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refactor: low hanging perf fixes to avoid v8 deopt #548

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720bbd7
refactor: low hanging perf fixes to avoid v8 deopt
barelyhuman Feb 3, 2026
e95070d
fix: existing behavioral tests
barelyhuman Feb 3, 2026
e101861
chore: reduce mem alloc for UpperCase
barelyhuman Feb 3, 2026
3df0a35
fix: optimize replaceInputRange for better performance
barelyhuman Feb 3, 2026
7e7e33e
refactor: manuall array allocation for `eval` hotpath
barelyhuman Feb 4, 2026
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39 changes: 33 additions & 6 deletions packages/ohm-js/src/Matcher.js
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Original file line number Diff line number Diff line change
Expand Up @@ -29,6 +29,14 @@ export class Matcher {
replaceInputRange(startIdx, endIdx, str) {
const prevInput = this._input;
const memoTable = this._memoTable;

// Grammar.match() creates a fresh Matcher and calls replaceInputRange(0, 0, input).
// In that scenario, there is nothing to preserve or invalidate, so avoid extra work.
if (startIdx === 0 && endIdx === 0 && prevInput.length === 0 && memoTable.length === 0) {
this._input = '' + str;
memoTable.length = str.length;
return this;
}
if (
startIdx < 0 ||
startIdx > prevInput.length ||
Expand All @@ -46,13 +54,32 @@ export class Matcher {
}

// update memo table (similar to the above)
const restOfMemoTable = memoTable.slice(endIdx);
memoTable.length = startIdx;
for (let idx = 0; idx < str.length; idx++) {
memoTable.push(undefined);
const oldLen = memoTable.length;
const deleteCount = endIdx - startIdx;
const insertCount = str.length;
const delta = insertCount - deleteCount;
const newLen = oldLen + delta;

// Move the tail segment to its new location.
if (oldLen > endIdx) {
if (delta > 0) {
// Grow first to make room, then shift the tail right.
memoTable.length = newLen;
memoTable.copyWithin(startIdx + insertCount, endIdx, oldLen);
} else if (delta < 0) {
// Shift the tail left first, then shrink (shrinking first would drop tail entries).
memoTable.copyWithin(startIdx + insertCount, endIdx, oldLen);
memoTable.length = newLen;
} else {
memoTable.copyWithin(startIdx + insertCount, endIdx, oldLen);
}
} else if (delta !== 0) {
memoTable.length = newLen;
}
for (const posInfo of restOfMemoTable) {
memoTable.push(posInfo);

// Clear the inserted range.
if (insertCount > 0) {
memoTable.fill(undefined, startIdx, startIdx + insertCount);
}

// Invalidate memoRecs
Expand Down
2 changes: 1 addition & 1 deletion packages/ohm-js/src/PosInfo.js
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Original file line number Diff line number Diff line change
Expand Up @@ -89,7 +89,7 @@ export class PosInfo {
Object.keys(memo).forEach(k => {
const memoRec = memo[k];
if (pos + memoRec.examinedLength > invalidatedIdx) {
delete memo[k];
memo[k] = undefined;
} else {
this.maxExaminedLength = Math.max(this.maxExaminedLength, memoRec.examinedLength);
this.maxRightmostFailureOffset = Math.max(
Expand Down
12 changes: 6 additions & 6 deletions packages/ohm-js/src/Semantics.js
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Expand Up @@ -341,7 +341,7 @@ export class Semantics {
const thisThing = this._semantics[typePlural][name];

