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This file documents the internals of the GNU Fortran compiler, (gfortran).

Published by the Free Software Foundation 51 Franklin Street, Fifth Floor Boston, MA 02110-1301 USA

Copyright © 2007-2017 Free Software Foundation, Inc.

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with the Invariant Sections being ``Funding Free Software'', the Front-Cover Texts being (a) (see below), and with the Back-Cover Texts being (b) (see below).  A copy of the license is included in the section entitled ``GNU Free Documentation License''.

(a) The FSF's Front-Cover Text is:

A GNU Manual

(b) The FSF's Back-Cover Text is:

You have freedom to copy and modify this GNU Manual, like GNU software.  Copies published by the Free Software Foundation raise funds for GNU development.

Introduction
************

This manual documents the internals of gfortran, the GNU Fortran compiler.

Introduction
************

At present, this manual is very much a work in progress, containing miscellaneous notes about the internals of the compiler.  It is hoped that at some point in the future it will become a reasonably complete guide; in the interim, GNU Fortran developers are strongly encouraged to contribute to it as a way of keeping notes while working on the compiler.

Code that Interacts with the User
*********************************

Command-Line Options
====================

Command-line options for gfortran involve four interrelated pieces within the Fortran compiler code.

The relevant command-line flag is defined in lang.opt, according to the documentation in Options (GNU Compiler Collection Internals).  This is then processed by the overall GCC machinery to create the code that enables gfortran and gcc to recognize the option in the command-line arguments and call the relevant handler function.

This generated code calls the gfc_handle_option code in options.c with an enumerator variable indicating which option is to be processed, and the relevant integer or string values associated with that option flag.  Typically, gfc_handle_option uses these arguments to set global flags which record the option states.

The global flags that record the option states are stored in the gfc_option_t struct, which is defined in gfortran.h. Before the options are processed, initial values for these flags are set in gfc_init_option in options.c; these become the default values for the options.

Error Handling
==============

The GNU Fortran compiler's parser operates by testing each piece of source code against a variety of matchers.  In some cases, if these matchers do not match the source code, they will store an error message in a buffer.  If the parser later finds a matcher that does correctly match the source code, then the buffered error is discarded.  However, if the parser cannot find a match, then the buffered error message is reported to the user.  This enables the compiler to provide more meaningful error messages even in the many cases where (erroneous) Fortran syntax is ambiguous due to things like the absence of reserved keywords.

As an example of how this works, consider the following line:

     IF = 3

Hypothetically, this may get passed to the matcher for an IF statement.  Since this could plausibly be an erroneous IF statement, the matcher will buffer an error message reporting the absence of an expected ( following an IF.  Since no matchers reported an error-free match, however, the parser will also try matching this against a variable assignment.  When IF is a valid variable, this will be parsed as an assignment statement, and the error discarded.  However, when IF is not a valid variable, this buffered error message will be reported to the user.

The error handling code is implemented in error.c.  Errors are normally entered into the buffer with the gfc_error function. Warnings go through a similar buffering process, and are entered into the buffer with gfc_warning.  There is also a special-purpose function, gfc_notify_std, for things which have an error/warning status that depends on the currently-selected language standard.

The gfc_error_check function checks the buffer for errors, reports the error message to the user if one exists, clears the buffer, and returns a flag to the user indicating whether or not an error existed.  To check the state of the buffer without changing its state or reporting the errors, the gfc_error_flag_test function can be used.  The gfc_clear_error function will clear out any errors in the buffer, without reporting them.  The gfc_warning_check and gfc_clear_warning functions provide equivalent functionality for the warning buffer.

Only one error and one warning can be in the buffers at a time, and buffering another will overwrite the existing one.  In cases where one may wish to work on a smaller piece of source code without disturbing an existing error state, the gfc_push_error, gfc_pop_error, and gfc_free_error mechanism exists to implement a stack for the error buffer.

