[flang][NFC] Update module file documentation (#135107)

The current module file documentation antedates the current
implementation of module files and contains many aspirational and
conditional statements, all of which can now be resolved with
descriptions of how things actually work.

Author: klausler

flang/docs/ModFiles.md

@@ -14,138 +14,136 @@ local:
 ---
 ```
 
-Module files hold information from a module that is necessary to compile 
-program units that depend on the module.
+Module files hold information from a module (or submodule) that is
+necessary to compile program units in other source files that depend on that module.
+Program units in the same source file as the module do not read
+module files, as this compiler parses entire source files and processes
+the program units it contains in dependency order.
 
 ## Name
 
-Module files must be searchable by module name. They are typically named
-`<modulename>.mod`. The advantage of using `.mod` is that it is consistent with
-other compilers so users will know what they are. Also, makefiles and scripts
-often use `rm *.mod` to clean up.
+Module files are named according to the module's name, suffixed with `.mod`.
+This is consistent with other compilers and expected by makefiles and
+other build systems.
+
+Module files for submodules are named with their ancestor module's name
+as a prefix, separated by a hyphen.
+E.g., `module-submod.mod` is generated for submodule `submod' of module
+`module`.
+Some other compilers use a distinct filename suffix for submodules,
+but this one doesn't.
 
 The disadvantage of using the same name as other compilers is that it is not
 clear which compiler created a `.mod` file and files from multiple compilers
-cannot be in the same directory. This could be solved by adding something
-between the module name and extension, e.g. `<modulename>-f18.mod`.  If this
-is needed, Flang's fc1 accepts the option `-module-suffix` to alter the suffix
-used for the module file.
+cannot be in the same directory. This can be solved by adding something
+between the module name and extension, e.g. `<modulename>-f18.mod`.  When
+this is needed, Flang accepts the option `-module-suffix` to alter the suffix.
 
 ## Format
 
-Module files will be Fortran source.
-Declarations of all visible entities will be included, along with private
-entities that they depend on.
-Entity declarations that span multiple statements will be collapsed into
-a single *type-declaration-statement*.
-Executable statements will be omitted.
+Module files are Fortran free form source code.
+(One can, in principle, copy `foo.mod` into `tmp.f90`, recompile it,
+and obtain a matching `foo.mod` file.)
+They include the declarations of all visible locally defined entities along
+with the private entities on which they depend.
 
 ### Header
 
-There will be a header containing extra information that cannot be expressed
-in Fortran. This will take the form of a comment or directive
-at the beginning of the file.
-
-If it's a comment, the module file reader would have to strip it out and
-perform *ad hoc* parsing on it. If it's a directive the compiler could
-parse it like other directives as part of the grammar.
-Processing the header before parsing might result in better error messages
-when the `.mod` file is invalid.
-
-Regardless of whether the header is a comment or directive we can use the
-same string to introduce it: `!mod$`.
+Module files begin with a UTF-8 byte order mark and a few lines of
+Fortran comments.
+(Pro tip: use `dd if=foo.mod bs=1 skip=3 2>/dev/null` to skip the byte order
+mark and dump the rest of the module.)
+The first comment begins `!mod$` and contains a version number
+and hash code.
+Further `!need$` comments contain the names and hash codes of other modules
+on which this module depends, and whether those modules are intrinsic
+or not to Fortran.
 
-Information in the header:
-- Magic string to confirm it is an f18 `.mod` file
-- Version information: to indicate the version of the file format, in case it changes,
-  and the version of the compiler that wrote the file, for diagnostics.
-- Checksum of the body of the current file
-- Modules we depend on and the checksum of their module file when the current
-  module file is created
-- The source file that produced the `.mod` file? This could be used in error messages.
+The header comments do not contain timestamps or original source file paths.
 
 ### Body
 
-The body will consist of minimal Fortran source for the required declarations.
-The order will match the order they first appeared in the source.
-
-Some normalization will take place:
-- extraneous spaces will be removed
-- implicit types will be made explicit
-- attributes will be written in a consistent order
-- entity declarations will be combined into a single declaration
-- function return types specified in a *prefix-spec* will be replaced by
-  an entity declaration
-- etc.
+The body comprises minimal Fortran source for the required declarations.
+Their order generally matches the order they appeared in the original
+source code for the module.
+All types are explicit, and all non-character literal constants are
+marked with explicit kind values.
+
+Declarations of objects, interfaces, types, and other entities are
+regenerated from the compiler's symbol table.
+So entity declarations that spanned multiple statements in the source
+program are effectively collapsed into a single *type-declaration-statement*.
+Constant expressions that appear in initializers, bounds, and other sites
+appear in the module file as their folded values.
+Any compiler directives (`!omp$`, `!acc$`, &c.) relevant to the declarations
