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Estimated hours taken: 0.1 COMPILER_DESIGN: Documented the fact that HLDS is now defined in four files.
390 lines
13 KiB
Plaintext
390 lines
13 KiB
Plaintext
-----------------------------------------------------------------------------
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This file contains various notes about the design of the compiler.
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-----------------------------------------------------------------------------
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OUTLINE
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The top-level of the compiler is in the file mercury_compile.m.
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The basic design is that compilation is broken into the following
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stages:
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1. parsing (source files -> HLDS)
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2. semantic analysis and error checking (HLDS -> annotated HLDS)
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3. high-level transformations (annotated HLDS -> annotated HLDS)
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4. code generation (annotated HLDS -> LLDS)
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5. low-level optimizations (LLDS -> LLDS)
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6. output C code (LLDS -> C)
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Note that in reality the seperation is not quite as simple as that.
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Although parsing is listed as step 1 and semantic analysis is listed
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as step 2, the last stage of parsing actually includes some semantic checks.
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And although optimization is listed as steps 3 and 5, it also occurs in
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steps 2, 4, and 6. For example, elimination of assignments to dead
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variables is done in mode analysis; middle-recursion optimization and
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the use of static constants for ground terms is done in code
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generation; and a few low-level optimizations are done in llds.m
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as we are spitting out the C code.
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-----------------------------------------------------------------------------
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DETAILED DESIGN (well, more detailed than the OUTLINE anyway ;-)
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The action is co-ordinated from mercury_compile.m.
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0. Option handling
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The command-line options are defined in the module options.m.
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mercury_compile.pp calls library/getopt.m, passing the predicates
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defined in options.m as arguments, to parse them. It then invokes
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handle_options.m to postprocess the option set. The results are
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stored in the io__state, using the type globals defined in globals.m.
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1. Parsing
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* lexical analysis (library/lexer.m)
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* stage 1 parsing - convert strings to terms.
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library/parser.m contains the code to do this, while
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library/term.m and library/varset.m contain the term and varset
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data structures that result, and predicates for manipulating them.
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* stage 2 parsing - convert terms to `items' (declarations, clauses, etc.)
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The result of this stage is a parse tree that has a one-to-one
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correspondence with the source code. Both the parse tree data
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structure and code to create it are in prog_io.m. The module
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prog_out.m and mercury_to_mercury.m contain predicates for
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printing the parse tree.
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prog_util.m contains some utility predicates for manipulating
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the parse tree.
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* imports and exports are handled at this point (modules.m)
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modules.m has the code to write out `.int', `.int2', `.d'
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and `.dep' files.
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* expansion of equivalence types (prog_util.m)
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This is really part of type-checking, but is done
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on the item_list rather than on the HLDS because it
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turned out to be much easier to implement that way.
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* simplification
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make_hlds.m transforms the code into superhomogenous form,
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and at the same time converts the parse tree into the HLDS.
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make_hlds.m also calls make_tags.m which chooses the data
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representation for each discriminated union type by
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assigning tags to each functor.
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The result at this stage is the High Level Data Structure,
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which is defined in four files:
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- hlds_data.m defines the parts of the HLDS concerned with
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function symbols, types, insts, modes and determinisms;
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- hlds_goal.m defines the part of the HLDS concerned with the
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structure of goals, including the annotations on goals;
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- hlds_pred.m defines the part of the HLDS concerning
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predicates and procedures;
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- hlds_module.m defines the top-level parts of the HLDS,
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including the type module_info.
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The module hlds_out.m contains predicates to dump the HLDS to a file.
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The module goal_util.m contains predicates for renaming variables
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in an HLDS goal.
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2. Semantic analysis and error checking
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* implicit quantification
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quantification.m handles implicit quantification and computes
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the set of non-local variables for each sub-goal
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* type checking
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- undef_types.m checks for undefined types.
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- typecheck.m handles type checking, overloading resolution &
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module name resolution, and fully qualifies all predicate and
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functor names. It sets the map(var, type) field in the pred_info.
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When it has finished, typecheck.m calls
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clause_to_proc.m to make duplicate copies of the clauses for
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each different mode of a predicate; all later stages work on
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procedures, not predicates.
