Semantics: Resolving Labels and Construct Names¶
Overview¶
After the Fortran input file(s) has been parsed into a syntax tree, the compiler must check that the program checks semantically. Target labels must be checked and violations of legal semantics should be reported to the user.
This is the detailed design document on how these labels will be semantically checked. Legal semantics may result in rewrite operations on the syntax tree. Semantics violations will be reported as errors to the user.
Requirements¶
Input: a parse tree that decomposes the Fortran program unit
Output:
Success returns true (Additionally, the parse tree may be rewritten on success to capture the nested DO loop structure explicitly from any label-do-stmt type loops.)
Failure returns false, instantiates (a container of) error message(s) to indicate the problem(s)
Label generalities (6.2.5)¶
Enforcement of the general label constraints. There are three sorts of label usage. Labels can serve
as a label-do-stmt block range marker
as branching (control flow) targets
as specification annotations (
FORMAT
statements) for data transfer statements (I/O constructs)
Labels are related to the standard definition of inclusive scope. For example, control-flow arcs are not allowed to originate from one inclusive scope and target statements outside of that inclusive scope.
Inclusive scope is defined as a tree structure of nested scoping constructs. A statement, s, is said to be in the same inclusive scope as another statement, t, if and only if s and t are in the same scope or t is in one of the enclosing scopes of s, otherwise s is not in the same inclusive scope as t. (Inclusive scope is unidirectional and is always from innermost scopes to outermost scopes.)
Semantic Checks¶
labels range from 1 to 99999, inclusive (6.2.5 note 2)
handled automatically by the parser, but add a range check
labels must be pairwise distinct within their program unit scope (6.2.5 para 2)
if redundant labels appear
→
error redundant labelsthe total number of unique statement labels may have a limit
Labels Used for DO
Loop Ranging¶
label-do-stmt (R1121)¶
A label-do-stmt is a control construct that results in the iterative execution of a number of statements. A label-do-stmt has a (possibly shared, nonblock-do-construct) label that will be called the loop target label. The statements to be executed will be the range from the label-do-stmt to the statement identified by the loop target label, inclusive. This range of statements will be called the loop’s body and logically forms a do-block.
A label-do-stmt is quite similar to a block-do-construct in semantics, but the parse tree is different in that the parser does not impose a do-block structure on the loop body.
In F18, the nonblock DO
construct has been removed. For legacy support (through F08), we will need to handle nonblock DO
constructs. In F18, the following legacy code is an error.
DO 100 I = 1, 100
DO 100 J = 1, 100
...
100 CONTINUE
Semantic Checks¶
the loop body target label must exist in the scope (F18:C1133; F08:C815, C817, C819)
if the label does not appear, error of missing label
the loop body target label must be, lexically, after the label-do-stmt (R1119)
if the label appears lexically preceding the
DO
, error of malformedDO
control cannot transfer into the body from outside the do-block
Exceptions (errors demoted to warnings)
some implementations relax enforcement of this and allow
GOTO
s from the loop body to “extended ranges” and back again (PGI & gfortan appear to allow, NAG & Intel do not.)should some form of “extended ranges” for do-constructs be supported, it should still be limited and not include parallel loops such as
DO CONCURRENT
or loops annotated with OpenACC or OpenMP directives.
GOTO
s into theDO
s inclusive scope, error/warn of invalid transfer of control
requires that the loop terminating statement for a label-do-stmt be either an
END DO
or aCONTINUE
Exception
earlier standards allowed other statements to be terminators
Semantics for F08 and earlier that support sharing the loop terminating statement in a nonblock-do-construct between multiple loops
some statements cannot be do-term-action-stmt (F08:C816)
a do-term-action-stmt is an action-stmt but does not include arithmetic-if-stmt, continue-stmt, cycle-stmt, end-function-stmt, end-mp-subprogram-stmt, end-program-stmt, end-subroutine-stmt, error-stop-stmt, exit-stmt, goto-stmt, return-stmt, or stop-stmt
if the term action statement is forbidden, error invalid statement in
DO
loop term position
some statements cannot be do-term-shared-stmt (F08:C818)
this is the case as in our above example where two different nested loops share the same terminating statement (
100 continue
)a do-term-shared-stmt is an action-stmt with all the same exclusions as a do-term-action-stmt except a continue-stmt is allowed
if the term shared action statement is forbidden, error invalid statement in term position
If the DO
loop is a DO CONCURRENT
construct, there are additional constraints (11.1.7.5).
a return-stmt is not allowed (C1136)
image control statements are not allowed (C1137)
branches must be from a statement and to a statement that both reside within the
DO CONCURRENT
(C1138)impure procedures shall not be called (C1139)
deallocation of polymorphic objects is not allowed (C1140)
references to
IEEE_GET_FLAG
,IEEE_SET_HALTING_MODE
, andIEEE_GET_HALTING_MODE
cannot appear in the body of aDO CONCURRENT
(C1141)the use of the
ADVANCE=
specifier by an I/O statement in the body of aDO CONCURRENT
is not allowed (11.1.7.5, para 5)
Labels Used in Branching¶
goto-stmt (11.2.2, R1157)¶
A GOTO
statement is a simple, direct transfer of control from the GOTO
to the labelled statement.
