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ctl::string small-string optimization (#1199)

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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.
This commit is contained in:
Jōshin 2024-06-06 17:50:51 -07:00 committed by GitHub
parent df6b384e31
commit 8b3e368e9a
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GPG key ID: B5690EEEBB952194
4 changed files with 333 additions and 139 deletions
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11
ctl/strcat.cc
View file

@ -34,10 +34,15 @@ strcat(const string_view lhs, const string_view rhs) noexcept
__builtin_trap(); __builtin_trap();
res.reserve(need); res.reserve(need);
if (lhs.n) if (lhs.n)
memcpy(res.p, lhs.p, lhs.n); memcpy(res.data(), lhs.p, lhs.n);
if (rhs.n) if (rhs.n)
memcpy(res.p + lhs.n, rhs.p, rhs.n); memcpy(res.data() + lhs.n, rhs.p, rhs.n);
res.p[res.n = lhs.n + rhs.n] = 0; if (res.isbig()) {
res.big()->n = lhs.n + rhs.n;
} else {
res.small()->rem = __::sso_max - lhs.n - rhs.n;
}
res.data()[res.size()] = 0;
return res; return res;
} }

247
ctl/string.cc
View file

@ -23,7 +23,9 @@
namespace ctl { namespace ctl {
string::~string() noexcept namespace __ {
big_string::~big_string() /* noexcept */
{ {
if (n) { if (n) {
if (n >= c) if (n >= c)
@ -36,27 +38,36 @@ string::~string() noexcept
free(p); free(p);
} }
string::string(const char* s) noexcept } // namespace __
string::~string() /* noexcept */
{
if (isbig()) {
big()->~big_string();
}
}
string::string(const char* s) noexcept : string()
{ {
append(s, strlen(s)); append(s, strlen(s));
} }
string::string(const string& s) noexcept string::string(const string& s) noexcept : string()
{
append(s.data(), s.size());
}
string::string(const string_view s) noexcept : string()
{ {
append(s.p, s.n); append(s.p, s.n);
} }
string::string(const string_view s) noexcept string::string(size_t size, char ch) noexcept : string()
{
append(s.p, s.n);
}
string::string(size_t size, char ch) noexcept
{ {
resize(size, ch); resize(size, ch);
} }
string::string(const char* s, size_t size) noexcept string::string(const char* s, size_t size) noexcept : string()
{ {
append(s, size); append(s, size);
} }
@ -64,29 +75,39 @@ string::string(const char* s, size_t size) noexcept
const char* const char*
string::c_str() const noexcept string::c_str() const noexcept
{ {
if (!n) if (!size())
return ""; return "";
if (n >= c) if (size() >= capacity())
__builtin_trap(); __builtin_trap();
if (p[n]) if (data()[size()])
__builtin_trap(); __builtin_trap();
return p; return data();
} }
void void
string::reserve(size_t c2) noexcept string::reserve(size_t c2) noexcept
{ {
char* p2; char* p2;
if (c2 < n) size_t n = size();
c2 = n; if (c2 < n + 1)
c2 = n + 1;
if (c2 <= __::string_size)
return;
if (ckd_add(&c2, c2, 15)) if (ckd_add(&c2, c2, 15))
__builtin_trap(); __builtin_trap();
c2 &= -16; c2 &= -16;
if (!(p2 = (char*)realloc(p, c2))) if (!isbig()) {
__builtin_trap(); if (!(p2 = (char*)malloc(c2)))
__builtin_trap();
__builtin_memcpy(p2, data(), size() + 1);
} else {
if (!(p2 = (char*)realloc(big()->p, c2)))
__builtin_trap();
}
std::atomic_signal_fence(std::memory_order_seq_cst); std::atomic_signal_fence(std::memory_order_seq_cst);
c = c2; set_big_capacity(c2);
p = p2; big()->n = n;
big()->p = p2;
} }
void void
@ -96,41 +117,54 @@ string::resize(size_t n2, char ch) noexcept
if (ckd_add(&c2, n2, 1)) if (ckd_add(&c2, n2, 1))
__builtin_trap(); __builtin_trap();
reserve(c2); reserve(c2);
if (n2 > n) if (n2 > size())
memset(p + n, ch, n2 - n); memset(data() + size(), ch, n2 - size());
p[n = n2] = 0; if (isbig()) {
big()->p[big()->n = n2] = 0;
} else {
set_small_size(n2);
data()[size()] = 0;
}
} }
void void
string::append(char ch) noexcept string::append(char ch) noexcept
{ {
if (n + 2 > c) { size_t n2;
size_t c2 = c + 2; if (ckd_add(&n2, size(), 2))
c2 += c2 >> 1; __builtin_trap();
if (n2 > capacity()) {
size_t c2 = capacity() + 2;
if (ckd_add(&c2, c2, c2 >> 1))
__builtin_trap();
reserve(c2); reserve(c2);
} }
p[n++] = ch; // XXX do we care to fence this?
