One-Pass, Optimal Tree Parsing - With Or Without Trees

This paper describes the theory behind and implementation of wburg, a code-generator generator that accepts tree grammars as input and produces a code generator that emits an optimal parse of an IR tree in just a single bottom-up pass. Furthermore, wburg eliminates the need for an explicit IR tree altogether. The grammars that wburg-generated parsers can parse are a proper subset of those that two-pass systems can handle. However, analysis indicates that wburg can optimally handle grammars for most instruction sets (e.g., SPARC, MIPS R3000, and x86). 1 I n t r o d u c t i o n Compilers often use intermediate representation (IR) trees to represent expressions. A compiler's front end generates an IR tree, and the back end walks the tree, emitting appropriate assembly language instructions and operands. Several automatically generated code generators perform pattern matching on IR trees to emit optimal code. The code-generator generator consumes a costaugmented tree grammar with associated semantic actions. The resulting code generator requires two passes over the IR tree to determine a least-cost parse and to execute the associated semantic actions. Code-generator generators based on this model include BEG [ESL89], twig [AGT89], burg [FHP92b, Pro92], i bu rg [FHP92a], and i b u r g [FH95]. This paper describes the theory behind and implementation of wburg, a codegenerator generator that accepts tree grammars as input and produces a code generator that emits an optimal parse of an IR tree in just a single bot tom-up pass. Furthermore, wburg eliminates the need for an explicit IR tree altogether. The generated parser emits optimal code, and can do so without retaining an entire IR tree during its single pass. The grammars that wburg-generated parsers can parse are a proper subset of those that the two-pass systems can handle. However, analysis indicates that wburg can optimally handle grammars for most major instruction sets, including the SPARC, the MIPS R3000, and the x86.