Overhead of casting double to float?

Question

So I have megabytes of data stored as doubles that need to be sent over a network... now I don't need the precision that a double offers, so I want to convert these to a float before sending them over the network. What is the overhead of simply doing:

float myFloat = (float)myDouble;

I'll be doing this operation several million times every few seconds and don't want to slow anything down. Thanks

EDIT: My platform is x64 with Visual Studio 2008.

EDIT 2: I have no control over how they are stored.

Answer 1

As Michael Burr said, while the overhead strongly depends on your platform, the overhead is definitely less than the time needed to send them over the wire.

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a rough estimate:

800MBit/s payload on a excellent Gigabit wire, 25M-floats/second.

On a 2GHz single core, that gives you a whopping 80 clock cycles for each value converted to break even - anythign less, and you will save time. That should be more than enough on all architectures :)

A simple load-store cycle (barring all caching delays) should be below 5 cycles per value. With instruction interleaving, SIMD extensions and/or parallelizing on multiple cores, you are likely to do multiple conversions in a single cycle.

Also, the receiver will be happy having to handle only half the data. Remember that memory access time is nonlinear.

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The only thing arguing against the conversion would be is if the transfer should have minimal CPU load: a modern architecture could transfer the data from disk/memory to bus without CPU intervention. However, with above numbers I'd say that doesn't matter in practice.

[edit]
I checked some numbers, the 387 coprocessor would indeed have taken around 70 cycles for a load-store cycle. On the initial pentium, you are down to 3 cycles without any parallelization.

So, unless you run a gigabit network on a 386...