p[n] = 0; data()[size()] = ch;
data()[size() + 1] = 0;
if (isbig()) {
++big()->n;
} else {
--small()->rem;
}
} }
void void
string::grow(size_t size) noexcept string::grow(size_t size) noexcept
{ {
size_t need; size_t need;
if (ckd_add(&need, n, size)) if (ckd_add(&need, this->size(), size))
__builtin_trap(); __builtin_trap();
if (ckd_add(&need, need, 1)) if (ckd_add(&need, need, 1))
__builtin_trap(); __builtin_trap();
if (need <= c) if (need <= capacity())
return; return;
size_t c2 = c; size_t c2 = capacity();
if (!c2) { if (!c2)
c2 = need; __builtin_trap();
} else { while (c2 < need)
while (c2 < need) if (ckd_add(&c2, c2, c2 >> 1))
if (ckd_add(&c2, c2, c2 >> 1)) __builtin_trap();
__builtin_trap();
}
reserve(c2); reserve(c2);
} }
@ -139,8 +173,14 @@ string::append(char ch, size_t size) noexcept
{ {
grow(size); grow(size);
if (size) if (size)
memset(p + n, ch, size); memset(data() + this->size(), ch, size);
p[n += size] = 0; // XXX fence?
if (isbig()) {
big()->n += size;
} else {
small()->rem -= size;
}
data()[this->size()] = 0;
} }
void void
@ -148,55 +188,53 @@ string::append(const void* data, size_t size) noexcept
{ {
grow(size); grow(size);
if (size) if (size)
memcpy(p + n, data, size); memcpy(this->data() + this->size(), data, size);
p[n += size] = 0; if (isbig()) {
big()->n += size;
} else {
small()->rem -= size;
}
this->data()[this->size()] = 0;
} }
void void
string::pop_back() noexcept string::pop_back() noexcept
{ {
if (!n) if (!size())
__builtin_trap(); __builtin_trap();
p[--n] = 0; if (isbig()) {
--big()->n;
} else {
++small()->rem;
}
data()[size()] = 0;
} }
string& string&
string::operator=(string&& s) noexcept string::operator=(string s) noexcept
{ {
if (p != s.p) { swap(s);
if (p) {
clear();
append(s.p, s.n);
} else {
p = s.p;
n = s.n;
c = s.c;
s.p = nullptr;
s.n = 0;
s.c = 0;
}
}
return *this; return *this;
} }
bool bool
string::operator==(const string_view s) const noexcept string::operator==(const string_view s) const noexcept
{ {
if (n != s.n) if (size() != s.n)
return false; return false;
if (!n) if (!s.n)
return true; return true;
return !memcmp(p, s.p, n); return !memcmp(data(), s.p, s.n);
} }
bool bool
string::operator!=(const string_view s) const noexcept string::operator!=(const string_view s) const noexcept
{ {
if (n != s.n) if (size() != s.n)
return true; return true;
if (!n) if (!s.n)
return false; return false;
return !!memcmp(p, s.p, n); return !!memcmp(data(), s.p, s.n);
} }
bool bool
@ -204,35 +242,35 @@ string::contains(const string_view s) const noexcept
{ {
if (!s.n) if (!s.n)
return true; return true;
return !!memmem(p, n, s.p, s.n); return !!memmem(data(), size(), s.p, s.n);
} }
bool bool
string::ends_with(const string_view s) const noexcept string::ends_with(const string_view s) const noexcept
{ {
if (n < s.n) if (size() < s.n)
return false; return false;
if (!s.n) if (!s.n)
return true; return true;
return !memcmp(p + n - s.n, s.p, s.n); return !memcmp(data() + size() - s.n, s.p, s.n);
} }
bool bool
string::starts_with(const string_view s) const noexcept string::starts_with(const string_view s) const noexcept
{ {
if (n < s.n) if (size() < s.n)
return false; return false;
if (!s.n) if (!s.n)
return true; return true;
return !memcmp(p, s.p, s.n); return !memcmp(data(), s.p, s.n);
} }
size_t size_t
string::find(char ch, size_t pos) const noexcept string::find(char ch, size_t pos) const noexcept
{ {
char* q; char* q;
if ((q = (char*)memchr(p, ch, n))) if ((q = (char*)memchr(data(), ch, size())))
return q - p; return q - data();
return npos; return npos;
