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本期文章由 Amniesia HNY Damon 赞助
最近的热门是windows蓝屏事件了，其实国内外安全都有关系户
本周内容较少
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 264期
文章
Safer code in C with lifetime bounds
https://lemire.me/blog/2024/07/26/safer-code-in-c-with-lifetime-bounds/
llvm和msvc支持生命周期检查，返回string_view有概率悬空，用错
代码语言：javascript复制std::string_view my_get_host(std::string_view url_string) {
auto url = ada::parse(url_string).value();
return url.get_host();
}比如这种用法明显就是错的，加上编译检查能抓出来
代码语言：javascript复制#ifndef __has_cpp_attribute
#define ada_lifetime_bound
#elif __has_cpp_attribute(msvc::lifetimebound)
#define ada_lifetime_bound [[msvc::lifetimebound]]
#elif __has_cpp_attribute(clang::lifetimebound)
#define ada_lifetime_bound [[clang::lifetimebound]]
#elif __has_cpp_attribute(lifetimebound)
#define ada_lifetime_bound [[lifetimebound]]
#else
#define ada_lifetime_bound
#endif
...
std::string_view get_host() const noexcept ada_lifetime_bound编译报错
代码语言：javascript复制fun.cpp:8:10: warning: address of stack memory associated with local variable 'url' returned [-Wreturn-stack-address]
8 | return url.get_host();想要了解可以看这里 https://clang.llvm.org/docs/AttributeReference.html#lifetimebound
strlcpy and how CPUs can defy common sense strlcpy and how CPUs can defy common sense
https://nrk.neocities.org/articles/cpu-vs-common-sense
strlcpy 实现openbsd和glibc实现不同，openbsd是这样的
代码语言：javascript复制size_t strlcpy(char *dst, const char *src, size_t dsize)
{
const char *osrc = src;
size_t nleft = dsize;
if (nleft != 0) while (--nleft != 0) { /* Copy as many bytes as will fit. */
if ((*dst = *src ) == ' ')
break;
}
if (nleft == 0) { /* Not enough room in dst, add NUL and traverse rest of src. */
if (dsize != 0) *dst = ' '; /* NUL-terminate dst */
while (*src ) ;
}
return(src - osrc - 1); /* count does not include NUL */
}能看到是一边复制一边移动的，没有提前算出src边界，而glibc是用strlen先计算src长度的，相当于重复计算了
所以openbsd版本应该比glibc版本快是不是？并不
考虑到strlen和memcpy有可能优化，咱们手写一个版本
代码语言：javascript复制size_t bespoke_strlcpy(char *dst, const char *src, size_t size)
{
size_t len = 0;
for (; src[len] != ' '; len) {} // strlen() loop
if (size > 0) {
size_t to_copy = len < size ? len : size - 1;
for (size_t i = 0; i < to_copy; i) // memcpy() loop
dst[i] = src[i];
dst[to_copy] = ' ';
}
return len;
}编译使用 -fno-builtin避免strlen memcpy优化
这个也比openbsd快
实际上没有长度信息 每次都要判断 ，严重影响优化，循环出现依赖，没法彻底优化
What's so hard about constexpr allocation?
https://brevzin.github.io/c /2024/07/24/constexpr-alloc/
讨论constexpr vector难做的原因，先从unique_ptr开始讨论，constexpr导致相关的传递语义发生变化，不好优化
考虑引入新关键字propconst 标记常量传递 讨论的还是比较有深度的，感兴趣的可以读一下
Does C allow template specialization by concepts?
https://lemire.me/blog/2024/07/22/does-c-allow-template-specialization-by-concepts/
用require实现函数偏特化
代码语言：javascript复制template <typename T>
void clear(T & t);
template <typename T>
concept not_string =
!std::is_same_v<T, std::string>;
template <>
void clear(std::string & t) {
t.clear();
}
template <class T>
void clear(T& container) requires not_string<T> {
for(auto& i : container) {
i = typename T::value_type{};
}
}看一乐
Scan HTML even faster with SIMD instructions (C and C#)
https://lemire.me/blog/2024/07/20/scan-html-even-faster-with-simd-instructions-c-and-c/
实现特殊版本find_first_of 向量化。代码不贴了，感兴趣的看一下
https://github.com/lemire/Code-used-on-Daniel-Lemire-s-blog/tree/master/2024/06/08