For cases where an error or warning should be reported immediately rather than buffered, the gfc_error_now and gfc_warning_now functions can be used.  Normally, the compiler will continue attempting to parse the program after an error has occurred, but if this is not appropriate, the gfc_fatal_error function should be used instead.  For errors that are always the result of a bug somewhere in the compiler, the gfc_internal_error function should be used.

The syntax for the strings used to produce the error/warning message in the various error and warning functions is similar to the printf syntax, with %-escapes to insert variable values.  The details, and the allowable codes, are documented in the error_print function in error.c.

Frontend Data Structures
************************

This chapter should describe the details necessary to understand how the various gfc_* data are used and interact.  In general it is advisable to read the code in dump-parse-tree.c as its routines should exhaust all possible valid combinations of content for these structures.

gfc_code
========

The executable statements in a program unit are represented by a nested chain of gfc_code structures.  The type of statement is identified by the op member of the structure, the different possible values are enumerated in gfc_exec_op.  A special member of this enum is EXEC_NOP which is used to represent the various END statements if they carry a label. Depending on the type of statement some of the other fields will be filled in.  Fields that are generally applicable are the next and here fields.  The former points to the next statement in the current block or is NULL if the current statement is the last in a block, here points to the statement label of the current statement.

If the current statement is one of IF, DO, SELECT it starts a block, i.e. a nested level in the program.  In order to represent this, the block member is set to point to a gfc_code structure whose next member starts the chain of statements inside the block; this structure's op member should be set to the same value as the parent structure's op member.  The SELECT and IF statements may contain various blocks (the chain of ELSE IF and ELSE blocks or the various CASEs, respectively).  These chains are linked-lists formed by the block members.

Consider the following example code:

     IF (foo < 20) THEN
       PRINT *, "Too small"
       foo = 20
     ELSEIF (foo > 50) THEN
       PRINT *, "Too large"
       foo = 50
     ELSE
       PRINT *, "Good"
     END IF

This statement-block will be represented in the internal gfortran tree as follows, were the horizontal link-chains are those induced by the next members and vertical links down are those of block. ==| and --| mean NULL pointers to mark the end of a chain:

     ... ==> IF ==> ...
             |
             +--> IF foo < 20 ==> PRINT *, "Too small" ==> foo = 20 ==|
                  |
                  +--> IF foo > 50 ==> PRINT *, "Too large" ==> foo = 50 ==|
                       |
                       +--> ELSE ==> PRINT *, "Good" ==|
                            |
                            +--|

IF Blocks
---------

Conditionals are represented by gfc_code structures with their op member set to EXEC_IF.  This structure's block member must point to another gfc_code node that is the header of the if-block.  This header's op member must be set to EXEC_IF, too, its expr member holds the condition to check for, and its next should point to the code-chain of the statements to execute if the condition is true.

If in addition an ELSEIF or ELSE block is present, the block member of the if-block-header node points to yet another gfc_code structure that is the header of the elseif- or else-block.  Its structure is identical to that of the if-block-header, except that in case of an ELSE block without a new condition the expr member should be NULL.  This block can itself have its block member point to the next ELSEIF or ELSE block if there's a chain of them.

Loops
-----

DO loops are stored in the tree as gfc_code nodes with their op set to EXEC_DO for a DO loop with iterator variable and to EXEC_DO_WHILE for infinite DOs and DO WHILE blocks. Their block member should point to a gfc_code structure heading the code-chain of the loop body; its op member should be set to EXEC_DO or EXEC_DO_WHILE, too, respectively.

For DO WHILE loops, the loop condition is stored on the top gfc_code structure's expr member; DO forever loops are simply DO WHILE loops with a constant .TRUE. loop condition in the internal representation.

Similarly, DO loops with an iterator have instead of the condition their ext.iterator member set to the correct values for the loop iterator variable and its range.

SELECT Statements
-----------------

A SELECT block is introduced by a gfc_code structure with an op member of EXEC_SELECT and expr containing the expression to evaluate and test.  Its block member starts a list of gfc_code structures linked together by their block members that stores the various CASE parts.

