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The semantic analysis pass determines if a syntactically correct Fortran program is is legal by enforcing the constraints of the language.
The input is a parse tree with a
Program node at the root; and a “cooked” character stream, a contiguous stream of characters containing a normalized form of the Fortran source.
The semantic analysis pass takes a parse tree for a syntactically correct Fortran program and determines whether it is legal by enforcing the constraints of the language.
If the program is not legal, the results of the semantic pass will be a list of errors associated with the program.
If the program is legal, the semantic pass will produce a (possibly modified) parse tree for the semantically correct program with each name mapped to a symbol and each expression fully analyzed.
All user errors are detected either prior to or during semantic analysis. After it completes successfully the program should compile with no error messages. There may still be warnings or informational messages.
Name::symboldata member in the parse tree
Variable::typedExprwith analyzed expressions; fix incorrect parses based on the result of this analysis
.modfiles for modules and submodules
If phase 1 or phase 2 encounter an error on any of the program units, compilation terminates. Otherwise, phases 3-6 are all performed even if errors occur. Module files are written (phase 7) only if there are no errors.
Perform semantic checks related to labels and branches:
DOloops are properly nested
This phase normalizes the parse tree by removing all unstructured
DO loops and replacing them with
The name resolution phase walks the parse tree and constructs the symbol table.
The symbol table consists of a tree of
Scope objects rooted at the global scope. The global scope is owned by the
SemanticsContext object. It contains a
Scope for each program unit in the compilation.
Scope in the scope tree contains child scopes representing other scopes lexically nested in it. Each
Scope also contains a map of
Symbol representing names declared in that scope. (All names in the symbol table are represented as
CharBlock objects, i.e. as substrings of the cooked character stream.)
Symbol objects are owned by the symbol table data structures. They should be accessed as
Symbol * or
Symbol & outside of the symbol table classes as they can't be created, copied, or moved. The
Symbol class has functions and data common across all symbols, and a
details field that contains more information specific to that type of symbol. Many symbols also have types, represented by
DeclTypeSpec. Types are also owned by scopes.
Name resolution happens on the parse tree in this order:
After the completion of this phase, every
Name corresponds to a
Symbol unless an error occurred.
The parser cannot build a completely correct parse tree without symbol information. This phase corrects mis-parses based on symbols:
a(i) = ...
NML=may be parsed as format expressions
This phase also produces an internal error if it finds a
Name that does not have its
symbol data member filled in. This error is suppressed if other errors have occurred because in that case a
Name corresponding to an erroneous symbol may not be resolved.
Expressions that occur in the specification part are analyzed during name resolution, for example, initial values, array bounds, type parameters. Any remaining expressions are analyzed in this phase.
Variable and top-level
Expr (i.e. one that is not nested below another
Expr in the parse tree) the analyzed form of the expression is saved in the
typedExpr data member. After this phase has completed, the analyzed expression can be accessed using
This phase also corrects mis-parses based on the result of expression analysis:
a(b)is parsed as a function reference but may need to be rewritten to an array element reference (if
ais an object entity) or to a structure constructor (if
ais a derive type)
a(b:c)is parsed as an array section but may need to be rewritten as a substring if
ais an object with type CHARACTER
Multiple independent checkers driven by the
SemanticsVisitor framework perform the remaining semantic checks. By this phase, all names and expressions that can be successfully resolved have been. But there may be names without symbols or expressions without analyzed form if errors occurred earlier.
Fortran supports many means of specifying static initializers for variables, object pointers, and procedure pointers, as well as default initializers for derived type object components, pointers, and type parameters.
Non-pointer static initializers of variables and named constants are scanned, analyzed, folded, scalar-expanded, and validated as they are traversed during declaration processing in name resolution. So are the default initializers of non-pointer object components in non-parameterized derived types. Name constant arrays with implied shapes take their actual shape from the initialization expression.
Default initializers of non-pointer components and type parameters in distinct parameterized derived type instantiations are similarly processed as those instances are created, as their expressions may depend on the values of type parameters. Error messages produced during parameterized derived type instantiation are decorated with contextual attachments that point to the declarations or other type specifications that caused the instantiation.
Static initializations in
DATA statements are collected, validated, and converted into static initialization in the symbol table, as if the initialized objects had used the newer style of static initialization in their entity declarations.
All statically initialized pointers, and default component initializers for pointers, are processed late in name resolution after all specification parts have been traversed. This allows for forward references even in the presence of
IMPLICIT NONE. Object pointer initializers in parameterized derived type instantiations are also cloned and folded at this late stage. Validation of pointer initializers takes place later in declaration checking (below).
Whenever possible, the enforcement of constraints and “shalls” pertaining to properties of symbols is deferred to a single read-only pass over the symbol table that takes place after all name resolution and typing is complete.
Separate compilation information is written out on successful compilation of modules and submodules. These are used as input to name resolution in program units that
USE the modules.
Module files are stripped down Fortran source for the module. Parts that aren't needed to compile dependent program units (e.g. action statements) are omitted.
The module file for module
m is named
m.mod and the module file for submodule
s of module
m is named