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perf(gsub): writeChainFormat3 coverage 槽改 writeInt16At 回填

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type6 format3 的 coverage 偏移槽原用 allHolders: number[][] + reserveOffset16
闭包模式（每槽 [0] 单元素数组 + patches push + flush 回填）。改为 writeUint16(0)
占位 + covSlotPositions 记录字节位置 + writeInt16At 直接回填（与优化329 subtable
slot 同思路），消除每槽的 [0] 数组分配 + reserveOffset16 闭包。

node --prof 显示 GrowFastSmiOrObjectElements 3.7% + ArrayPrototypePush 3.2%
主来自 writeChainFormat3/serializeChainedContextSubst 的 allHolders grow。
FiraCode subsetGSUB min 0.229→0.216ms（-5.7%），端到端 avg 3.4→2.9ms（-15%），
29 用例输出字节全相同 SSIM 一致。

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
This commit is contained in:
崮生（子虚） 2026-07-25 07:39:06 +08:00
parent e8989c7fa4
commit 98102bc3be
1 changed files with 15 additions and 11 deletions
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26
backend/font_util/gsub-subset.ts
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@ -1209,14 +1209,17 @@ function writeChainFormat3(
parsed: { backCovs: number[][]; inputCovs: number[][]; lookCovs: number[][]; records: Array<{ seq: number; lookup: number }> },
): void {
const subStart = w.length;
const allHolders: number[][] = [];
/** 预留一个 Offset16 槽，flush 时回填 allHolders[slotIdx] 的值。
* 必须用 IIFE 捕获当前 slotIdx——闭包直接引用 allHolders.length-1 会在 flush 时（循环已结束）
* 统一取到最后一个槽，导致所有 coverage 偏移指向同一个 coverage（FiraCode === 连字断裂的根因）。 */
/** 每个 coverage 偏移槽的字节位置（writeUint16(0) 占位，emitCoverage 后 writeInt16At 回填）。
* 替代旧 allHolders: number[][] + reserveOffset16 闭包模式：消除每槽的 [0] 单元素数组分配 +
* reserveOffset16 的 patches 闭包 push + flush 回填遍历（优化329 同思路，对 chain coverage slot 复用）。
* coverage 槽数 = backCovs + inputCovs + lookCovs，一次性预分配容量避免 grow。 */
const totalCovSlots = parsed.backCovs.length + parsed.inputCovs.length + parsed.lookCovs.length;
const covSlotPositions: number[] = new Array<number>(totalCovSlots);
/** 占位写入 coverage 偏移槽并记录其字节位置 */
let slotIdx = 0;
const reserveCovSlot = () => {
const slotIdx = allHolders.length;
allHolders.push([0]);
w.reserveOffset16(subStart, () => allHolders[slotIdx][0]);
covSlotPositions[slotIdx++] = w.length;
w.writeUint16(0);
};
w.writeUint16(3);
w.writeUint16(parsed.backCovs.length);
@ -1231,15 +1234,16 @@ function writeChainFormat3(
w.writeUint16(rc.lookup);
}
let holderIdx = 0;
/** 写出各组 coverage 并回填偏移槽（相对 subStart） */
let posIdx = 0;
for (const cov of parsed.backCovs) {
allHolders[holderIdx++][0] = emitCoverage(w, cov);
w.writeInt16At(covSlotPositions[posIdx++], emitCoverage(w, cov) - subStart);
}
for (const cov of parsed.inputCovs) {
allHolders[holderIdx++][0] = emitCoverage(w, cov);
w.writeInt16At(covSlotPositions[posIdx++], emitCoverage(w, cov) - subStart);
}
for (const cov of parsed.lookCovs) {
allHolders[holderIdx++][0] = emitCoverage(w, cov);
w.writeInt16At(covSlotPositions[posIdx++], emitCoverage(w, cov) - subStart);
}
}