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https://github.com/jart/cosmopolitan.git
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8b3e368e9a
A small-string optimization is a way of reusing inline storage space for sufficiently small strings, rather than allocating them on the heap. The current approach takes after an old Facebook string class: it reuses the highest-order byte for flags and small-string size, in such a way that a maximally-sized small string will have its last byte zeroed, making it a null terminator for the C string. The only flag we have is in the highest-order bit, that says whether the string is big (set) or small (cleared.) Most of the logic switches based on the value of this bit; e.g. data() returns big()->p if it's set, else small()->buf if it's cleared. For a small string, the capacity is always fixed at sizeof(string) - 1 bytes; we store the length in the last byte, but we store it as the number of remaining bytes of capacity, so that at max size, the last byte will read zero and serve as our null terminator. Morally speaking, our class's storage is a union over two POD C structs. For now I gravitated towards a slightly more obtuse approach: the string class itself contains a blob of the right size, and we alias that blob's pointer for the two structs, taking some care not to run afoul of object lifetime rules in C++. If anyone wants to improve on this, contributions are welcome. This commit also introduces the `ctl::__` namespace. It can't be legally spelled by library users, and serves as our version of boost's "detail". We introduced a string::swap function, and we now use that in operator=. operator= now takes its argument by value, so we never need to check for the case where the pointers are equal and can just swap the entire store of the argument with our own, leaving the C++ destructor to free our old storage afterwards. There are probably still a few places where our capacity is slightly off and we grow too fast, although there don't appear to be any where we are too slow. I will leave these to be fixed in future changes.
365 lines
8 KiB
C++
365 lines
8 KiB
C++
// -*-mode:c++;indent-tabs-mode:nil;c-basic-offset:4;tab-width:8;coding:utf-8-*-
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// vi: set et ft=cpp ts=4 sts=4 sw=4 fenc=utf-8 :vi
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#ifndef COSMOPOLITAN_CTL_STRING_H_
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#define COSMOPOLITAN_CTL_STRING_H_
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#include "string_view.h"
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namespace ctl {
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class string;
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string
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strcat(const string_view, const string_view) noexcept __wur;
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namespace __ {
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constexpr size_t string_size = 3 * sizeof(size_t);
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constexpr size_t sso_max = string_size - 1;
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constexpr size_t big_mask = ~(1ull << (8ull * sizeof(size_t) - 1ull));
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struct small_string
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{
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char buf[sso_max];
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// interpretation is: size == sso_max - rem
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unsigned char rem;
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#if 0
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size_t rem : 7;
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size_t big : 1 /* = 0 */;
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#endif
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};
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struct big_string
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{
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char* p;
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size_t n;
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// interpretation is: capacity == c & big_mask
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size_t c;
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#if 0
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size_t c : sizeof(size_t) * 8 - 1;
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size_t big : 1 /* = 1 */;
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#endif
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~big_string() /* noexcept */;
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};
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} // namespace __
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class string
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{
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public:
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using iterator = char*;
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using const_iterator = const char*;
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static constexpr size_t npos = -1;
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~string() /* noexcept */;
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string(const string_view) noexcept;
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string(const char*) noexcept;
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string(const string&) noexcept;
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string(const char*, size_t) noexcept;
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explicit string(size_t, char = 0) noexcept;
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string& operator=(string) noexcept;
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const char* c_str() const noexcept;
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void pop_back() noexcept;
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void grow(size_t) noexcept;
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void reserve(size_t) noexcept;
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void resize(size_t, char = 0) noexcept;
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void append(char) noexcept;
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void append(char, size_t) noexcept;
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void append(unsigned long) noexcept;
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void append(const void*, size_t) noexcept;
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string& insert(size_t, const string_view) noexcept;
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string& erase(size_t = 0, size_t = npos) noexcept;
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string substr(size_t = 0, size_t = npos) const noexcept;
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string& replace(size_t, size_t, const string_view&) noexcept;
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bool operator==(const string_view) const noexcept;
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bool operator!=(const string_view) const noexcept;
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bool contains(const string_view) const noexcept;
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bool ends_with(const string_view) const noexcept;
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bool starts_with(const string_view) const noexcept;
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size_t find(char, size_t = 0) const noexcept;
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size_t find(const string_view, size_t = 0) const noexcept;
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string() noexcept
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{
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set_small_size(0);
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#if 0
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small()->buf[0] = 0;
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#endif
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}
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void swap(string& s) noexcept
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{
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char tmp[__::string_size];
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__builtin_memcpy(tmp, __builtin_launder(blob), sizeof(tmp));
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__builtin_memcpy(
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__builtin_launder(blob), __builtin_launder(s.blob), sizeof(tmp));
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__builtin_memcpy(__builtin_launder(s.blob), tmp, sizeof(tmp));
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}
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string(string&& s) noexcept
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{
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__builtin_memcpy(blob, __builtin_launder(s.blob), sizeof(blob));
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s.set_small_size(0);
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#if 0
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s.small()->buf[0] = 0;
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#endif
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}
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void clear() noexcept
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{
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if (isbig()) {
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big()->n = 0;
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} else {
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set_small_size(0);
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}
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}
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bool empty() const noexcept
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{
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return isbig() ? !big()->n : small()->rem >= __::sso_max;
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}
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inline char* data() noexcept
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{
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return isbig() ? big()->p : small()->buf;
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}
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inline const char* data() const noexcept
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{
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return isbig() ? big()->p : small()->buf;
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}
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inline size_t size() const noexcept
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{
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#if 0
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if (!isbig() && small()->rem > __::sso_max)
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__builtin_trap();
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#endif
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return isbig() ? big()->n : __::sso_max - small()->rem;
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}
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size_t length() const noexcept
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{
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return size();
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}
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size_t capacity() const noexcept
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{
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#if 0
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if (isbig() && big()->c <= __::sso_max)
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__builtin_trap();
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#endif
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return isbig() ? __::big_mask & big()->c : __::sso_max;
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}
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iterator begin() noexcept