Semantic Checks¶
the labelled statement that is the target of a
GOTO
(11.2.1 constraints)must refer to a label that is in inclusive scope of the computed
GOTO
statement (C1169)if a label does not exist, error nonexistent label
if a label is out of scope, error out of inclusive scope
the branch target statement must be valid
if the statement is not allowed as a branch target, error not a valid branch target
the labelled statement must be a branch target statement
a branch target statement is any of action-stmt, associate-stmt, end-associate-stmt, if-then-stmt, end-if-stmt, select-case-stmt, end-select-stmt, select-rank-stmt, end-select-rank-stmt, select-type-stmt, end-select-type-stmt, do-stmt, end-do-stmt, block-stmt, end-block-stmt, critical-stmt, end-critical-stmt, forall-construct-stmt, forall-stmt, where-construct-stmt, end-function-stmt, end-mp-subprogram-stmt, end-program-stmt, or end-subroutine-stmt. (11.2.1)
Some deleted features that were action-stmt in older standards include arithmetic-if-stmt, assign-stmt, assigned-goto-stmt, and pause-stmt. For legacy mode support, these statements should be considered action-stmt.
computed-goto-stmt (11.2.3, R1158)¶
The computed GOTO
statement is analogous to a switch
statement in C++.
GOTO ( label-list ) [,] scalar-int-expr
Semantics Checks¶
each label in label-list (11.2.1 constraints, same as
GOTO
)must refer to a label that is in inclusive scope of the computed
GOTO
statement (C1170)if a label does not exist, error nonexistent label
if a label is out of scope, error out of inclusive scope
the branch target statement must be valid
if the statement is not allowed as a branch target, error not a valid branch target
the scalar-int-expr needs to have
INTEGER
typecheck the type of the expression (type checking done elsewhere)
R853 arithmetic-if-stmt (F08:8.2.4)¶
This control-flow construct is deleted in F18.
IF (scalar-numeric-expr) label1,label2,label3
The arithmetic if statement is like a three-way branch operator. If the scalar numeric expression is less than zero goto label-1, else if the variable is equal to zero goto label-2, else if the variable is greater than zero goto label-3.
Semantics Checks¶
the labels in the arithmetic-if-stmt triple must all be present in the inclusive scope (F08:C848)
if a label does not exist, error nonexistent label
if a label is out of scope, error out of inclusive scope
the scalar-numeric-expr must not be
COMPLEX
(F08:C849)check the type of the expression (type checking done elsewhere)
alt-return-spec (15.5.1, R1525)¶
These are a Fortran control-flow construct for combining a return from a subroutine with a branch to a labelled statement in the calling routine all in one operation. A typical implementation is for the subroutine to return a hidden integer, which is used as a key in the calling code to then, possibly, branch to a labelled statement in inclusive scope.
The labels are passed by the calling routine. We want to check those labels at the call-site, that is instances of alt-return-spec.
Semantics Checks¶
each alt-return-spec (11.2.1 constraints, same as
GOTO
)must refer to a label that is in inclusive scope of the
CALL
statementif a label does not exist, error nonexistent label
if a label is out of scope, error out of inclusive scope
the branch target statement must be valid
if the statement is not allowed as a branch target, error not a valid branch target
END, EOR, ERR specifiers (12.11)¶
These specifiers can appear in I/O statements and can transfer control to specific labelled statements under exceptional conditions like end-of-file, end-of-record, and other error conditions. (The PGI compiler adds code to test the results from the runtime routines to determine if these branches should take place.)
Semantics Checks¶
each END, EOR, and ERR specifier (11.2.1 constraints, same as
GOTO
)must refer to a label that is in inclusive scope of the I/O statement
if a label does not exist, error nonexistent label
if a label is out of scope, error out of inclusive scope
the branch target statement must be valid
if the statement is not allowed as a branch target, error not a valid branch target
assigned-goto-stmt and assign-stmt (F90:8.2.4)¶
Deleted feature since Fortran 95.
The assigned-goto-stmt and assign-stmt were action-stmt in the Fortran 90 standard. They are included here for completeness. This pair of obsolete statements can (will) be enabled as part of the compiler’s legacy Fortran support.
