108. Prologue and Rollback Helpers

• Status: Implemented
• Date: 2026-10-22

#Problem and Mechanism

Every generated rule body began with the same four lines (count one more rule call, fail beyond the nesting limit) and every backtracking point and failure exit ended with the same two statements (return to the saved position, drop the errors recovered since). Together they were a large part of the generated source.

The runtime now has Parser.Enter (the depth count and the nesting check) and Parser.Restore (the position and the recovered errors), and the generator calls them. Both are inlinable, so the compiled code of a call site is what the statements were; runtime/inline_test.go fails if either stops being inlinable.

#Results

Bytes of parser.go with -typed-only:

The duckdb test binary grows from 17.76 MB to 18.26 MB (+2.8%): the machine code is the same (+0.4% text), and each inlined call adds an inline-tree entry to the function tables the binary keeps. The cold compile time of the duckdb test binary (best of three, clean build cache) is unchanged at 6.7 to 6.9 s. Parse speed (30 interleaved rounds, minimum, with an identical copy as the noise control) is within noise: python -1.1% (control -0.5%), cue -0.1% (control +1.6%), duckdb -0.2% (control -0.2%), at most 1.7 points from the control.

#Decision and Limits

Adopted. Making Restore a call instead (not inlined) shrinks the binary by a further 0.8% but costs 1.6% to 3% of parse time on the language grammars, so the helpers stay inlinable.