On 4/8/12 4:55 AM, Vladimir Panteleev wrote:
On Sunday, 8 April 2012 at 09:46:28 UTC, Vladimir Panteleev wrote:
On Sunday, 8 April 2012 at 05:56:36 UTC, Andrei Alexandrescu wrote:
Walter and I discussed today about using the small string
optimization in string and other arrays of immutable small objects.
On 64 bit machines, string occupies 16 bytes. We could use the first
byte as discriminator, which means that all strings under 16 chars
need no memory allocation at all.
Don't use the first byte. Use the last byte.
The last byte is the highest-order byte of the length. Limiting arrays
to 18.37 exabytes, as opposed to 18.45 exabytes, is a much nicer
limitation than making assumptions about the memory layout.
Erm... never mind, I thought the pointer was the first field.
Me too, actually.
Even so, how would you use the lowest-order byte of the length as a
discriminator? Unless you allocated bytes 2-8 for the length (which
would be an unaligned read, or a shift every time...)
It's amazing how fast shift works :o).
"making assumptions about the memory layout" now seems like the only
solution.
Also, what will be .ptr for such arrays? It can't point inside the
string, because the type is immutable and the array is on the stack.
Duplication can be expensive as well.
Once anyone asks for .ptr a conservative copy will be made.
Andrei
