[Mono-dev] implementation of BitConverter [possible perf improvements?]

[Alan]

Hi,

You probably still need the null check and you definitely still need the
array bounds checking. Both of these will slow things down a bit. Other
than that there's no reason not to do things as you describe, i'm surprised
it's not done already. If you want to create a patch on github which passes
the regression tests (and any new ones which may be required), I'd be happy
to merge it.

Alan

On 17 November 2011 23:44, Jonathan Shore <jonathan.shore at gmail.com> wrote:

>
> I was looking at the code for the mono implementation of BitConverter and
> was surprised to see that common types (such as long) are converted by
> writing to a byte* a byte at a time.   I don't  know why it was done this
> way unless this was done to avoid a temporary pin of the byte[].
>
> The current code is:
>
> 		unsafe static void PutBytes (byte *dst, byte[] src, int start_index, int count)
> 		{
> 			if (src == null)
> 				throw new ArgumentNullException ("value");
>
> 			if (start_index < 0 || (start_index > src.Length - 1))
> 				throw new ArgumentOutOfRangeException ("startIndex", "Index was"
> 					+ " out of range. Must be non-negative and less than the"
> 					+ " size of the collection.");
>
> 			// avoid integer overflow (with large pos/neg start_index values)
> 			if (src.Length - count < start_index)
> 				throw new ArgumentException ("Destination array is not long"
> 					+ " enough to copy all the items in the collection."
> 					+ " Check array index and length.");
>
> 			for (int i = 0; i < count; i++)
> 				dst[i] = src[i + start_index];
> 		}
>
> 		unsafe public static long ToInt64 (byte[] value, int startIndex)
> 		{
> 			long ret;
>
> 			PutBytes ((byte *) &ret, value, startIndex, 8);
>
> 			return ret;
> 		}
>
>
>
> The above code does avoid pinning the byte[] buffer, however is 3-4x
> slower than, say doing this:
>
> unsafe public static long ToLong (byte[] buffer, int offset)
> {
> fixed (byte* pbuf = buffer)
> {
> long* vlong = (long*)(pbuf + offset);
> return *vlong;
> }
> }
>
>
> Any reason why we would not want to do the above?
>
> Jonathan
>
>
>
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