} }
@ -240,10 +278,10 @@ size_t
string::find(const string_view s, size_t pos) const noexcept string::find(const string_view s, size_t pos) const noexcept
{ {
char* q; char* q;
if (pos > n) if (pos > size())
__builtin_trap(); __builtin_trap();
if ((q = (char*)memmem(p + pos, n - pos, s.p, s.n))) if ((q = (char*)memmem(data() + pos, size() - pos, s.p, s.n)))
return q - p; return q - data();
return npos; return npos;
} }
@ -251,15 +289,15 @@ string
string::substr(size_t pos, size_t count) const noexcept string::substr(size_t pos, size_t count) const noexcept
{ {
size_t last; size_t last;
if (pos > n) if (pos > size())
__builtin_trap(); __builtin_trap();
if (count > n - pos) if (count > size() - pos)
count = n - pos; count = size() - pos;
if (ckd_add(&last, pos, count)) if (ckd_add(&last, pos, count))
last = n; last = size();
if (last > n) if (last > size())
__builtin_trap(); __builtin_trap();
return string(p + pos, count); return string(data() + pos, count);
} }
string& string&
@ -268,12 +306,12 @@ string::replace(size_t pos, size_t count, const string_view& s) noexcept
size_t last; size_t last;
if (ckd_add(&last, pos, count)) if (ckd_add(&last, pos, count))
__builtin_trap(); __builtin_trap();
if (last > n) if (last > size())
__builtin_trap(); __builtin_trap();
size_t need; size_t need;
if (ckd_add(&need, pos, s.n)) if (ckd_add(&need, pos, s.n))
__builtin_trap(); __builtin_trap();
size_t extra = n - last; size_t extra = size() - last;
if (ckd_add(&need, need, extra)) if (ckd_add(&need, need, extra))
__builtin_trap(); __builtin_trap();
size_t c2; size_t c2;
@ -281,42 +319,57 @@ string::replace(size_t pos, size_t count, const string_view& s) noexcept
__builtin_trap(); __builtin_trap();
reserve(c2); reserve(c2);
if (extra) if (extra)
memmove(p + pos + s.n, p + last, extra); memmove(data() + pos + s.n, data() + last, extra);
memcpy(p + pos, s.p, s.n); memcpy(data() + pos, s.p, s.n);
p[n = need] = 0; if (isbig()) {
big()->p[big()->n = need] = 0;
} else {
set_small_size(need);
data()[size()] = 0;
}
return *this; return *this;
} }
string& string&
string::insert(size_t i, const string_view s) noexcept string::insert(size_t i, const string_view s) noexcept
{ {
if (i > n) if (i > size())
__builtin_trap(); __builtin_trap();
size_t extra = n - i; size_t extra = size() - i;
size_t need; size_t need;
if (ckd_add(&need, n, s.n)) if (ckd_add(&need, size(), s.n))
__builtin_trap(); __builtin_trap();
if (ckd_add(&need, need, 1)) if (ckd_add(&need, need, 1))
__builtin_trap(); __builtin_trap();
reserve(need); reserve(need);
if (extra) if (extra)
memmove(p + i + s.n, p + i, extra); memmove(data() + i + s.n, data() + i, extra);
memcpy(p + i, s.p, s.n); memcpy(data() + i, s.p, s.n);
p[n += s.n] = 0; if (isbig()) {
big()->n += s.n;
} else {
small()->rem -= s.n;
}
data()[size()] = 0;
return *this; return *this;
} }
string& string&
string::erase(size_t pos, size_t count) noexcept string::erase(size_t pos, size_t count) noexcept
{ {
if (pos > n) if (pos > size())
__builtin_trap(); __builtin_trap();
if (count > n - pos) if (count > size() - pos)
count = n - pos; count = size() - pos;
size_t extra = n - (pos + count); size_t extra = size() - (pos + count);
if (extra) if (extra)
memmove(p + pos, p + pos + count, extra); memmove(data() + pos, data() + pos + count, extra);
p[n = pos + extra] = 0; if (isbig()) {
big()->n = pos + extra;
} else {
set_small_size(pos + extra);
}
data()[size()] = 0;
return *this; return *this;
} }

207
ctl/string.h
View file

@ -6,29 +6,59 @@
namespace ctl { namespace ctl {
struct string; class string;
