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Make circular buffer use only power-of-two sizes, cheaper arithmetic.

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This commit is contained in:
Michael Bebenita 2010-07-27 23:38:16 -07:00 committed by Graydon Hoare
parent d786469785
commit 60b0486c11
2 changed files with 15 additions and 3 deletions
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14
src/rt/circular_buffer.cpp
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@ -4,6 +4,14 @@
#include "rust_internal.h"
bool
is_power_of_two(size_t value) {
if (value > 0) {
return (value & (value - 1)) == 0;
}
return false;
}
circular_buffer::circular_buffer(rust_dom *dom, size_t unit_sz) :
dom(dom),
_buffer_sz(INITIAL_CIRCULAR_BUFFFER_SIZE_IN_UNITS * unit_sz),
@ -37,9 +45,10 @@ circular_buffer::~circular_buffer() {
void
circular_buffer::transfer(void *dst) {
I(dom, dst);
I(dom, is_power_of_two(_buffer_sz));
uint8_t *ptr = (uint8_t *) dst;
for (size_t i = 0; i < _unread; i += _unit_sz) {
memcpy(&ptr[i], &_buffer[(_next + i) % _buffer_sz], _unit_sz);
memcpy(&ptr[i], &_buffer[(_next + i) & (_buffer_sz - 1)], _unit_sz);
}
}
@ -67,11 +76,12 @@ circular_buffer::enqueue(void *src) {
"unread: %d, buffer_sz: %d, unit_sz: %d",
_unread, _buffer_sz, _unit_sz);
I(dom, is_power_of_two(_buffer_sz));
I(dom, _unread < _buffer_sz);
I(dom, _unread + _unit_sz <= _buffer_sz);
// Copy data
size_t i = (_next + _unread) % _buffer_sz;
size_t i = (_next + _unread) & (_buffer_sz - 1);
memcpy(&_buffer[i], src, _unit_sz);
_unread += _unit_sz;

4
src/rt/circular_buffer.h
View file

@ -20,7 +20,9 @@ public:
bool is_empty();
private:
// Size of the buffer in bytes.
// Size of the buffer in bytes, should always be a power of two so that
// modulo arithmetic (x % _buffer_sz) can optimized away with
// (x & (_buffer_sz - 1)).
size_t _buffer_sz;
// Size of the data unit in bytes.