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perf(gsub-reachable): type1 fmt1 + type3 条件反转遍历方向

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延续 9d6e982（type1 fmt2 反转）的思路，对 SingleSubst format1 与
AlternateSubst（type3）同样反转遍历方向：遍历 reachable 二分查 covGids
（按 gid 升序）得 index，替代遍历 covGids 逐个查 reachable。

初夏明朝纯标点实测 fmt1 covLen 405、type3 covLen 987（命中率 2.3%），
均属「展开全量命中极少」模式，反转后遍历量从 covGids.length 降到
reachable.size。

关键：加条件 covGids.length > reachable.size 才反转。无条件反转在
霞鹜/FiraCode 上负优化（covGids 短，FiraCode type1 avg 5.5，二分开销
> Set.has 遍历）。条件反转使短 covGids 走原 Set.has 路径。

type4（Ligature）不反转：firstHit 判定反转后遍历 reachable（53）× 18
calls = 954 次二分 > 原 684 次 Set.has，必负。

A/B vs 9d6e982（3000 次 p50，reachable sha 全一致）：
- 初夏标点 102→92μs（-10%）
- 初夏8字 54→51μs（-5%）
- 霞鹜标点 12→10μs（-12%，无条件版 +8% 已修正）
- 霞鹜8字 6.8→6.4μs（-7%）
- FiraCode 标点 70→69μs（-1%，无条件版 +2% 已修正）

相比最初 baseline（fee9e49 前）初夏标点 129→92μs（-29%）。基准测试
SSIM 全不变。

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
This commit is contained in:
崮生（子虚） 2026-07-24 17:07:13 +08:00
parent 9d6e982682
commit f053112e44
1 changed files with 59 additions and 18 deletions
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77
backend/font_util/gsub-reachable.ts
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@ -271,25 +271,46 @@ function collectSubtableTargets(
const covGids = readCoverageGids(r, covOff, covCache);
if (format === 1) {
const delta = r.i16(off + 4);
for (const g of covGids) {
if (inSubset(g)) targets.push((g + delta) & 0xffff);
/** 条件反转：仅当 covGids 明显多于 reachable 时反转遍历方向（遍历小集合二分查大集合）。
* covGids 短（FiraCode type1 avg 5.5）时原 Set.has 路径更快（二分开销 > 遍历）。 */
if (covGids.length > reachable.size) {
const lim = covGids.length;
for (const g of reachable) {
let lo = 0, hi = lim;
while (lo < hi) {
const mid = (lo + hi) >> 1;
const mg = covGids[mid];
if (mg < g) lo = mid + 1;
else if (mg > g) hi = mid;
else { targets.push((g + delta) & 0xffff); break; }
}
}
} else {
for (const g of covGids) {
if (inSubset(g)) targets.push((g + delta) & 0xffff);
}
}
} else if (format === 2) {
/** 反转遍历方向：coverage（avg 103 gid，命中率 <4%）远大于 reachable（初夏标点 53）。
* 遍历 covGids 查 reachable 是 9180 次 Set.has/round；改为遍历 reachable 二分查 covGids 得 index。
/** 条件反转遍历方向：coverage 命中率低（初夏 fmt2 covLen 9180/round，reachable 53，命中率<4%）。
* 仅当 covGids 明显多于 reachable 时遍历 reachable 二分查 covGids 得 index；否则原 Set.has 路径。
* covGids 按 gid 升序（fmt1 list / fmt2 range 展开），可二分。 */
const count = r.u16(off + 4);
const lim = covGids.length < count ? covGids.length : count;
const gidArrBase = off + 6;
for (const g of reachable) {
/** 二分 covGids[0..lim) 找 g 的 index */
let lo = 0, hi = lim;
while (lo < hi) {
const mid = (lo + hi) >> 1;
const mg = covGids[mid];
if (mg < g) lo = mid + 1;
else if (mg > g) hi = mid;
else { targets.push(r.u16(gidArrBase + mid * 2)); break; }
if (lim > reachable.size) {
for (const g of reachable) {
let lo = 0, hi = lim;
while (lo < hi) {
const mid = (lo + hi) >> 1;
const mg = covGids[mid];
if (mg < g) lo = mid + 1;
else if (mg > g) hi = mid;
else { targets.push(r.u16(gidArrBase + mid * 2)); break; }
}
}
} else {
for (let i = 0; i < lim; i++) {
if (inSubset(covGids[i])) targets.push(r.u16(gidArrBase + i * 2));
}
}
}
@ -304,14 +325,34 @@ function collectSubtableTargets(
for (let k = 0; k < glyphCount; k++) targets.push(r.u16(seqOff + 2 + k * 2));
}
} else if (type === LT_ALTERNATE) {
/** 条件反转：初夏 type3 covLen 987/3 calls（avg 329），命中率 2.3%；covGids 短时走原路径。 */
const covOff = off + r.u16(off + 2);
const altCount = r.u16(off + 4);
const covGids = readCoverageGids(r, covOff, covCache);
for (let i = 0; i < covGids.length && i < altCount; i++) {
if (!inSubset(covGids[i])) continue;
const altOff = off + r.u16(off + 6 + i * 2);
const cnt = r.u16(altOff);
for (let k = 0; k < cnt; k++) targets.push(r.u16(altOff + 2 + k * 2));
const lim = covGids.length < altCount ? covGids.length : altCount;
if (covGids.length > reachable.size) {
for (const g of reachable) {
let lo = 0, hi = lim;
while (lo < hi) {
const mid = (lo + hi) >> 1;
const mg = covGids[mid];
if (mg < g) lo = mid + 1;
else if (mg > g) hi = mid;
else {
const altOff = off + r.u16(off + 6 + mid * 2);
const cnt = r.u16(altOff);
for (let k = 0; k < cnt; k++) targets.push(r.u16(altOff + 2 + k * 2));
break;
}
}
}
} else {
for (let i = 0; i < lim; i++) {
if (!inSubset(covGids[i])) continue;
const altOff = off + r.u16(off + 6 + i * 2);
const cnt = r.u16(altOff);
for (let k = 0; k < cnt; k++) targets.push(r.u16(altOff + 2 + k * 2));
}
}
} else if (type === LT_LIGATURE) {
/** LigatureSubst: 全部 component 在子集 → target gid */