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第123期

C++ 中文周刊 第123期

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本周内容不多

资讯

标准委员会动态/ide/编译器信息放在这里

编译器信息最新动态推荐关注hellogcc公众号 本周更新 2023-07-19 第211期

文章

Inside boost::concurrent_flat_map

boost 1.83会有个 boost::concurrent_flat_map, 这篇文章带你了解设计思路。还是开地址法，还并发，还快

测了一下是吊锤tbb的。没测和folly::concurrentHashMap的比较

其实这个文章可以展开讲讲

浏览器安卓端崩溃最终怀疑是CPU的问题

总之就是利用CPU流水线先走一个快速路径，再检查条件，也就是所谓的分支预测

你可能想问了，我能不能让CPU别预测，别显得你多聪明了老老实实算就完了

__builtin_unpredictable() https://clang.llvm.org/docs/LanguageExtensions.html#builtin-unpredictable

基于async_simple的。有点意思

了解一下PGO流程

看个热闹

手把手教你写个grpc server

SIMD时间，字符转数字

常规, 一个一个比

if (ch >= '0' && ch <= '9')
  d = ch - '0';
else if (ch >= 'A' && ch <= 'V')
  d = ch - 'A' + 10;
else if (ch >= 'a' && ch <= 'v')
  d = ch - 'a' + 10;
else
  return -1;

进化版本，唉我会打表了

size_t base32hex_simple(uint8_t *dst, const uint8_t *src) {
  static const uint8_t table[256] = {
      32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32,
      32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32,
      32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32,
      0,  1,  2,  3,  4,  5,  6,  7,  8,  9,  32, 32, 32, 32, 32, 32,
      32, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24,
      25, 26, 27, 28, 29, 30, 31, 32, 32, 32, 32, 32, 32, 32, 32, 32,
      32, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24,
      25, 26, 27, 28, 29, 30, 31, 32, 32, 32, 32, 32, 32, 32, 32, 32,

      32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32,
      32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32,
      32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32,
      32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32,
      32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32,
      32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32,
      32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32,
      32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32,
  };
  bool valid = true;
  const uint8_t *srcinit = src;

  do {
    uint64_t r = 0;
    for (size_t i = 0; i < 8; i++) {
      uint8_t x = table[*src];
      src++;
      if (x > 31) {
        r <<= (5 * (8 - i));
        valid = false;
        break;
      }
      r <<= 5;
      r |= x;
    }
    r = (unsigned long int)_bswap64((long long int)r);
    uint64_t rs = ((uint64_t)r >> (3 * 8));
    memcpy(dst, (const char *)&rs, 8);
    dst += 5;
  } while (valid);
  return (size_t)(src - srcinit);
}

超神版本 SIMD直接贴代码

size_t base32hex_simd(uint8_t *dst, const uint8_t *src) {
  bool valid = true;
  const __m128i delta_check =
      _mm_setr_epi8(-16, -32, -48, 70, -65, 41, -97, 9, 0, 0, 0, 0, 0, 0, 0, 0);
  const __m128i delta_rebase =
      _mm_setr_epi8(0, 0, 0, -48, -55, -55, -87, -87, 0, 0, 0, 0, 0, 0, 0, 0);
  const uint8_t *srcinit = src;
  do {
    __m128i v = _mm_loadu_si128((__m128i *)src);

    __m128i hash_key = _mm_and_si128(_mm_srli_epi32(v, 4), _mm_set1_epi8(0x0F));
    __m128i check = _mm_add_epi8(_mm_shuffle_epi8(delta_check, hash_key), v);
    v = _mm_add_epi8(v, _mm_shuffle_epi8(delta_rebase, hash_key));
    unsigned int m = (unsigned)_mm_movemask_epi8(check);

    if (m) {
      int length = __builtin_ctz(m);
      if (length == 0) {
        break;
      }
      src += length;
      __m128i zero_mask =
          _mm_loadu_si128((__m128i *)(zero_masks + 16 - length));
      v = _mm_andnot_si128(zero_mask, v);
      valid = false;
    } else { // common case
      src += 16;
    }
    v = _mm_maddubs_epi16(v, _mm_set1_epi32(0x01200120));
    v = _mm_madd_epi16(
        v, _mm_set_epi32(0x00010400, 0x00104000, 0x00010400, 0x00104000));
    // ...00000000`0000eeee`efffffgg`ggghhhhh`00000000`aaaaabbb`bbcccccd`dddd0000
    v = _mm_or_si128(v, _mm_srli_epi64(v, 48));
    v = _mm_shuffle_epi8(
        v, _mm_set_epi8(0, 0, 0, 0, 0, 0, 12, 13, 8, 9, 10, 4, 5, 0, 1, 2));

    /* decoded 10 bytes... but write 16 cause why not? */
    _mm_storeu_si128((__m128i *)dst, v);
    dst += 10;
  } while (valid);

  return (size_t)(src - srcinit);
}

还有SWAR版本，我直接贴仓库连接，不贴代码了 https://github.com/lemire/Code-used-on-Daniel-Lemire-s-blog/blob/master/2023/07/20/src/base32.c

raymond chen的window时间，看不懂

开源项目需要人手

  • asteria 一个脚本语言，可嵌入，长期找人，希望胖友们帮帮忙，也可以加群753302367和作者对线
  • Unilang deepin的一个通用编程语言，点子有点意思，也缺人，感兴趣的可以github讨论区或者deepin论坛看一看。这里也挂着长期推荐了
  • gcc-mcf 懂的都懂

新项目介绍/版本更新

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