string string
strcat(const string_view, const string_view) noexcept __wur; strcat(const string_view, const string_view) noexcept __wur;
struct string namespace __ {
{
char* p = nullptr;
size_t n = 0;
size_t c = 0;
constexpr size_t string_size = 3 * sizeof(size_t);
constexpr size_t sso_max = string_size - 1;
constexpr size_t big_mask = ~(1ull << (8ull * sizeof(size_t) - 1ull));
struct small_string
{
char buf[sso_max];
// interpretation is: size == sso_max - rem
unsigned char rem;
#if 0
size_t rem : 7;
size_t big : 1 /* = 0 */;
#endif
};
struct big_string
{
char* p;
size_t n;
// interpretation is: capacity == c & big_mask
size_t c;
#if 0
size_t c : sizeof(size_t) * 8 - 1;
size_t big : 1 /* = 1 */;
#endif
~big_string() /* noexcept */;
};
} // namespace __
class string
{
public:
using iterator = char*; using iterator = char*;
using const_iterator = const char*; using const_iterator = const char*;
static constexpr size_t npos = -1; static constexpr size_t npos = -1;
~string() noexcept; ~string() /* noexcept */;
string() = default;
string(const string_view) noexcept; string(const string_view) noexcept;
string(const char*) noexcept; string(const char*) noexcept;
string(const string&) noexcept; string(const string&) noexcept;
string(const char*, size_t) noexcept; string(const char*, size_t) noexcept;
explicit string(size_t, char = 0) noexcept; explicit string(size_t, char = 0) noexcept;
string& operator=(string&&) noexcept;
string& operator=(string) noexcept;
const char* c_str() const noexcept; const char* c_str() const noexcept;
void pop_back() noexcept; void pop_back() noexcept;
@ -51,103 +81,139 @@ struct string
size_t find(char, size_t = 0) const noexcept; size_t find(char, size_t = 0) const noexcept;
size_t find(const string_view, size_t = 0) const noexcept; size_t find(const string_view, size_t = 0) const noexcept;
string(string&& s) noexcept : p(s.p), n(s.n), c(s.c) string() noexcept
{ {
s.p = nullptr; set_small_size(0);
s.n = 0; #if 0
s.c = 0; small()->buf[0] = 0;
#endif
}
void swap(string& s) noexcept
{
char tmp[__::string_size];
__builtin_memcpy(tmp, __builtin_launder(blob), sizeof(tmp));
__builtin_memcpy(
__builtin_launder(blob), __builtin_launder(s.blob), sizeof(tmp));
__builtin_memcpy(__builtin_launder(s.blob), tmp, sizeof(tmp));
}
string(string&& s) noexcept
{
__builtin_memcpy(blob, __builtin_launder(s.blob), sizeof(blob));
s.set_small_size(0);
#if 0
s.small()->buf[0] = 0;
#endif
} }
void clear() noexcept void clear() noexcept
{ {
n = 0; if (isbig()) {
big()->n = 0;
} else {
set_small_size(0);
}
} }
bool empty() const noexcept bool empty() const noexcept
{ {
return !n; return isbig() ? !big()->n : small()->rem >= __::sso_max;
} }
char* data() const noexcept inline char* data() noexcept
{ {
return p; return isbig() ? big()->p : small()->buf;
} }
size_t size() const noexcept inline const char* data() const noexcept
{ {
return n; return isbig() ? big()->p : small()->buf;
}
inline size_t size() const noexcept
{
#if 0
if (!isbig() && small()->rem > __::sso_max)
__builtin_trap();
#endif
return isbig() ? big()->n : __::sso_max - small()->rem;
} }
size_t length() const noexcept size_t length() const noexcept
{ {
return n; return size();
} }
size_t capacity() const noexcept size_t capacity() const noexcept
{ {
return c; #if 0
if (isbig() && big()->c <= __::sso_max)
__builtin_trap();
#endif
return isbig() ? __::big_mask & big()->c : __::sso_max;
} }
iterator begin() noexcept iterator begin() noexcept
{ {
return p; return data();
} }
iterator end() noexcept iterator end() noexcept
