PEGO
G040: Copy-on-Change Attribute Normalization
Preparing a grammar should not copy every expression node when its attributes and children already have the normalized form. Copy-on-change reuses unchanged paths.
Mechanism and Invariants
NormalizeAttributes returns an original node when no supported child or
attribute changes. A changed descendant requires copying each modified ancestor
before replacement, preserving the source AST. Attribute order, binding,
validation, diagnostics and legacy nil/empty container shapes remain exact.
Opaque extension values remain opaque. The prepared grammar can share unchanged
nodes with its source; it is not a detached deep copy.
Scope and State
Default shared compilation/generation preprocessing. Matching in all six backends is unchanged. No standalone runtime attribute-normalization path is implied.
Reference Control
Internal compilation option EagerAttributeCopies restores the eager same-source
normalizer. Selection occurs during compilation and adds no matching-loop
branch.
Evidence and Implementation
Record 095 reports workload-specific allocation reductions and a longer recheck that did not reproduce an initial synthetic slowdown. Generated parsers remain byte-identical. These compilation measurements do not establish a universal preparation speedup or a matching benefit.
Evidence records: 095.
Source owners:
NormalizeAttributes,
Options.EagerAttributeCopies, source immutability and reference checks.
The comparison policy distinguishes same-source controls, historical changes and full checkpoints.