Reading text files using pointers?

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From: Einar Høst (anonymous_at_discussions.microsoft.com)
Date: 02/12/04 

Date: Thu, 12 Feb 2004 03:46:33 -0800

Hi,

I'm trying to learn a bit about performance, hope someone can help me out.

I have a text file with 8-bit characters in it. In order to improve performance, I'm using a BinaryReader instead of a StreamReader. I've made two versions of my method, one which uses typesafe code, and one which uses unsafe code with pointers. I've read several places that direct pointer access will eliminate bounds-checking when accessing an array, and would like to see the effect myself. However, the typesafe code is as fast or faster than the unsafe code. The methods are practically identical, except for the access bit:

Shared code:
BinaryReader reader = ... // Open data reader.
int fileSize = ... // Get size of file.
int bufSize = 32768;
byte[] buf = null;
int bytesRead = 0;
int totalBytesRead = 0;
byte msgStartCode = (byte) '$';

Typesafe version is like:
do
{
  buf = reader.ReadBytes(bufSize);
  bytesRead = buf.Length;
  totalBytesRead += bytesRead;
 
  for (int bufIndex = 0; bufIndex < buf.Length; bufIndex++)
  {
    if (buf[bufIndex] == msgStartCode)
    {
      // Parse message.
    }
  } // for

} while (totalBytesRead < fileSize);

wheras the unsafe version is like:

do
{
  buf = reader.ReadBytes(bufSize);
  bytesRead = buf.Length;
  totalBytesRead += bytesRead;

  // Pin memory.
  fixed (byte* bufPtrUnsigned = &buf[0])
  {
    sbyte* bufPtr = (sbyte*) bufPtrUnsigned; // Use to build message string
 
    for (int bufIndex = 0; bufIndex < buf.Length; bufIndex++)
    {
      if (buf[bufIndex] == msgStartCode)
      {
        // Parse message, using e.g. new String(bufPtr, bufIndex, msgLength);
      }
    } // for

  } // fixed

} while (totalBytesRead < fileSize);

Shouldn't the last version be faster?

Thanks in advance for any help!

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