUT) non-pointer effective arguments (see below).
5. For interoperable procedures called from C: compact discontiguous dummy argument values when necessary (CONTIGUOUS &/or explicit-shape/assumed-size arrays of assumed-length CHARACTER(*)).
6. Optionally compact assumed-shape arguments for contiguity on one or more leading dimensions to improve SIMD vectorization, if not TARGET and not already sufficiently contiguous. (PGI does this in the caller, whether the callee needs it or not.)
7. Complete allocation of function result storage, if that has not been done by the caller.
8. Initialize components of derived type local variables, including the function result.

Execute the callee, populating the function result or selecting the subroutine's alternate return.

On exit:

1. Clean up local scope (finalization, deallocation)
2. Deallocate VALUE argument temporaries. (But don't finalize them; see 7.5.6.3(3)).
3. Replace any assumed-shape argument data that were compacted on entry for contiguity when the data were possibly modified across the call (never when INTENT(IN) or VALUE).
4. Identify alternate RETURN to caller.
5. Marshal results.
6. Jump

On return to the caller:

1. Save the result registers, if any.
2. Copy effective argument array designator data that was copied into a temporary back into its original storage (see below).
3. Complete deallocation of effective argument temporaries (not VALUE).
4. Reload definable host-escaping local objects from memory, if they were saved to memory by the host before the call.
5. GO TO alternate return, if any.
6. Use the function result in an expression.
7. Eventually, finalize &/or deallocate the function result.

(I've omitted some obvious steps, like preserving/restoring callee-saved registers on entry/exit, dealing with caller-saved registers before/after calls, and architecture-dependent ABI requirements.)

The messy details

Copying effective argument values into temporary storage

There are several conditions that require the compiler to generate code that allocates and populates temporary storage for an actual argument.

First, effective arguments that are expressions, not designators, obviously need to be computed and captured into memory in order to be passed by reference. This includes parenthesized designators like (X), which are expressions in Fortran, as an important special case. (This case also technically includes unparenthesized constants, but those are better implemented by passing addresses in read-only memory.) The dummy argument cannot be known to have INTENT(OUT) or INTENT(IN OUT).

Small scalar or elemental VALUE arguments may be passed in registers, as should arguments wrapped in the legacy VMS %VAL() notation. Multiple elemental VALUE arguments might be packed into SIMD registers.

Effective arguments that are designators, not expressions, must also be copied into temporaries in the following situations.

1. Coindexed objects need to be copied into the local image. This can get very involved if they contain ALLOCATABLE components, which also need to be copied, along with their ALLOCATABLE components, and may be best implemented with a runtime library routine working off a description of the type.
2. Effective arguments associated with dummies with the VALUE attribute need to be copied; this can be done on either side of the call, but there are optimization opportunities available when the caller's side bears the responsibility.
3. In non-elemental calls, the values of array sections with vector-valued subscripts need to be gathered into temporaries. These effective arguments are not definable, and they are not allowed to be associated with non-VALUE dummy arguments with the attributes INTENT(OUT), INTENT(IN OUT), ASYNCHRONOUS, or VOLATILE (15.5.2.4(21)); INTENT() can't always be checked.
4. Non-simply-contiguous (9.5.4) arrays being passed to non-POINTER dummy arguments that must be contiguous (due to a CONTIGUOUS attribute, or not being assumed-shape or assumed-rank; this is always the case for F77ish procedures). This should be a runtime decision, so that effective arguments that turn out to be contiguous can be passed cheaply. This rule does not apply to coarray dummies, whose effective arguments are required to be simply contiguous when this rule would otherwise force the use of a temporary (15.5.2.8); neither does it apply to ASYNCHRONOUS and VOLATILE effective arguments, which are disallowed when copies would be necessary (C1538 - C1540). Only temporaries created by this contiguity requirement are candidates for being copied back to the original variable after the call (see below).

