llvm-project/flang/docs/LabelResolution.md

Semantics: Resolving Labels and Construct Names

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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

1. as a label-do-stmt block range marker
2. as branching (control flow) targets
3. 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 labels
  • the 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 malformed DO
• control cannot transfer into the body from outside the do-block
  • Exceptions (errors demoted to warnings)
    • some implementations relax enforcement of this and allow GOTOs 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.
  • GOTOs into the DOs 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 a CONTINUE
  • 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, and IEEE_GET_HALTING_MODE cannot appear in the body of a DO CONCURRENT (C1141)
• the use of the ADVANCE= specifier by an I/O statement in the body of a DO 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 type
  • check 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 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

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 integer
  • an 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 statement
  • the 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 statement
    • if the statement is not a FORMAT, error statement must be a FORMAT
  • 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 procedure
    • Possible relaxation: the scope of the referenced FORMAT statement may be ignored, allowing a FORMAT 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 additional ELSEWHERE clauses
• IF has additional ELSE IF and ELSE clauses
• SELECT CASE has additional CASE clauses
• SELECT RANK has additional RANK clauses
• SELECT TYPE has additional type-guard-stmt These additional statements must meet the same constraint as the END 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 the CYCLE statement must appear within that named do-construct (C1134)
• if the CYCLE does not have a do-construct-name, the CYCLE 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 the EXIT statement must appear within that named construct (C1166)
• if the EXIT does not have a construct-name, the EXIT statement must appear within a do-construct (C1166)
• an exit-stmt must not appear in a DO CONCURRENT if the EXIT belongs to the DO CONCURRENT or an outer construct enclosing the DO CONCURRENT (C1167)
• an exit-stmt must not appear in a CHANGE TEAM (CRITICAL) if the EXIT belongs to an outer construct enclosing the CHANGE TEAM (CRITICAL) (C1168)