Each CASE node has its op member set to EXEC_SELECT, too, its next member points to the code-chain to be executed in the current case-block, and extx.case_list contains the case-values this block corresponds to.  The block member links to the next case in the list.

BLOCK and ASSOCIATE
-------------------

The code related to a BLOCK statement is stored inside an gfc_code structure (say C) with c.op set to EXEC_BLOCK.  The gfc_namespace holding the locally defined variables of the BLOCK is stored in c.ext.block.ns.  The code inside the construct is in c.code.

ASSOCIATE constructs are based on BLOCK and thus also have the internal storage structure described above (including EXEC_BLOCK). However, for them c.ext.block.assoc is set additionally and points to a linked list of gfc_association_list structures.  Those structures basically store a link of associate-names to target expressions. The associate-names themselves are still also added to the BLOCK's namespace as ordinary symbols, but they have their gfc_symbol's member assoc set also pointing to the association-list structure. This way associate-names can be distinguished from ordinary variables and their target expressions identified.

For association to expressions (as opposed to variables), at the very beginning of the BLOCK construct assignments are automatically generated to set the corresponding variables to their target expressions' values, and later on the compiler simply disallows using such associate-names in contexts that may change the value.

gfc_expr
========

Expressions and ``values'', including constants, variable-, array- and component-references as well as complex expressions consisting of operators and function calls are internally represented as one or a whole tree of gfc_expr objects.  The member expr_type specifies the overall type of an expression (for instance, EXPR_CONSTANT for constants or EXPR_VARIABLE for variable references).  The members ts and rank as well as shape, which can be NULL, specify the type, rank and, if applicable, shape of the whole expression or expression tree of which the current structure is the root.  where is the locus of this expression in the source code.

Depending on the flavor of the expression being described by the object (that is, the value of its expr_type member), the corresponding structure in the value union will usually contain additional data describing the expression's value in a type-specific manner.  The ref member is used to build chains of (array-, component- and substring-) references if the expression in question contains such references, see below for details.

Constants
---------

Scalar constants are represented by gfc_expr nodes with their expr_type set to EXPR_CONSTANT.  The constant's value shall already be known at compile-time and is stored in the logical, integer, real, complex or character struct inside value, depending on the constant's type specification.

Operators
---------

Operator-expressions are expressions that are the result of the execution of some operator on one or two operands.  The expressions have an expr_type of EXPR_OP.  Their value.op structure contains additional data.

op1 and optionally op2 if the operator is binary point to the two operands, and operator or uop describe the operator that should be evaluated on these operands, where uop describes a user-defined operator.

Function Calls
--------------

If the expression is the return value of a function-call, its expr_type is set to EXPR_FUNCTION, and symtree must point to the symtree identifying the function to be called.  value.function.actual holds the actual arguments given to the function as a linked list of gfc_actual_arglist nodes.

The other members of value.function describe the function being called in more detail, containing a link to the intrinsic symbol or user-defined function symbol if the call is to an intrinsic or external function, respectively.  These values are determined during resolution-phase from the structure's symtree member.

A special case of function calls are ``component calls'' to type-bound procedures; those have the expr_type EXPR_COMPCALL with value.compcall containing the argument list and the procedure called, while symtree and ref describe the object on which the procedure was called in the same way as a EXPR_VARIABLE expression would. See Type-bound Procedures.

Array- and Structure-Constructors
---------------------------------

Array- and structure-constructors (one could probably call them ``array-'' and ``derived-type constants'') are gfc_expr structures with their expr_type member set to EXPR_ARRAY or EXPR_STRUCTURE, respectively.  For structure constructors, symtree points to the derived-type symbol for the type being constructed.

The values for initializing each array element or structure component are stored as linked-list of gfc_constructor nodes in the value.constructor member.

Null
----

NULL is a special value for pointers; it can be of different base types. Such a NULL value is represented in the internal tree by a gfc_expr node with expr_type EXPR_NULL.  If the base type of the NULL expression is known, it is stored in ts (that's for instance the case for default-initializers of ALLOCATABLE components), but this member can also be set to BT_UNKNOWN if the information is not available (for instance, when the expression is a pointer-initializer NULL()).

