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perf(woff2): triplet 编码无分支 abs + 符号位提取

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transformGlyfAndLoca 的 y+triplet 循环每点原先 4 次三元判断（absDx/absDy/xSign/ySign），
中文字体 dx/dy 正负近似 50/50 为不可预测分支，致流水线冲刷。

改用 Int32 算术右移统一提取符号：
- dxSgn = dx >> 31（0 或 -1）
- absDx = (dx ^ dxSgn) - dxSgn（无分支 abs）
- xSignBit = dxSgn + 1（等价 dx >= 0 ? 1 : 0）

令东 74.6% 点走 case3，收益最大：woff2 端到端 2.197→2.078ms（-5.4%），
思源 0.662→0.653ms（-1.4%）。sha256 字节级一致，全基准 SSIM 零变化。

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
This commit is contained in:
崮生（子虚） 2026-07-24 12:08:48 +08:00
parent 60db8d226b
commit bcd4574b3f
1 changed files with 16 additions and 12 deletions
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28
vendor/fonteditor-core/woff2/woff2-encode.js vendored
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@ -507,20 +507,32 @@ function transformGlyfAndLoca(glyfData, locaData, indexFormat, numGlyphs) {
const curveBit = ((f & 1) ^ 1) << 7;
const dx = cx - prevX;
const dy = cy - prevY;
const absDx = dx < 0 ? -dx : dx;
const absDy = dy < 0 ? -dy : dy;
/**
* 优化317: 无分支 abs + 符号位提取。
* dx/dy 为 SmI（Int32 路径），dx >> 31 在非负时为 0、负时为 -1（算术右移）。
* absDx = (dx ^ sgn) - sgn —— sgn=0 时不变，sgn=-1 时 dx^(-1)=~dx，~dx-(-1)=~dx+1=-dx
* xSignBit = sgn + 1 —— 0→1（正/零）、-1→0（负），与原 `dx >= 0 ? 1 : 0` 等价
* 中文字体 dx/dy 正负近似 50/50，原三元是不可预测分支致流水线冲刷；位运算无分支。
* 每点消除 4 次三元（absDx/absDy/xSign/ySign），令东 74.6% 点走 case3 收益最大。
*/
const dxSgn = dx >> 31;
const dySgn = dy >> 31;
const absDx = (dx ^ dxSgn) - dxSgn;
const absDy = (dy ^ dySgn) - dySgn;
const xSignBit = dxSgn + 1;
const ySignBit = dySgn + 1;
const wpos = gsBase + gsbi;
let flag;
/** 优化313: 各分支内直接累加 gsbi（字节数由分支确定：1/1/1/2/3/4），消除 TRIPLET_DATA_SIZES 查表 */
let adv;
if (dx === 0 && absDy < 1280) {
_gs[wpos] = absDy & 0xFF;
flag = curveBit + ((absDy & 0xF00) >> 7) + (dy >= 0 ? 1 : 0);
flag = curveBit + ((absDy & 0xF00) >> 7) + ySignBit;
adv = 1;
} else if (dy === 0 && absDx < 1280) {
/** 优化316: case2 去掉冗余 `dx !== 0`——case1 已排除 dx===0，走到此分支 dx 必非 0 */
_gs[wpos] = absDx & 0xFF;
flag = curveBit + 10 + ((absDx & 0xF00) >> 7) + (dx >= 0 ? 1 : 0);
flag = curveBit + 10 + ((absDx & 0xF00) >> 7) + xSignBit;
adv = 1;
} else if (absDx < 65 && absDy < 65) {
/** 优化316: case3 去掉冗余 `dx !== 0 && dy !== 0`——case1/case2 已排除任一轴为 0，
@ -528,8 +540,6 @@ function transformGlyfAndLoca(glyfData, locaData, indexFormat, numGlyphs) {
const ax = absDx - 1;
const ay = absDy - 1;
_gs[wpos] = ((ax & 0xF) << 4) | (ay & 0xF);
const xSignBit = dx >= 0 ? 1 : 0;
const ySignBit = dy >= 0 ? 1 : 0;
flag = curveBit + 20 + (ax & 0x30) + ((ay & 0x30) >> 2) + xSignBit + 2 * ySignBit;
adv = 1;
} else if (absDx < 769 && absDy < 769) {
@ -538,16 +548,12 @@ function transformGlyfAndLoca(glyfData, locaData, indexFormat, numGlyphs) {
const ay = absDy - 1;
_gs[wpos] = ax & 0xFF;
_gs[wpos + 1] = ay & 0xFF;
const xSignBit = dx >= 0 ? 1 : 0;
const ySignBit = dy >= 0 ? 1 : 0;
flag = curveBit + 84 + 12 * ((ax & 0x300) >> 8) + ((ay & 0x300) >> 6) + xSignBit + 2 * ySignBit;
adv = 2;
} else if (absDx < 4096 && absDy < 4096) {
_gs[wpos] = absDx >> 4;
_gs[wpos + 1] = ((absDx & 0xF) << 4) | (absDy >> 8);
_gs[wpos + 2] = absDy & 0xFF;
const xSignBit = dx >= 0 ? 1 : 0;
const ySignBit = dy >= 0 ? 1 : 0;
flag = curveBit + 120 + xSignBit + 2 * ySignBit;
adv = 3;
} else {
@ -555,8 +561,6 @@ function transformGlyfAndLoca(glyfData, locaData, indexFormat, numGlyphs) {
_gs[wpos + 1] = absDx & 0xFF;
_gs[wpos + 2] = (absDy >> 8) & 0xFF;
_gs[wpos + 3] = absDy & 0xFF;
const xSignBit = dx >= 0 ? 1 : 0;
const ySignBit = dy >= 0 ? 1 : 0;
flag = curveBit + 124 + xSignBit + 2 * ySignBit;
adv = 4;
}