{ {
return p + n; return data() + size();
} }
const_iterator cbegin() const noexcept const_iterator cbegin() const noexcept
{ {
return p; return data();
} }
const_iterator cend() const noexcept const_iterator cend() const noexcept
{ {
return p + n; return data() + size();
} }
char& front() char& front()
{ {
if (!n) if (!size())
__builtin_trap(); __builtin_trap();
return p[0]; return data()[0];
} }
const char& front() const const char& front() const
{ {
if (!n) if (!size())
__builtin_trap(); __builtin_trap();
return p[0]; return data()[0];
} }
char& back() char& back()
{ {
if (!n) if (!size())
__builtin_trap(); __builtin_trap();
return p[n - 1]; return data()[size() - 1];
} }
const char& back() const const char& back() const
{ {
if (!n) if (!size())
__builtin_trap(); __builtin_trap();
return p[n - 1]; return data()[size() - 1];
} }
char& operator[](size_t i) noexcept char& operator[](size_t i) noexcept
{ {
if (i >= n) if (i >= size())
__builtin_trap(); __builtin_trap();
return p[i]; return data()[i];
} }
const char& operator[](size_t i) const noexcept const char& operator[](size_t i) const noexcept
{ {
if (i >= n) if (i >= size())
__builtin_trap(); __builtin_trap();
return p[i]; return data()[i];
} }
void push_back(char ch) noexcept void push_back(char ch) noexcept
@ -160,9 +226,10 @@ struct string
append(s.p, s.n); append(s.p, s.n);
} }
inline constexpr operator string_view() const noexcept // TODO(mrdomino): explicit?
inline operator string_view() const noexcept
{ {
return string_view(p, n); return string_view(data(), size());
} }
string& operator=(const char* s) noexcept string& operator=(const char* s) noexcept
@ -220,8 +287,70 @@ struct string
{ {
return compare(s) >= 0; return compare(s) >= 0;
} }
private:
inline bool isbig() const noexcept
{
return *(__builtin_launder(blob) + __::sso_max) & 0x80;
}
inline void set_small_size(size_t size) noexcept
{
if (size > __::sso_max)
__builtin_trap();
*(__builtin_launder(blob) + __::sso_max) = (__::sso_max - size);
}
inline void set_big_capacity(size_t c2) noexcept
{
if (c2 > __::big_mask)
__builtin_trap();
*(__builtin_launder(blob) + __::sso_max) = 0x80;
big()->c &= ~__::big_mask;
big()->c |= c2;
}
inline __::small_string* small() noexcept
{
if (isbig())
__builtin_trap();
return __builtin_launder(reinterpret_cast<__::small_string*>(blob));
}
inline const __::small_string* small() const noexcept
{
if (isbig())
__builtin_trap();
return __builtin_launder(
reinterpret_cast<const __::small_string*>(blob));
}
inline __::big_string* big() noexcept
{
if (!isbig())
__builtin_trap();
return __builtin_launder(reinterpret_cast<__::big_string*>(blob));
}
inline const __::big_string* big() const noexcept
{
if (!isbig())
__builtin_trap();
return __builtin_launder(reinterpret_cast<const __::big_string*>(blob));
}
friend string strcat(const string_view, const string_view);
alignas(union {
__::big_string a;
__::small_string b;
}) char blob[__::string_size];
}; };
static_assert(sizeof(string) == __::string_size);
static_assert(sizeof(__::small_string) == __::string_size);
static_assert(sizeof(__::big_string) == __::string_size);
} // namespace ctl } // namespace ctl
#pragma GCC diagnostic push #pragma GCC diagnostic push

7
test/ctl/string_test.cc
View file

@ -308,6 +308,13 @@ main()
s.append(" world"); s.append(" world");
if (s != "hello world") if (s != "hello world")
return 63; return 63;
for (int i = 0; i < 10; ++i) {
s.append(" world");
}
if (s != "hello world world world world world world world world world "
"world world") {
return 64;
}
} }
// { // {