Fortran requires (18.3.6(5)) that calls to interoperable procedures with dummy argument arrays with contiguity requirements handle the compaction of discontiguous data in the Fortran callee, at least when called from C. And discontiguous data must be compacted on the caller's side when passed from Fortran to C (18.3.6(6)).

We could perform all argument compaction (discretionary or required) in the callee, but there are many cases where the compiler knows that the effective argument data are contiguous when compiling the caller (a temporary is needed for other reasons, or the effective argument is simply contiguous) and a run-time test for discontiguity in the callee can be avoided by using a caller-compaction convention when we have the freedom to choose.

While we are unlikely to want to needlessly use a temporary for an effective argument that does not require one for any of these reasons above, we are specifically disallowed from doing so by the standard in cases where pointers to the original target data are required to be valid across the call (15.5.2.4(9-10)). In particular, compaction of assumed-shape arrays for discretionary contiguity on the leading dimension to ease SIMD vectorization cannot be done safely for TARGET dummies without VALUE.

Effective arguments associated with known INTENT(OUT) dummies that require allocation of a temporary -- and this can only be for reasons of contiguity -- don't have to populate it, but they do have to perform minimal initialization of any ALLOCATABLE components so that the runtime doesn't crash when the callee finalizes and deallocates them. ALLOCATABLE coarrays are prohibited from being affected by INTENT(OUT) (see C846). Note that calls to implicit interfaces must conservatively allow for the use of INTENT(OUT) by the callee.

Except for VALUE and known INTENT(IN) dummy arguments, the original contents of local designators that have been compacted into temporaries could optionally have their ALLOCATABLE components invalidated across the call as an aid to debugging.

Except for VALUE and known INTENT(IN) dummy arguments, the contents of the temporary storage will be copied back into the effective argument designator after control returns from the procedure, and it may be necessary to preserve addresses (or the values of subscripts and cosubscripts needed to recalculate them) of the effective argument designator, or its elements, in additional temporary storage if they can't be safely or quickly recomputed after the call.

INTENT(OUT) preparation

Effective arguments that are associated with INTENT(OUT) dummy arguments are required to be definable. This cannot always be checked, as the use of INTENT(OUT) does not by itself mandate the use of an explicit interface.

INTENT(OUT) arguments are finalized (as if) on entry to the called procedure. In particular, in calls to elemental procedures, the elements of an array are finalized by a scalar or elemental FINAL procedure (7.5.6.3(7)).

Derived type components that are ALLOCATABLE are finalized and deallocated; they are prohibited from being coarrays. Components with initializers are (re)initialized.

The preparation of effective arguments for INTENT(OUT) could be done on either side of the call. If the preparation is done by the caller, there is an optimization opportunity in situations where unmodified incoming INTENT(OUT) dummy arguments whose types lack FINAL procedures are being passed onward as outgoing INTENT(OUT) arguments.

Arguments and function results requiring descriptors

Dummy arguments are represented with the addresses of new descriptors when they have any of the following characteristics:

1. assumed-shape array (DIMENSION::A(:))
2. assumed-rank array (DIMENSION::A(..))
3. parameterized derived type with assumed LEN parameters
4. polymorphic (CLASS(T), CLASS(*))
5. assumed-type (TYPE(*))
6. coarray dummy argument
7. INTENT(IN) POINTER argument (15.5.2.7, C.10.4)

ALLOCATABLE and other POINTER arguments can be passed by simple address.

Non-F77ish procedures use descriptors to represent two further kinds of dummy arguments:

1. assumed-length CHARACTER(*)
2. dummy procedures

F77ish procedures use other means to convey character length and host instance links (respectively) for these arguments.

