On Friday, 5 February 2016 at 17:33:55 UTC, Era Scarecrow wrote:
On Friday, 5 February 2016 at 15:41:11 UTC, Fool wrote:
One swap usually decomposes into three moves.
Recently from reading some interesting hacks and super code, a
good swap can also be done via 3 xor operations (and avoiding
an intermediate storage unit).
http://aggregate.ee.engr.uky.edu/MAGIC/#Swap%20Values%20Without%20a%20Temporary
x ^= y;
y ^= x;
x ^= y;
Since these can be used directly as registers it might be
faster (assuming all 5 are mapped to registers until the final
write out, which might not work).
It's a cool trick but I would be surprised if this were actually
faster. Modern x64 processors have 16 "general purpose" registers
so saving a single register for a simple set of instructions is
likely to not have any benefit. My other thought is that the
compiler may not recognize this pattern, making it more difficult
to optimize. On the other hand, compilers are very good at
rearranging simple reads and writes to avoid stalling the
pipeline in the CPU.