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- type_util.m contains utility predicates dealing with types
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that are used in a variety of different places within the compiler
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* mode analysis
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- undef_modes.m checks for undefined insts and modes
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- modes.m is the main mode analysis module.
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It checks that the code is mode-correct, reordering it
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if necessary, and annotates each goal with a delta-instmap
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that specifies the changes in instantiatedness of each
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variable over that goal. It also converts higher-order
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pred terms into lambda expressions.
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It use the following sub-modules:
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mode_info.m (the main data structure for mode analysis)
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delay_info.m (a sub-component of the mode_info data
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structure used for storing the information
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for scheduling: which goals are currently
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delayed, what variables they are delayed on, etc.)
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inst_match.m
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This contains the code for dealing with insts:
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abstractly unifying them, checking whether two
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insts match, etc.
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mode_errors.m
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This module contains all the code to
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print error messages for mode errors
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- mode_util.m contains miscellaneous useful predicates dealing
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with modes (many of these are used by lots of later stages
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of the compiler)
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* indexing and determinism analysis
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switch detection (switch_detection.m),
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common goal hoisting (cse_detection.m)
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Note that if cse_detection.m modifies the code,
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it will re-run mode analysis and switch detection
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determinism analysis (det_analysis.m and det_report.m)
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* checking of unique modes (unique_modes.m)
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unique_modes.m checks that non-backtrackable unique modes were
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not used in a context which might require backtracking.
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Note that what unique_modes.m does is quite similar to
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what modes.m does, and unique_modes calls lots of predicates
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defined in modes.m to do it.
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3. High-level transformations
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The first two passes of this stage are code simplifications.
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* introduction of type_info arguments for polymorphic predicates
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(polymorphism.m)
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* removal of lambda expressions (lambda.m)
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lambda.m converts lambda expressions into higher-order predicate
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terms referring to freshly introduced separate predicates.
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This pass needs to come after unique_modes.m to ensure that
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the modes we give to the introduced predicates are correct.
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It also needs to come after polymorphism.m since polymorphism.m
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doesn't handle higher-order predicate constants.
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To improve efficiency, the above two passes are actually combined into
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one - polymorphism.m calls calls lambda__transform_lambda directly.
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The remaining HLDS-to-HLDS transformations are optimizations:
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* specialization of higher-order predicates where the value of the
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higher-order arguments are known (higher_order.m)
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* inlining (i.e. unfolding) of simple procedures (inlining.m)
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* detection of common terms constructions (common.m)
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common.m looks for construction unifications which
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constructs a term that is the same as one that already exists,
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and replaces them with assignment unifications
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* constraint propagation (constraint.m)
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Not yet working.
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* issue warnings about unused arguments from predicates, and create
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specialized versions without them (unused_args.m); type_infos are
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often unused
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* elimination of dead procedures (dead_proc_elim.m); inlining, higher-order
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specialization and the elimination of unused args can make procedures dead
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even the user doesn't
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* removal of excess assignment unifications (excess.m)
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* migration of builtins following branched structures (follow_code.m)
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The module transform.m contains stuff that is supposed to be useful
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for high-level optimizations (but which is not yet used).
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4. Code generation
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* pre-passes to annotate the HLDS
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Before code generation there are a few more passes which
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annotate the HLDS with information used for code generation:
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choosing registers for procedure arguments (arg_info.m)
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Currently uses a very simple algorithm, but
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we may change this later.
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annotation of goals with liveness information (liveness.m)
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This records the birth and death of each variable
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in the HLDS goal_info.
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allocation of stack slots
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This is done by live_vars.m, which works
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out which variables need to be saved on the
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stack when, and then uses graph_colour.m to determine
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a good allocation of variables to stack slots.
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allocating the follow vars (follow_vars.m)
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Traverses backwards over the HLDS,
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annotating each call with the variable target
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locations for the following call, so we can
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generate efficient code by putting variables in
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the right spot.
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allocating the store map (store_alloc.m)
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Allocates locations for variables at the end of
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branched goals. Annotates the goal_info for
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each branched goal with allocation of variable
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target locations before so that we can generate
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correct code by putting variables in the same
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spot in each branch.