Function results are described by the caller & callee in a caller-supplied descriptor when they have any of the following characteristics, some which necessitate an explicit interface:

1. deferred-shape array (so ALLOCATABLE or POINTER)
2. derived type with any non-constant LEN parameter (C795 prohibit assumed lengths)
3. procedure pointer result (when the interface must be explicit)

Storage for a function call's result is allocated by the caller when possible: the result is neither ALLOCATABLE nor POINTER, the shape is scalar or explicit, and the type has LEN parameters that are constant expressions. In other words, the result doesn't require the use of a descriptor but can't be returned in registers. This allows a function result to be written directly into a local variable or temporary when it is safe to treat the variable as if it were an additional INTENT(OUT) argument. (Storage for CHARACTER results, assumed or explicit, is always allocated by the caller, and the length is always passed so that an assumed-length external function will work when eventually called from a scope that declares the length that it will use (15.5.2.9 (2)).)

Note that the lower bounds of the dimensions of non-POINTER non-ALLOCATABLE dummy argument arrays are determined by the callee, not the caller. (A Fortran pitfall: declaring A(0:9), passing it to a dummy array D(:), and assuming that LBOUND(D,1) will be zero in the callee.) If the declaration of an assumed-shape dummy argument array contains an explicit lower bound expression (R819), its value needs to be computed by the callee; it may be captured and saved in the incoming descriptor as long as we assume that argument descriptors can be modified by callees. Callers should fill in all of the fields of outgoing non-POINTER non-ALLOCATABLE argument descriptors with the assumption that the callee will use 1 for lower bound values, and callees can rely on them being 1 if not modified.

Copying temporary storage back into argument designators

Except for VALUE and known INTENT(IN) dummy arguments and array sections with vector-valued subscripts (15.5.2.4(21)), temporary storage into which effective argument data were compacted for contiguity before the call must be redistributed back to its original storage by the caller after the return.

In conjunction with saved cosubscript values, a standard descriptor would suffice to represent a pointer to the original storage into which the temporary data should be redistributed; the descriptor need not be fully populated with type information.

Note that coindexed objects with ALLOCATABLE ultimate components are required to be associated only with dummy arguments with the VALUE &/or INTENT(IN) attributes (15.6.2.4(6)), so there is no requirement that the local image somehow reallocate remote storage when copying the data back.

Polymorphic bindings

Calls to the type-bound procedures of monomorphic types are resolved at compilation time, as are calls to NON_OVERRIDABLE type-bound procedures. The resolution of calls to overridable type-bound procedures of polymorphic types must be completed at execution (generic resolution of type-bound procedure bindings from effective argument types, kinds, and ranks is always a compilation-time task (15.5.6, C.10.6)).

Each derived type that declares or inherits any overridable type-bound procedure bindings must correspond to a static constant table of code addresses (or, more likely, a static constant type description containing or pointing to such a table, along with information used by the runtime support library for initialization, copying, finalization, and I/O of type instances). Each overridable type-bound procedure in the type corresponds to an index into this table.

Host instance linkage

Calls to dummy procedures and procedure pointers that resolve to internal procedures need to pass an additional "host instance" argument that addresses a block of storage in the stack frame of the their host subprogram that was active at the time they were passed as an effective argument or associated with a procedure pointer. This is similar to a static link in implementations of programming languages with nested subprograms, although Fortran only allows one level of nesting. The 64-bit x86 and little-endian OpenPower ABIs reserve registers for this purpose (%r10 & R11); 64-bit ARM has a reserved register that can be used (x18).

The host subprogram objects that are visible to any of their internal subprograms need to be resident in memory across any calls to them (direct or not). Any host subprogram object that might be defined during a call to an internal subprogram needs to be reloaded after a call or reside permanently in memory. A simple conservative analysis of the internal subprograms can identify all of these escaping objects and their definable subset.

The address of the host subprogram storage used to hold the escaping objects needs to be saved alongside the code address(es) that represent a procedure pointer. It also needs to be conveyed alongside the text address for a dummy procedure.

For F77ish procedures, we cannot use a "procedure pointer descriptor" to pass a procedure argument -- they expect to receive a single address argument. We will need to package the host instance link in a trampoline that loads its address into the designated register.