Variables and Reference Expressions
-----------------------------------

Variable references are gfc_expr structures with their expr_type set to EXPR_VARIABLE; their symtree should point to the variable that is referenced.

For this type of expression, it's also possible to chain array-, component- or substring-references to the original expression to get something like struct%component(2:5), where component is either an array or a CHARACTER member of struct that is of some derived-type.  Such a chain of references is achieved by a linked list headed by ref of the gfc_expr node.  For the example above it would be (==| is the last NULL pointer):

     EXPR_VARIABLE(struct) ==> REF_COMPONENT(component) ==> REF_ARRAY(2:5) ==|

If component is a string rather than an array, the last element would be a REF_SUBSTRING reference, of course.  If the variable itself or some component referenced is an array and the expression should reference the whole array rather than being followed by an array-element or -section reference, a REF_ARRAY reference must be built as the last element in the chain with an array-reference type of AR_FULL. Consider this example code:

     TYPE :: mytype
       INTEGER :: array(42)
     END TYPE mytype

     TYPE(mytype) :: variable
     INTEGER :: local_array(5)

     CALL do_something (variable%array, local_array)

The gfc_expr nodes representing the arguments to the do_something call will have a reference-chain like this:

     EXPR_VARIABLE(variable) ==> REF_COMPONENT(array) ==> REF_ARRAY(FULL) ==|
     EXPR_VARIABLE(local_array) ==> REF_ARRAY(FULL) ==|

Constant Substring References
-----------------------------

EXPR_SUBSTRING is a special type of expression that encodes a substring reference of a constant string, as in the following code snippet:

     x = "abcde"(1:2)

In this case, value.character contains the full string's data as if it was a string constant, but the ref member is also set and points to a substring reference as described in the subsection above.

Internals of Fortran 2003 OOP Features
**************************************

Type-bound Procedures
=====================

Type-bound procedures are stored in the tb_sym_root of the namespace f2k_derived associated with the derived-type symbol as gfc_symtree nodes.  The name and symbol of these symtrees corresponds to the binding-name of the procedure, i.e. the name that is used to call it from the context of an object of the derived-type.

In addition, this type of symtrees stores in n.tb a struct of type gfc_typebound_proc containing the additional data needed:  The binding attributes (like PASS and NOPASS, NON_OVERRIDABLE or the access-specifier), the binding's target(s) and, if the current binding overrides or extends an inherited binding of the same name, overridden points to this binding's gfc_typebound_proc structure.

Specific Bindings
-----------------

For specific bindings (declared with PROCEDURE), if they have a passed-object argument, the passed-object dummy argument is first saved by its name, and later during resolution phase the corresponding argument is looked for and its position remembered as pass_arg_num in gfc_typebound_proc. The binding's target procedure is pointed-to by u.specific.

DEFERRED bindings are just like ordinary specific bindings, except that their deferred flag is set of course and that u.specific points to their ``interface'' defining symbol (might be an abstract interface) instead of the target procedure.

At the moment, all type-bound procedure calls are statically dispatched and transformed into ordinary procedure calls at resolution time; their actual argument list is updated to include at the right position the passed-object argument, if applicable, and then a simple procedure call to the binding's target procedure is built.  To handle dynamic dispatch in the future, this will be extended to allow special code generation during the trans-phase to dispatch based on the object's dynamic type.

Generic Bindings
----------------

Bindings declared as GENERIC store the specific bindings they target as a linked list using nodes of type gfc_tbp_generic in u.generic. For each specific target, the parser records its symtree and during resolution this symtree is bound to the corresponding gfc_typebound_proc structure of the specific target.

Calls to generic bindings are handled entirely in the resolution-phase, where for the actual argument list present the matching specific binding is found and the call's target procedure (value.compcall.tbp) is re-pointed to the found specific binding and this call is subsequently handled by the logic for specific binding calls.

