The Structure of the GNAT Compiler

A target-independent Ada95 front-end for GCC Ada components C components SyntaxSemExpandgigiGCC AST Annotated AST GCC tree Your Project!

Lexical Scanner Hand-written for speed Must handle different encodings for extended character sets (ISO642). Input file is read in full to minimize I/O Data structure: global name table Scanner is subroutine for parser, delivers tokens on demand Preprocessor may be integrated with scanner (but Ada has no defined preprocessor).

Parser Hand-written for speed and error recovery Organized according to chapters of the language reference (ARM): par-ch3 handles declarations, par-ch9 handles tasking, par-ch12 handles generics, etc. Recursive-descent with lookahead Follows exactly ARM grammar (ambiguous!) Builds AST: main data structure for front-end Symbol table is integrated into AST.

Semantic Analysis Legality rules (1436 error and warning messages) Tree annotations: add structure to AST to localize semantic information Name resolution (which x?) Type and overload resolution (which “+”?) Dispatching and polymorphism Static expression evaluation Simple optimizations: dead code elimination, constant folding, static conditions.

Expansion Replace complex constructs with simpler one (map Ada semantics into C semantics) Aggregates: (1..10 => 42, others => 0) Equality on composite types: if A (x.. y) = B (x..y) then.. Tasking (thread communication) Many others… Expander builds AST fragments, calls semantics to annotate them: process is recursive

Gigi: GNAT to GCC Impractical to use GCC data structures in front- end Gigi traverses GNAT tree fragments and calls tree-building procedures (just like other GCC front-ends) Each tree fragment is immediately translated into Register Transfer Language (RTL) Gigi must reflect Ada semantics (syntax and semantic information packages) and must interface to GCC generators: written in C.

Other components Library management: program is assembled from compilation of multiple files. Need tools to insure coherence (like make) gather object files, record dependencies between units. gnatbind, gnatlink Runtime management: Input/Output, numeric libraries, tasking, real time, distributed computing.

The data structures of the AST Nodes are variant records of fixed size. Each variant corresponds to one non-terminal A node has at most 4 syntactic descendants A descendant is either a node or a list Terminal nodes are identifiers and literals Node kind is discriminant: determines meaning of descendants for each node 220 kinds: Compilation_unit, package_declaration…loop_statement… Syntactic structure described in sinfo.ads

Semantic annotations Each tree node carries semantic information Node-specific: expressions have types procedures have scope array types have index and component types Literals have a value Full description in einfo.ads

Symbol table is integrated into AST Identifiers in declarations are special: defining_occurrence accumulates semantic information for an entity References to an identifier are pointers to corresponding defining_occurrence: Entity. Symbol table is set of defining_occurrences. Identifiers point to names table Symbol table contains no strings