// Check that the caller passed the correct number of arguments.
if (arguments.length !== thisThing.formals.length) {
if (args.length !== thisThing.formals.length) {
throw new Error(
'Invalid number of arguments passed to ' +
name +
Expand All @@ -358,9 +358,9 @@ export class Semantics {
// Create an "arguments object" from the arguments that were passed to this
// operation / attribute.
const argsObj = Object.create(null);
for (const [idx, val] of Object.entries(args)) {
for (let idx = 0; idx < args.length; idx += 1) {
const formal = thisThing.formals[idx];
argsObj[formal] = val;
argsObj[formal] = args[idx];
}

const oldArgs = this.args;
Expand Down Expand Up @@ -569,12 +569,12 @@ Semantics.createSemantics = function (grammar, optSuperSemantics) {
let semantic;
if (operationOrAttributeName in s.operations) {
semantic = s.operations[operationOrAttributeName];
delete s.operations[operationOrAttributeName];
s.operations[operationOrAttributeName] = undefined;
} else if (operationOrAttributeName in s.attributes) {
semantic = s.attributes[operationOrAttributeName];
delete s.attributes[operationOrAttributeName];
s.attributes[operationOrAttributeName] = undefined;
}
delete s.Wrapper.prototype[operationOrAttributeName];
s.Wrapper.prototype[operationOrAttributeName] = undefined;
return semantic;
};
proxy.getOperationNames = function () {
Expand Down
157 changes: 137 additions & 20 deletions packages/ohm-js/src/common.js
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Original file line number Diff line number Diff line change
Expand Up @@ -4,19 +4,136 @@

// Helpers

const escapeStringFor = {};
for (let c = 0; c < 128; c++) {
escapeStringFor[c] = String.fromCharCode(c);
}
escapeStringFor["'".charCodeAt(0)] = "\\'";
escapeStringFor['"'.charCodeAt(0)] = '\\"';
escapeStringFor['\\'.charCodeAt(0)] = '\\\\';
escapeStringFor['\b'.charCodeAt(0)] = '\\b';
escapeStringFor['\f'.charCodeAt(0)] = '\\f';
escapeStringFor['\n'.charCodeAt(0)] = '\\n';
escapeStringFor['\r'.charCodeAt(0)] = '\\r';
escapeStringFor['\t'.charCodeAt(0)] = '\\t';
escapeStringFor['\u000b'.charCodeAt(0)] = '\\v';
const escapeStringFor = {
0: '\x00',
1: '\x01',
2: '\x02',
3: '\x03',
4: '\x04',
5: '\x05',
6: '\x06',
7: '\x07',
8: '\\b',
9: '\\t',
10: '\\n',
11: '\\v',
12: '\\f',
13: '\\r',
14: '\x0E',
15: '\x0F',
16: '\x10',
17: '\x11',
18: '\x12',
19: '\x13',
20: '\x14',
21: '\x15',
22: '\x16',
23: '\x17',
24: '\x18',
25: '\x19',
26: '\x1A',
27: '\x1B',
28: '\x1C',
29: '\x1D',
30: '\x1E',
31: '\x1F',
32: ' ',
33: '!',
34: '\\"',
35: '#',
36: '$',
37: '%',
38: '&',
39: "\\'",
40: '(',
41: ')',
42: '*',
43: '+',
44: ',',
45: '-',
46: '.',
47: '/',
48: '0',
49: '1',
50: '2',
51: '3',
52: '4',
53: '5',
54: '6',
55: '7',
56: '8',
57: '9',
58: ':',
59: ';',
60: '<',
61: '=',
62: '>',
63: '?',
64: '@',
65: 'A',
66: 'B',
67: 'C',
68: 'D',
69: 'E',
70: 'F',
71: 'G',
72: 'H',
73: 'I',
74: 'J',
75: 'K',
76: 'L',
77: 'M',
78: 'N',
79: 'O',
80: 'P',
81: 'Q',
82: 'R',
83: 'S',
84: 'T',
85: 'U',
86: 'V',
87: 'W',
88: 'X',
89: 'Y',
90: 'Z',
91: '[',
92: '\\\\',
93: ']',
94: '^',
95: '_',
96: '`',
97: 'a',
98: 'b',
99: 'c',
100: 'd',
101: 'e',
102: 'f',
103: 'g',
104: 'h',
105: 'i',
106: 'j',
107: 'k',
108: 'l',
109: 'm',
110: 'n',
111: 'o',
112: 'p',
113: 'q',
114: 'r',
115: 's',
116: 't',
117: 'u',
118: 'v',
119: 'w',
120: 'x',
121: 'y',
122: 'z',
123: '{',
124: '|',
125: '}',
126: '~',
127: '\x7F',
};