Calls to Type-bound Procedures
------------------------------

Calls to type-bound procedures are stored in the parse-tree as gfc_expr nodes of type EXPR_COMPCALL.  Their value.compcall.actual saves the actual argument list of the call and value.compcall.tbp points to the gfc_typebound_proc structure of the binding to be called.  The object in whose context the procedure was called is saved by combination of symtree and ref, as if the expression was of type EXPR_VARIABLE.

For code like this:

     CALL myobj%procedure (arg1, arg2)

the CALL is represented in the parse-tree as a gfc_code node of type EXEC_COMPCALL.  The expr member of this node holds an expression of type EXPR_COMPCALL of the same structure as mentioned above except that its target procedure is of course a SUBROUTINE and not a FUNCTION.

Expressions that are generated internally (as expansion of a type-bound operator call) may also use additional flags and members. value.compcall.ignore_pass signals that even though a PASS attribute may be present the actual argument list should not be updated because it already contains the passed-object. value.compcall.base_object overrides, if it is set, the base-object (that is normally stored in symtree and ref as mentioned above); this is needed because type-bound operators can be called on a base-object that need not be of type EXPR_VARIABLE and thus representable in this way. Finally, if value.compcall.assign is set, the call was produced in expansion of a type-bound assignment; this means that proper dependency-checking needs to be done when relevant.

Type-bound Operators
====================

Type-bound operators are in fact basically just GENERIC procedure bindings and are represented much in the same way as those (see Type-bound Procedures).

They come in two flavours: User-defined operators (like .MYOPERATOR.) are stored in the f2k_derived namespace's tb_uop_root symtree exactly like ordinary type-bound procedures are stored in tb_sym_root; their symtrees' names are the operator-names (e.g. myoperator in the example). Intrinsic operators on the other hand are stored in the namespace's array member tb_op indexed by the intrinsic operator's enum value.  Those need not be packed into gfc_symtree structures and are only gfc_typebound_proc instances.

When an operator call or assignment is found that can not be handled in another way (i.e. neither matches an intrinsic nor interface operator definition) but that contains a derived-type expression, all type-bound operators defined on that derived-type are checked for a match with the operator call.  If there's indeed a relevant definition, the operator call is replaced with an internally generated GENERIC type-bound procedure call to the respective definition and that call is further processed.

The LibGFortran Runtime Library
*******************************

Symbol Versioning
=================

In general, this capability exists only on a few platforms, thus there is a need for configure magic so that it is used only on those targets where it is supported.

The central concept in symbol versioning is the so-called map file, which specifies the version node(s) exported symbols are labeled with. Also, the map file is used to hide local symbols.

Some relevant references:

  * GNU ld manual (https://sourceware.org/binutils/docs/ld/VERSION.html)

  * ELF Symbol Versioning - Ulrich Depper (https://www.akkadia.org/drepper/symbol-versioning)

  * How to Write Shared Libraries - Ulrich Drepper (see Chapter 3) (https://www.akkadia.org/drepper/dsohowto.pdf)

If one adds a new symbol to a library that should be exported, the new symbol should be mentioned in the map file and a new version node defined, e.g., if one adds a new symbols foo and bar to libgfortran for the next GCC release, the following should be added to the map file:

     GFORTRAN_1.1 {
         global:
             foo;
             bar;
     } GFORTRAN_1.0;

where GFORTRAN_1.0 is the version node of the current release, and GFORTRAN_1.1 is the version node of the next release where foo and bar are made available.

If one wants to change an existing interface, it is possible by using some asm trickery (from the ld manual referenced above):

     __asm__(".symver original_foo,foo@");
     __asm__(".symver old_foo,foo@VERS_1.1");
     __asm__(".symver old_foo1,foo@VERS_1.2");
     __asm__(".symver new_foo,foo@VERS_2.0");

In this example, foo@ represents the symbol foo bound to the unspecified base version of the symbol. The source file that contains this example would define 4 C functions: original_foo, old_foo, old_foo1, and new_foo.

In this case the map file must contain foo in VERS_1.1 and VERS_1.2 as well as in VERS_2.0.

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