The Structure of the GNAT Compiler

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The Structure of the GNAT Compiler
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A target-independent Ada95 front-end for GCC

• Ada components
  
• C components

GCC tree
Syntax
Sem
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gigi
GCC
AST
Annotated AST
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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).

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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.

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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.

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Expansion

• Replace complex constructs with simpler one (map
  Ada semantics into C semantics)
• Aggregates (1..10 gt 42, others gt 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

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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.

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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.

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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_declarationloop_stateme
  nt
• Syntactic structure described in sinfo.ads

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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

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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