+of names are also included in the module file.
+
+Executable statements are omitted.
+If we ever want to do Fortran-level inline expansion of procedures
+in the future,
+we will have to "unparse" the executable parts of their definitions.
 
 #### Symbols included
 
-All public symbols from the module need to be included.
+All public symbols from the module are included.
 
 In addition, some private symbols are needed:
 - private types that appear in the public API
 - private components of non-private derived types
 - private parameters used in non-private declarations (initial values, kind parameters)
-- others?
-
-It might be possible to anonymize private names if users don't want them exposed
-in the `.mod` file. (Currently they are readable in PGI `.mod` files.)
 
 #### USE association
 
-A module that contains `USE` statements needs them represented in the
-`.mod` file.
-Each use-associated symbol will be written as a separate *use-only* statement,
-possibly with renaming.
-
-Alternatives:
-- Emit a single `USE` for each module, listing all of the symbols that were
-  use-associated in the *only-list*.
-- Detect when all of the symbols from a module are imported (either by a *use-stmt*
-  without an *only-list* or because all of the public symbols of the module
-  have been listed in *only-list*s). In that case collapse them into a single *use-stmt*.
-- Emit the *use-stmt*s that appeared in the original source.
+Entities that have been included in a module by means of USE association
+are represented in the module file with `USE` statements.
+Name aliases are sometimes necessary when an entity from another
+module is needed for a declaration and conflicts with another
+entity of the same name.
 
 ## Reading and writing module files
 
 ### Options
 
-The compiler will have command-line options to specify where to search
+The compiler has command-line options to specify where to search
 for module files and where to write them. By default it will be the current
 directory for both.
 
-For PGI, `-I` specifies directories to search for include files and module
-files. `-module` specifics a directory to write module files in as well as to
-search for them. gfortran is similar except it uses `-J` instead of `-module`.
+`-I` specifies directories to search for include files and module
+files.
+`-J`, and its alias `-module-dir`, specify a directory into which module files are written
+as well as to search for them.
 
-The search order for module files is:
-1. The `-module` directory (Note: for gfortran the `-J` directory is not searched).
-2. The current directory
-3. The `-I` directories in the order they appear on the command line
+`-fintrinsic-modules-path` is available to specify an alternative location
+for Fortran's intrinsic modules.
 
 ### Writing module files
 
 When writing a module file, if the existing one matches what would be written,
 the timestamp is not updated.
 
-Module files will be written after semantics, i.e. after the compiler has
-determined the module is valid Fortran.<br>
-**NOTE:** PGI does create `.mod` files sometimes even when the module has a
-compilation error.
-
-Question: If the compiler can get far enough to determine it is compiling a module
-but then encounters an error, should it delete the existing `.mod` file?
-PGI does not, gfortran does.
+Module files are written only after semantic analysis completes without
+a fatal error message.
 
 ### Reading module files
 
 When the compiler finds a `.mod` file it needs to read, it firsts checks the first
-line and verifies it is a valid module file. It can also verify checksums of
-modules it depends on and report if they are out of date.
-
-If the header is valid, the module file will be run through the parser and name
-resolution to recreate the symbols from the module. Once the symbol table is
-populated the parse tree can be discarded.
+line and verifies it is a valid module file.
+The header checksum must match the file's contents.
+(Pro tip: if a developer needs to hack the contents of a module file, they can
+recompile it afterwards as Fortran source to regenerate it with its new hash.)
+
+The known hashes of dependent modules are used to disambiguate modules whose
+names match module files in multiple search directories, as well as to
+detect dependent modules whose recompilation has rendered a module file
+obsolete.
+
+The hash codes used in module files also serve as a means of protection from
+updates to code in other packages.
+If a project A uses module files from package B, and package B is updated in
+a way that causes its module files to be updated, then the modules in A that
+depend on those modules in B will no longer be accepted for use until they
+have also been regenerated.
+This feature can catch errors that other compilers cannot.
 
 When processing `.mod` files we know they are valid Fortran with these properties:
 1. The input (without the header) is already in the "cooked input" format.
@@ -155,15 +153,7 @@ When processing `.mod` files we know they are valid Fortran with these propertie
 ## Error messages referring to modules
 
 With this design, diagnostics can refer to names in modules and can emit a
-normalized declaration of an entity but not point to its location in the
-source.
-
-If the header includes the source file it came from, that could be included in
-a diagnostic but we still wouldn't have line numbers.
-
-To provide line numbers and character positions or source lines as the user
-wrote them we would have to save some amount of provenance information in the
-module file as well.
+normalized declaration of an entity.
 
 ## Hermetic modules files