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Use memset() to optimize prevector::resize()

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Further optimize prevector::resize() (which is called by a number of
other prevector methods) to use memset to initialize memory when the
prevector contains trivial types.
This commit is contained in:
Evan Klitzke 2018-02-26 21:39:22 -08:00
parent e46be25f0e
commit 5aad635b78
1 changed files with 25 additions and 6 deletions
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31
src/prevector.h
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@ -10,9 +10,12 @@
#include <stdint.h> #include <stdint.h>
#include <string.h> #include <string.h>
#include <cstddef>
#include <iterator> #include <iterator>
#include <type_traits> #include <type_traits>
#include <compat.h>
#pragma pack(push, 1) #pragma pack(push, 1)
/** Implements a drop-in replacement for std::vector<T> which stores up to N /** Implements a drop-in replacement for std::vector<T> which stores up to N
* elements directly (without heap allocation). The types Size and Diff are * elements directly (without heap allocation). The types Size and Diff are
@ -194,8 +197,21 @@ private:
T* item_ptr(difference_type pos) { return is_direct() ? direct_ptr(pos) : indirect_ptr(pos); } T* item_ptr(difference_type pos) { return is_direct() ? direct_ptr(pos) : indirect_ptr(pos); }
const T* item_ptr(difference_type pos) const { return is_direct() ? direct_ptr(pos) : indirect_ptr(pos); } const T* item_ptr(difference_type pos) const { return is_direct() ? direct_ptr(pos) : indirect_ptr(pos); }
void fill(T* dst, size_type count, const T& value) { void fill(T* dst, ptrdiff_t count) {
for (size_type i = 0; i < count; ++i) { if (IS_TRIVIALLY_CONSTRUCTIBLE<T>::value) {
// The most common use of prevector is where T=unsigned char. For
// trivially constructible types, we can use memset() to avoid
// looping.
::memset(dst, 0, count * sizeof(T));
} else {
for (auto i = 0; i < count; ++i) {
new(static_cast<void*>(dst + i)) T();
}
}
}
void fill(T* dst, ptrdiff_t count, const T& value) {
for (auto i = 0; i < count; ++i) {
new(static_cast<void*>(dst + i)) T(value); new(static_cast<void*>(dst + i)) T(value);
} }
} }
@ -310,16 +326,19 @@ public:
void resize(size_type new_size) { void resize(size_type new_size) {
size_type cur_size = size(); size_type cur_size = size();
if (cur_size == new_size) {
return;
}
if (cur_size > new_size) { if (cur_size > new_size) {
erase(item_ptr(new_size), end()); erase(item_ptr(new_size), end());
return;
} }
if (new_size > capacity()) { if (new_size > capacity()) {
change_capacity(new_size); change_capacity(new_size);
} }
for (T* p = item_ptr(0); cur_size < new_size; cur_size++) { ptrdiff_t increase = new_size - cur_size;
_size++; fill(item_ptr(cur_size), increase);
new(static_cast<void*>(p + cur_size)) T(); _size += increase;
}
} }
void reserve(size_type new_capacity) { void reserve(size_type new_capacity) {