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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 2024-03-13 第245期
c 26 东京会议如火如荼，详情Mick235711已经发了，公众号也发了，这里不再赘述
文章
Introduction To Low Latency Programming: External Processing https://tech.davidgorski.ca/introduction-to-low-latency-programming-external-processing/
其实就是提前算好，包括不限于利用编译期 利用脚本生成以及constexpr
异步拆分，让别人算
Condvars and atomics do not mix https://zeux.io/2024/03/23/condvars-atomic/
用condvar 条件就要放到mutex下修改，即便这个变量是atomic，也要放到mutex下修改
Jumbled Protocol Buffer Message Layout https://eason-zhan.github.io/posts/jumbled-protocol-buffer-message-layout/
protoc会重排你定义的字段布局
代码语言：javascript复制static void OptimizeLayoutHelper(std::vector<const FieldDescriptor*>* fields,
const Options& options,
MessageSCCAnalyzer* scc_analyzer) {
if (fields->empty()) return;
// The sorted numeric order of Family determines the declaration order in the
// memory layout.
enum Family {
REPEATED = 0,
STRING = 1,
// Laying out LAZY_MESSAGE before MESSAGE allows a single memset to zero
// MESSAGE and ZERO_INITIALIZABLE fields together.
LAZY_MESSAGE = 2,
MESSAGE = 3,
ZERO_INITIALIZABLE = 4,
OTHER = 5,
kMaxFamily
};
作者观察到一个现象，本来字段很多，删掉一部份字段，性能应该有提升，结果并没有
代码语言：javascript复制message Stats {
int64 ts = 1;
int64 show = 2;
int64 click = 3;
int64 cost = 4;
int64 hour_show = 5;
int64 hour_click = 6;
int64 hour_cost = 7;
int64 acc_show = 8;
int64 acc_click = 9;
int64 acc_cost = 10;
repeated int64 bucket = 11;
repeated int64 hour_bucket = 12;
repeated int64 acc_bucket = 13;
}
优化成这样
代码语言：javascript复制// After remove the `hour_*` fields
message StatsOpt {
int64 ts = 1;
int64 show = 2;
int64 click = 3;
int64 cost = 4;
int64 acc_show = 5;
int64 acc_click = 6;
int64 acc_cost = 7;
repeated int64 bucket = 8;
repeated int64 acc_bucket = 9;
}
内存布局原来是这样
代码语言：javascript复制 ------ 16 BYTE ------ - 8 BYTE - ------ 16 BYTE ------ - 8 BYTE - ------ 16 BYTE ------
| (11)bucket | (11)size | (12)hour | (12)size | (13)acc_bucket |
- 8 BYTE - - 8 BYTE - - 8 BYTE - - 8 BYTE - - 8 BYTE - - 8 BYTE - - 8 BYTE - - 8 BYTE -
| (13.a) | (1)ts | (2)show | (3)click | (4)cost | (5) | (6) | (7) |
- 8 BYTE - - 8 BYTE - - 8 BYTE - - 8 BYTE - - 8 BYTE - - 8 BYTE - - 8 BYTE - - 8 BYTE -
| (8) | (9) | (10) | * | * | * | * | * |
- 8 BYTE - - 8 BYTE - - 8 BYTE - - 8 BYTE - - 8 BYTE - - 8 BYTE - - 8 BYTE - - 8 BYTE - 现在是这样
代码语言：javascript复制 ------ 16 BYTE ------ - 8 BYTE - ------ 16 BYTE ------ - 8 BYTE - - 8 BYTE - - 8 BYTE -
| (8)bucket | (8)size | (9)hour | (9)size | (13.a) | (1)ts |
- 8 BYTE - - 8 BYTE - - 8 BYTE - - 8 BYTE - - 8 BYTE - - 8 BYTE - - 8 BYTE - - 8 BYTE -
| (2)show | (3)click | (4)cost | (5) | (6) | (7) | * | * |
- 8 BYTE - - 8 BYTE - - 8 BYTE - - 8 BYTE - - 8 BYTE - - 8 BYTE - - 8 BYTE - - 8 BYTE -
生成文件是这样的
代码语言：javascript复制 struct Impl_ {
::PROTOBUF_NAMESPACE_ID::RepeatedField< int64_t > bucket_;
mutable std::atomic<int> _bucket_cached_byte_size_;
::PROTOBUF_NAMESPACE_ID::RepeatedField< int64_t > acc_bucket_;
mutable std::atomic<int> _acc_bucket_cached_byte_size_;
int64_t ts_;
int64_t show_;
int64_t click_;
int64_t cost_;
int64_t acc_show_;
int64_t acc_click_;
int64_t acc_cost_;
mutable ::PROTOBUF_NAMESPACE_ID::internal::CachedSize _cached_size_;
};
union { Impl_ _impl_; };能看到ts和cost跨cacheline了。以前的字段虽然大，但不是夸cacheline的，优化后反而跨cacheline导致变慢
C exit-time destructors https://maskray.me/blog/2024-03-17-c -exit-time-destructors
介绍析构 runtime细节，非常细，值得一看
Daily bit(e) of C | Coroutines: step by step https://simontoth.substack.com/p/daily-bite-of-c-coroutines-step-by
又一个协程教程
C 23: Encoding related changes https://www.sandordargo.com/blog/2024/03/20/cpp23-encoding-related-changes
介绍了一些编码方面的改进，包括多语言支持/unicode等等
代码语言：javascript复制
std::locale::global(std::locale("Russian.1251"));
auto s = std::format("День недели: {}", std::chrono::Monday);
Bug hunting in Btrfs https://tavianator.com/2024/btrfs_bug.html
调试代码发现了btrfs有bug，然后去找bug在哪里，很细，值得一看，这里标记TODO
C 20: Basic Chrono Terminology with Time Duration and Time Point https://www.modernescpp.com/index.php/c20-basic-chrono-terminology-with-time-duration-and-time-point/
std::chrono::months 你猜是多少？30.436875
std::chrono::years 你猜是多少？365.2425
非常之令人无语，这种傻逼接口有存在的必要吗
Two handy GDB breakpoint tricks https://nullprogram.com/blog/2024/01/28/
gdb小技巧
Daily bit(e) of C | Coroutines: step by step https://simontoth.substack.com/p/daily-bite-of-c-coroutines-step-by
coroutine介绍，代码很长
