"Tom Lane" <[EMAIL PROTECTED]> writes:
>
> Also, I would like to see the actual test code.  I wonder whether what
> you measured is the ability of the compiler to optimize references to
> successive elements of an array inside a loop; that has little or
> nothing to do with the typical usage of BufferGetBlock().
>

The source code is attached.

compiled with "gcc testbuf.c". I tried -O2 actually, and it turns out that
the timing is reduced a lot so not believable.
---

/*
 * testbuf.c
 */
#include <stdio.h>
#include <sys/file.h>
#include <sys/param.h>
#include <sys/stat.h>
#include <sys/time.h>
#include <unistd.h>
#include <fcntl.h>

#define BLCKSZ  8192
#define NBuffers 80000

typedef void* Block;

int main(void)
{
 int  i, round, method;
 Block k, start;
 struct timeval start_t, stop_t;
 long usecs;
 Block *array = (Block *) calloc(NBuffers, sizeof(Block));

 start = (Block)0xff3386;
 for (i = 0; i < NBuffers; i++)
  array[i] = start + BLCKSZ*i;

 for (method = 0; method < 3; method ++)
 {
  start = (Block)0xff3386;
  for (round = 0; round < 3; round ++)
  {
   gettimeofday(&start_t, NULL);
   if (method == 0)
   {
    for (i = 0; i < NBuffers; i++)
     k = array[i];
   }
   if (method == 1)
   {
    for (i = 0; i < NBuffers; i++)
     k = start + i*BLCKSZ;
   }
   if (method == 2)
   {
    for (i = 0; i < NBuffers; i++)
     k = start + (i<<13);
   }
   gettimeofday(&stop_t, NULL);

   if (stop_t.tv_usec < start_t.tv_usec)
   {
     stop_t.tv_sec--;
     stop_t.tv_usec += 1000000;
   }

   usecs = (long) (stop_t.tv_sec - start_t.tv_sec) * 1000000
     + (long) (stop_t.tv_usec - start_t.tv_usec);

   fprintf (stdout, "duration round %d of %s method: %ld.%03ld ms\n",
       round + 1,
       method==0?"array":method==1?"mul":"shift",
       (long) ((stop_t.tv_sec - start_t.tv_sec) * 1000 +
         (stop_t.tv_usec - start_t.tv_usec) / 1000),
       (long) (stop_t.tv_usec - start_t.tv_usec) % 1000);
  }
 }
}



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