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{
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return data();
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}
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iterator end() noexcept
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{
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return data() + size();
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}
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const_iterator cbegin() const noexcept
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{
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return data();
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}
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const_iterator cend() const noexcept
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{
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return data() + size();
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}
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char& front()
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{
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if (!size())
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__builtin_trap();
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return data()[0];
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}
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const char& front() const
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{
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if (!size())
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__builtin_trap();
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return data()[0];
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}
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char& back()
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{
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if (!size())
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__builtin_trap();
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return data()[size() - 1];
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}
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const char& back() const
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{
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if (!size())
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__builtin_trap();
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return data()[size() - 1];
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}
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char& operator[](size_t i) noexcept
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{
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if (i >= size())
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__builtin_trap();
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return data()[i];
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}
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const char& operator[](size_t i) const noexcept
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{
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if (i >= size())
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__builtin_trap();
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return data()[i];
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}
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void push_back(char ch) noexcept
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{
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append(ch);
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}
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void append(const string_view s) noexcept
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{
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append(s.p, s.n);
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}
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// TODO(mrdomino): explicit?
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inline operator string_view() const noexcept
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{
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return string_view(data(), size());
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}
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string& operator=(const char* s) noexcept
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{
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clear();
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append(s);
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return *this;
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}
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string& operator=(const string_view s) noexcept
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{
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clear();
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append(s);
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return *this;
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}
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string& operator+=(char x) noexcept
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{
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append(x);
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return *this;
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}
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string& operator+=(const string_view s) noexcept
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{
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append(s);
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return *this;
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}
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string operator+(const string_view s) const noexcept
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{
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return strcat(*this, s);
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}
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int compare(const string_view s) const noexcept
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{
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return strcmp(*this, s);
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}
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bool operator<(const string_view s) const noexcept
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{
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return compare(s) < 0;
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}
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bool operator<=(const string_view s) const noexcept
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{
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return compare(s) <= 0;
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}
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bool operator>(const string_view s) const noexcept
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{
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return compare(s) > 0;
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}
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bool operator>=(const string_view s) const noexcept
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{
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return compare(s) >= 0;
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}
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private:
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inline bool isbig() const noexcept
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{
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return *(__builtin_launder(blob) + __::sso_max) & 0x80;
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}
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inline void set_small_size(size_t size) noexcept
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{
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if (size > __::sso_max)
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__builtin_trap();
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*(__builtin_launder(blob) + __::sso_max) = (__::sso_max - size);
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}
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inline void set_big_capacity(size_t c2) noexcept
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{
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if (c2 > __::big_mask)
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__builtin_trap();
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*(__builtin_launder(blob) + __::sso_max) = 0x80;
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big()->c &= ~__::big_mask;
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big()->c |= c2;
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}
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inline __::small_string* small() noexcept
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{
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if (isbig())
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__builtin_trap();
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return __builtin_launder(reinterpret_cast<__::small_string*>(blob));
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}
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inline const __::small_string* small() const noexcept
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{
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if (isbig())
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__builtin_trap();
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return __builtin_launder(
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reinterpret_cast<const __::small_string*>(blob));
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}
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inline __::big_string* big() noexcept
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{
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if (!isbig())
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__builtin_trap();
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return __builtin_launder(reinterpret_cast<__::big_string*>(blob));
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}
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inline const __::big_string* big() const noexcept
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{
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if (!isbig())
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__builtin_trap();
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return __builtin_launder(reinterpret_cast<const __::big_string*>(blob));
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}
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friend string strcat(const string_view, const string_view);
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alignas(union {
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__::big_string a;
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__::small_string b;
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}) char blob[__::string_size];
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};
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static_assert(sizeof(string) == __::string_size);
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static_assert(sizeof(__::small_string) == __::string_size);
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static_assert(sizeof(__::big_string) == __::string_size);
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} // namespace ctl
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#pragma GCC diagnostic push
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#pragma GCC diagnostic ignored "-Wliteral-suffix"
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inline ctl::string
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operator"" s(const char* s, size_t n)
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{
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return ctl::string(s, n);
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}
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#pragma GCC diagnostic pop
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#endif // COSMOPOLITAN_CTL_STRING_H_
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