The assign-stmt stores a label in an integer variable. The assigned-goto-stmt will then transfer control to the label stored in that integer variable.
ASSIGN 10 TO i
...
GOTO i (10,20,30)
Semantic Checks¶
an assigned-goto-stmt cannot be a do-term-action-stmt (F90:R829)
an assigned-goto-stmt cannot be a do-term-shared-stmt (F90:R833)
constraints from (F90:R839)
each label in an optional label-list must be the statement label of a branch target statement that appears in the same scoping unit as the assigned-goto-stmt
scalar-int-variable (
i
in the example above) must be named and of type default integeran integer variable that has been assigned a label may only be referenced in an assigned-goto or as a format specifier in an I/O statement
when an I/O statement with a format-specifier that is an integer variable is executed or when an assigned-goto is executed, the variable must have been assigned a label
an integer variable can only be assigned a label via the
ASSIGN
statementthe label assigned to the variable must be in the same scoping unit as the assigned-goto that branches to the label value
if the parameterized list of labels is present, the label value assigned to the integer variable must appear in that label-list
a distinct label can appear more than once in the label-list
Some interpretation is needed as the terms of the older standard are different.
A “scoping unit” is defined as
a derived-type definition
a procedure interface body, excluding derived-types and interfaces contained within it
a program unit or subprogram, excluding derived-types, interfaces, and subprograms contained within it
This is a more lax definition of scope than inclusive scope.
A named variable distinguishes a variable such as, i
, from an element of an array, a(i)
, for example.
Labels used in I/O¶
Data transfer statements¶
In data transfer (I/O) statements (e.g., READ
), the user can specify a FMT=
specifier that can take a label as its argument. (R1215)
Semantic Checks¶
if the
FMT=
specifier has a label as its argument (C1230)the label must correspond to a
FORMAT
statementif the statement is not a
FORMAT
, error statement must be aFORMAT
the labelled
FORMAT
statement must be in the same inclusive scope as the originating data transfer statement (also in 2008)if the label statement does not exist, error label does not exist
if the label statement is not in scope, error label is not in inclusive scope
Exceptions (errors demoted to warnings)
PGI extension: referenced
FORMAT
statements may appear in a host procedurePossible relaxation: the scope of the referenced
FORMAT
statement may be ignored, allowing aFORMAT
to be referenced from any scope in the compilation.
Construct Name generalities¶
Various Fortran constructs can have names. These include
the
WHERE
construct (10.2.3)the
FORALL
construct (10.2.4)the
ASSOCIATE
construct (11.1.3)the
BLOCK
construct (11.1.4)the
CHANGE TEAM
construct (11.1.5)the
CRITICAL
construct (11.1.6)the
DO
construct (11.1.7)the
IF
construct (11.1.8)the
SELECT CASE
construct (11.1.9)the
SELECT RANK
construct (11.1.10)the
SELECT TYPE
construct (11.1.11)
Semantics Checks¶
A construct name is a name formed under 6.2.2. A name is an identifier. Identifiers are parsed by the parser.
the maximum length of a name is 63 characters (C601)
Names must either not be given for the construct or used throughout when specified.
if a construct is given a name, the construct’s
END
statement must also specify the same name (WHERE
C1033,FORALL
C1035, …)WHERE
has additionalELSEWHERE
clausesIF
has additionalELSE IF
andELSE
clausesSELECT CASE
has additionalCASE
clausesSELECT RANK
has additionalRANK
clausesSELECT TYPE
has additional type-guard-stmt These additional statements must meet the same constraint as theEND
of the construct. Names must match, if present, or there must be no names for any of the clauses.
CYCLE
statement (11.1.7.4.4)¶
The CYCLE
statement takes an optional do-construct-name.
Semantics Checks¶
if the
CYCLE
has a construct-name, then theCYCLE
statement must appear within that named do-construct (C1134)if the
CYCLE
does not have a do-construct-name, theCYCLE
statement must appear within a do-construct (C1134)
EXIT
statement (11.1.12)¶
The EXIT
statement takes an optional construct-name.
Semantics Checks¶
if the
EXIT
has a construct-name, then theEXIT
statement must appear within that named construct (C1166)if the
EXIT
does not have a construct-name, theEXIT
statement must appear within a do-construct (C1166)an exit-stmt must not appear in a
DO CONCURRENT
if theEXIT
belongs to theDO CONCURRENT
or an outer construct enclosing theDO CONCURRENT
(C1167)an exit-stmt must not appear in a
CHANGE TEAM
(CRITICAL
) if theEXIT
belongs to an outer construct enclosing theCHANGE TEAM
(CRITICAL
) (C1168)