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* code generation
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For code generation itself, the main module is code_gen.pp.
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It handles conjunctions and negations, but calls sub-modules
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to do most of the other work:
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ite_gen.m (if-then-elses)
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call_gen.m (predicate calls and also calls to
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out-of-line unification procedures)
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disj_gen.m (disjunctions)
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unify_gen.m (unifications)
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switch_gen.m (switches), which has sub-modules
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dense_switch.m
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string_switch.m
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tag_switch.m
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It also calls middle_rec.m to do middle recursion optimization.
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The code generation modules make use of
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code_info.m
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The main data structure for the code generator
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code_aux.m
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Various preds which use code_info
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code_exprn.m
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This defines the exprn_info type, which is
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a sub-component of the code_info data structure
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which holds the information about
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the contents of registers and
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the values/locations of variables.
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exprn_aux.m
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Various preds which use exprn_info
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code_util.m
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Some miscellaneous preds used for code generation
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The result of code generation is the Low Level Data Structure (llds.m).
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The code is generated as a tree of code fragments which is then
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flattened (tree.m).
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5. Low-level optimization
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The various LLDS-to-LLDS optimizations are invoked from optimize.m.
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They are:
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* optimization of jumps to jumps (jumpopt.m)
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* elimination of duplicate code sequences (dupelim.m)
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* optimization of stack frame allocation/deallocation (frameopt.m)
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* dead code and dead label removal (labelopt.m)
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* value numbering
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This is done by value_number.m, which has the following sub-modules:
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vn_block.m
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vn_cost.m
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vn_debug.m
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vn_flush.m
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vn_order.m
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atsort.m ("approximate topological sort" predicates)
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vn_table.m
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vn_temploc.m
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vn_type.m
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vn_util.m
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Several of these modules (and also frameopt, above) use livemap.m.
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* peephole optimization (peephole.m)
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Depending on which optimization flags are enabled,
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optimize.m may invoke many of these passes multiple times.
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Some of the low-level optimization passes use opt_util.m, which
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contains miscellaneous predicates for LLDS-to-LLDS optimization.
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6. Output C code
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Final generation of C code is done in llds.m.
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-----------------------------------------------------------------------------
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MISCELLANEOUS
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special_pred.m, unify_proc.m:
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These modules contain stuff for handling the special
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compiler-generated predicates which are generated for
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each type: unify/2, compare/3, index/1 (used in the
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implementation of compare/3), and also type_to_term/2
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and term_to_type/2 (but those last two are disabled
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at the moment).
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dependency_graph.m:
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This contains predicates to compute the call graph for a
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module, and to print it out to a file.
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(The call graph file is used by the profiler.)
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The call graph may eventually also be used by det_analysis.m,
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inlining.m, and other parts of the compiler which need
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to traverse the predicates in a module bottom-up or top-down.
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passes_aux.m
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Contains write_progress_message, which is called
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from various places to write progress messages.
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opt_debug.m:
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Utility routines for debugging the LLDS-to-LLDS optimizations.
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-----------------------------------------------------------------------------
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CURRENTLY USELESS
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The following modules do not serve any function at the moment.
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Some of them are obsolete; other are work-in-progress.
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(For some of them its hard to say which!)
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utils.m
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Higher-order predicates. Has suffered from software rot.
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Should go in library/{list,map,...}.m anyway.
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swi_lib.m, swi_builtin.m:
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Support for SWI-Prolog. Incomplete.
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Should go in the library directory anyway.
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no_builtin.m:
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An empty module, for debugging purposes.
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nit_builtin.m:
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Support for `nit', the NU-Prolog incompetence tester.
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We don't use nit anymore - the Mercury compiler does
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a better job.
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mercury_to_goedel.m:
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This converts from item_list to Goedel source code.
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It works for simple programs, but doesn't handle
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various Mercury constructs such as lambda expressions,
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higher-order predicates, and functor overloading.
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mercury_to_c.m:
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The very incomplete beginnings of an alternate
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code generator. When finished, it will convert HLDS
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to high-level C code (without going via LLDS).
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shapes.m, garbage_out.m:
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These two modules are used for the native garbage collector.
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-----------------------------------------------------------------------------
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