GNU Fortran and Intel Fortran construct trampolines by writing a sequence of machine instructions to a block of storage in the host's stack frame, which requires the stack to be executable, which seems inadvisable for security reasons; XLF manages trampolines in its runtime support library, which adds some overhead to their construction and a reclamation obligation; NAG Fortran manages a static fixed-sized stack of trampolines per call site, imposing a hidden limit on recursion and foregoing reentrancy; PGI passes host instance links in descriptors in additional arguments that are not always successfully forwarded across implicit interfaces, sometimes leading to crashes when they turn out to be needed.

F18 will manage a pool of trampolines in its runtime support library that can be used to pass internal procedures as effective arguments to F77ish procedures, so that a bare code address can serve to represent the effective argument. But targets that can only be called with an explicit interface have the option of using a "fat pointer" (or additional argument) to represent a dummy procedure closure so as to avoid the overhead of constructing and reclaiming a trampoline. Procedure descriptors can also support multiple code addresses.

Naming

External subroutines and functions (R503) and ENTRY points (R1541) with BIND(C) (R808) have linker-visible names that are either explicitly specified in the program or determined by straightforward rules. The names of other F77ish external procedures should respect the conventions of the target architecture for legacy Fortran '77 programs; this is typically something like foo_.

In other cases, however, we have fewer constraints on external naming, as well as some additional requirements and goals.

Module procedures need to be distinguished by the name of their module and (when they have one) the submodule where their interface was defined. Note that submodule names are distinct in their modules, not hierarchical, so at most two levels of qualification are needed.

Pure ELEMENTAL functions (15.8) must use distinct names for any alternate entry points used for packed SIMD arguments of various widths if we support calls to these functions in SIMD parallel contexts. There are already conventions for these names in libpgmath.

The names of non-F77ish external procedures should be distinguished as such so that incorrect attempts to call or pass them with an implicit interface will fail to resolve at link time. Fortran 2018 explicitly enables us to do this with a correction to Fortran 2003 in 4.3.4(5).

Last, there must be reasonably permanent naming conventions used by the F18 runtime library for those unrestricted specific intrinsic functions (table 16.2 in 16.8) and extensions that can be passed as arguments.

In these cases where external naming is at the discretion of the implementation, we should use names that are not in the C language user namespace, begin with something that identifies the current incompatible version of F18, the module, the submodule, and elemental SIMD width, and are followed by the external name. The parts of the external name can be separated by some character that is acceptable for use in LLVM IR and assembly language but not in user Fortran or C code, or by switching case (so long as there's a way to cope with extension names that don't begin with letters).

In particular, the period (.) seems safe to use as a separator character, so a Fa. prefix can serve to isolate these discretionary names from other uses and to identify the earliest link-compatible version. For examples: Fa.mod.foo, Fa.mod.submod.foo, and (for an external subprogram that requires an explicit interface) Fa.foo. When the ABI changes in the future in an incompatible way, the initial prefix becomes Fb., Fc., &c.

Summary of checks to be enforced in semantics analysis

8.5.10 INTENT attributes

• (C846) An INTENT(OUT) argument shall not be associated with an object that is or has an allocatable coarray.
• (C847) An INTENT(OUT) argument shall not have LOCK_TYPE or EVENT_TYPE.

8.5.18 VALUE attribute

• (C863) The argument cannot be assumed-size, a coarray, or have a coarray ultimate component.
• (C864) The argument cannot be ALLOCATABLE, POINTER, INTENT(OUT), INTENT(IN OUT), or VOLATILE.
• (C865) If the procedure is BIND(C), the argument cannot be OPTIONAL.