// --------------------------------------------------------------------
// Exports
Expand Down Expand Up @@ -59,7 +176,7 @@ export function defineLazyProperty(obj, propName, getterFn) {

export function clone(obj) {
if (obj) {
return Object.assign({}, obj);
return {...obj};
}
return obj;
}
Expand All @@ -81,14 +198,14 @@ export function repeat(x, n) {
}

export function getDuplicates(array) {
const duplicates = [];
const duplicates = new Set();
for (let idx = 0; idx < array.length; idx++) {
const x = array[idx];
if (array.lastIndexOf(x) !== idx && duplicates.indexOf(x) < 0) {
duplicates.push(x);
if (array.lastIndexOf(x) !== idx && !duplicates.has(x)) {
duplicates.add(x);
}
}
return duplicates;
return [...duplicates];
}

export function copyWithoutDuplicates(array) {
Expand All @@ -102,8 +219,8 @@ export function copyWithoutDuplicates(array) {
}

export function isSyntactic(ruleName) {
const firstChar = ruleName[0];
return firstChar === firstChar.toUpperCase();
const code = ruleName.charCodeAt(0);
return code >= 65 && code <= 90;
}

export function isLexical(ruleName) {
Expand Down
26 changes: 17 additions & 9 deletions packages/ohm-js/src/pexprs-eval.js
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Expand Up @@ -133,12 +133,15 @@ pexprs.Iter.prototype.eval = function (state) {
}
prevPos = inputStream.pos;
numMatches++;
const row = state._bindings.splice(state._bindings.length - arity, arity);
const rowOffsets = state._bindingOffsets.splice(
state._bindingOffsets.length - arity,
arity
);
for (idx = 0; idx < row.length; idx++) {
// Avoid using `splice` which allocates an array; use repeated `pop` which is faster
// in hot loops and preserves insertion order by filling from the end.
const row = new Array(arity);
const rowOffsets = new Array(arity);
for (let j = arity - 1; j >= 0; j--) {
row[j] = state._bindings.pop();
rowOffsets[j] = state._bindingOffsets.pop();
}
for (idx = 0; idx < arity; idx++) {
cols[idx].push(row[idx]);
colOffsets[idx].push(rowOffsets[idx]);
}
Expand Down Expand Up @@ -222,7 +225,7 @@ pexprs.Apply.prototype.eval = function (state) {
if (state.hasNecessaryInfo(memoRec)) {
return state.useMemoizedResult(state.inputStream.pos, memoRec);
}
delete posInfo.memo[memoKey];
posInfo.memo[memoKey] = undefined;
}
return app.reallyEval(state);
};
Expand Down Expand Up @@ -336,8 +339,13 @@ pexprs.Apply.prototype.evalOnce = function (expr, state) {

if (state.eval(expr)) {
const arity = expr.getArity();
const bindings = state._bindings.splice(state._bindings.length - arity, arity);
const offsets = state._bindingOffsets.splice(state._bindingOffsets.length - arity, arity);
// Avoid Array.prototype.splice allocation in hot path; use pop into preallocated arrays
const bindings = new Array(arity);
const offsets = new Array(arity);
for (let j = arity - 1; j >= 0; j--) {
bindings[j] = state._bindings.pop();
offsets[j] = state._bindingOffsets.pop();
}
const matchLength = inputStream.pos - origPos;
return new NonterminalNode(this.ruleName, bindings, offsets, matchLength);
} else {
Expand Down