qemu

buffer.c (4795B)

---

 /*
  * QEMU generic buffers
  *
  * Copyright (c) 2015 Red Hat, Inc.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Lesser General Public
  * License as published by the Free Software Foundation; either
  * version 2.1 of the License, or (at your option) any later version.
  *
  * This library is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  * Lesser General Public License for more details.
  *
  * You should have received a copy of the GNU Lesser General Public
  * License along with this library; if not, see <http://www.gnu.org/licenses/>.
  *
  */
 
 #include "qemu/osdep.h"
 #include "qemu/host-utils.h"
 #include "qemu/buffer.h"
 #include "trace.h"
 
 #define BUFFER_MIN_INIT_SIZE     4096
 #define BUFFER_MIN_SHRINK_SIZE  65536
 
 /* define the factor alpha for the exponential smoothing
  * that is used in the average size calculation. a shift
  * of 7 results in an alpha of 1/2^7. */
 #define BUFFER_AVG_SIZE_SHIFT       7
 
 static size_t buffer_req_size(Buffer *buffer, size_t len)
 {
     return MAX(BUFFER_MIN_INIT_SIZE,
                pow2ceil(buffer->offset + len));
 }
 
 static void buffer_adj_size(Buffer *buffer, size_t len)
 {
     size_t old = buffer->capacity;
     buffer->capacity = buffer_req_size(buffer, len);
     buffer->buffer = g_realloc(buffer->buffer, buffer->capacity);
     trace_buffer_resize(buffer->name ?: "unnamed",
                         old, buffer->capacity);
 
     /* make it even harder for the buffer to shrink, reset average size
      * to current capacity if it is larger than the average. */
     buffer->avg_size = MAX(buffer->avg_size,
                            buffer->capacity << BUFFER_AVG_SIZE_SHIFT);
 }
 
 void buffer_init(Buffer *buffer, const char *name, ...)
 {
     va_list ap;
 
     va_start(ap, name);
     buffer->name = g_strdup_vprintf(name, ap);
     va_end(ap);
 }
 
 static uint64_t buffer_get_avg_size(Buffer *buffer)
 {
     return buffer->avg_size >> BUFFER_AVG_SIZE_SHIFT;
 }
 
 void buffer_shrink(Buffer *buffer)
 {
     size_t new;
 
     /* Calculate the average size of the buffer as
      * avg_size = avg_size * ( 1 - a ) + required_size * a
      * where a is 1 / 2 ^ BUFFER_AVG_SIZE_SHIFT. */
     buffer->avg_size *= (1 << BUFFER_AVG_SIZE_SHIFT) - 1;
     buffer->avg_size >>= BUFFER_AVG_SIZE_SHIFT;
     buffer->avg_size += buffer_req_size(buffer, 0);
 
     /* And then only shrink if the average size of the buffer is much
      * too big, to avoid bumping up & down the buffers all the time.
      * realloc() isn't exactly cheap ...  */
     new = buffer_req_size(buffer, buffer_get_avg_size(buffer));
     if (new < buffer->capacity >> 3 &&
         new >= BUFFER_MIN_SHRINK_SIZE) {
         buffer_adj_size(buffer, buffer_get_avg_size(buffer));
     }
 
     buffer_adj_size(buffer, 0);
 }
 
 void buffer_reserve(Buffer *buffer, size_t len)
 {
     if ((buffer->capacity - buffer->offset) < len) {
         buffer_adj_size(buffer, len);
     }
 }
 
 gboolean buffer_empty(Buffer *buffer)
 {
     return buffer->offset == 0;
 }
 
 uint8_t *buffer_end(Buffer *buffer)
 {
     return buffer->buffer + buffer->offset;
 }
 
 void buffer_reset(Buffer *buffer)
 {
     buffer->offset = 0;
     buffer_shrink(buffer);
 }
 
 void buffer_free(Buffer *buffer)
 {
     trace_buffer_free(buffer->name ?: "unnamed", buffer->capacity);
     g_free(buffer->buffer);
     g_free(buffer->name);
     buffer->offset = 0;
     buffer->capacity = 0;
     buffer->buffer = NULL;
     buffer->name = NULL;
 }
 
 void buffer_append(Buffer *buffer, const void *data, size_t len)
 {
     memcpy(buffer->buffer + buffer->offset, data, len);
     buffer->offset += len;
 }
 
 void buffer_advance(Buffer *buffer, size_t len)
 {
     memmove(buffer->buffer, buffer->buffer + len,
             (buffer->offset - len));
     buffer->offset -= len;
     buffer_shrink(buffer);
 }
 
 void buffer_move_empty(Buffer *to, Buffer *from)
 {
     trace_buffer_move_empty(to->name ?: "unnamed",
                             from->offset,
                             from->name ?: "unnamed");
     assert(to->offset == 0);
 
     g_free(to->buffer);
     to->offset = from->offset;
     to->capacity = from->capacity;
     to->buffer = from->buffer;
 
     from->offset = 0;
     from->capacity = 0;
     from->buffer = NULL;
 }
 
 void buffer_move(Buffer *to, Buffer *from)
 {
     if (to->offset == 0) {
         buffer_move_empty(to, from);
         return;
     }
 
     trace_buffer_move(to->name ?: "unnamed",
                       from->offset,
                       from->name ?: "unnamed");
     buffer_reserve(to, from->offset);
     buffer_append(to, from->buffer, from->offset);
 
     g_free(from->buffer);
     from->offset = 0;
     from->capacity = 0;
     from->buffer = NULL;
 }