15.5.1 procedure references:

• (C1533) can't pass non-intrinsic ELEMENTAL as argument
• (C1536) alternate return labels must be in the inclusive scope
• (C1537) coindexed argument cannot have a POINTER ultimate component

15.5.2.4 requirements for non-POINTER non-ALLOCATABLE dummies:

• (2) dummy must be monomorphic for coindexed polymorphic actual
• (2) dummy must be polymorphic for assumed-size polymorphic actual
• (2) dummy cannot be TYPE(*) if effective is PDT or has TBPs or FINAL
• (4) character length of effective cannot be less than dummy
• (6) coindexed effective with ALLOCATABLE ultimate component requires INTENT(IN) &/or VALUE dummy
• (13) a coindexed scalar effective requires a scalar dummy
• (14) a non-conindexed scalar effective usually requires a scalar dummy, but there are some exceptions that allow elements of storage sequences to be passed and treated like explicit-shape or assumed-size arrays (see 15.5.2.11)
• (16) array rank agreement
• (20) INTENT(OUT) & INTENT(IN OUT) dummies require definable actuals
• (21) array sections with vector subscripts can't be passed to definable dummies (INTENT(OUT), INTENT(IN OUT), ASYNCHRONOUS, VOLATILE)
• (22) VOLATILE attributes must match when dummy has a coarray ultimate component
• (C1538 - C1540) checks for ASYNCHRONOUS and VOLATILE

15.5.2.5 requirements for ALLOCATABLE & POINTER arguments when both the dummy and effective arguments have the same attributes:

• (2) both or neither can be polymorphic
• (2) both are unlimited polymorphic or both have the same declared type
• (3) rank compatibility
• (4) effective argument must have deferred the same type parameters as the dummy

15.5.2.6 ALLOCATABLE dummy arguments:

• (2) effective must be ALLOCATABLE
• (3) corank must match
• (4) coindexed effective requires INTENT(IN) dummy
• (7) INTENT(OUT) & INTENT(IN OUT) dummies require definable actuals

15.5.2.7 POINTER dummy arguments:

• (C1541) CONTIGUOUS dummy requires simply contiguous actual
• (C1542) effective argument cannot be coindexed unless procedure is intrinsic
• (2) effective argument must be POINTER unless dummy is INTENT(IN) and effective could be the right-hand side of a pointer assignment statement

15.5.2.8 corray dummy arguments:

• (1) effective argument must be coarray
• (1) VOLATILE attributes must match
• (2) explicitly or implicitly contiguous dummy array requires a simply contiguous actual

15.5.2.9 dummy procedures:

• (1) explicit dummy procedure interface must have same characteristics as actual
• (5) dummy procedure POINTER requirements on effective arguments

15.6.2.1 procedure definitions:

• NON_RECURSIVE procedures cannot recurse.
• Assumed-length CHARACTER(*) functions cannot be declared as RECURSIVE, array-valued, POINTER, ELEMENTAL, or `PURE' (C723), and cannot be called recursively (15.6.2.1(3)).
• (C823) A function result cannot be a coarray or contain a coarray ultimate component.

PURE requirements (15.7): C1583 - C1599. These also apply to ELEMENTAL procedures that are not IMPURE.

ELEMENTAL requirements (15.8.1): C15100-C15103, and C1533 (can't pass as effective argument unless intrinsic)

For interoperable procedures and interfaces (18.3.6):

• C1552 - C1559
• function result is scalar and of interoperable type (C1553, 18.3.1-3)
• VALUE arguments are scalar and of interoperable type
• POINTER dummies cannot be CONTIGUOUS (18.3.6 paragraph 2(5))
• assumed-type dummies cannot be ALLOCATABLE, POINTER, assumed-shape, or assumed-rank (18.3.6 paragraph 2 (5))
• CHARACTER dummies that are ALLOCATABLE or POINTER must be deferred-length

Further topics to document

• Alternate return specifiers
• %VAL(), %REF(), and %DESCR() legacy VMS interoperability extensions
• Unrestricted specific intrinsic functions as effective arguments
• SIMD variants of ELEMENTAL procedures (& unrestricted specific intrinsics)
• Elemental subroutine calls with array arguments