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captacha

Bit Flags vs. Boolean

7 posts in this topic

I was trying to explain to a semi-programmer friend of mine today the purpose of Bit Flags vs. Booleans. And I couldn't think of any besides Memory Usage which seems rather weak seeing as it would save you a few 100 bytes of memory at most. So what's a could argument for Bit Flags I can tell him about, and to be honest I'm wondering myself right now too.
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It tends to be a bit more memory than that in most use cases. You often don't just have one bit flag, you have one bit flag per user/entity/product/etc. It adds up.

It also speeds certain operations since an equality check is 1 op rather than N. And/Or/Xor/Negate are also 1 op instead of N.

Reading and writing from disk/database/network is faster since there's less space and/or compression going on.

I'm sure there's a few others I'm missing.
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One example of a performance benefit would be doing something like render queue sorting based on a bitfield. It would be much better to sort based on a single field rather than a bunch of single flags.
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Bitfields were available in C because C was used to develop the Unix operating system, and operating systems deal with hardware. Hardware control registers often have bit-addressable functions.

C also did not have a boolean type. The PDP architeture it was first developed on has a (bit-addressable) flag register in which the Z flag gets set every time an operation yields a zero result (and cleared otherwise), so it was easier to have control-flow operations act on zero or non-zero integral values. BTW, that's also why null-terminated character strings were used instead of the more-common (at the time) Pascal-style counted strings.
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[quote name='SiCrane' timestamp='1349925185' post='4988945']
One particular application you might want to look at is Bitboards.
[/quote]
This is probably the best example and is also what I was hinting at by using the bitboard concept to sort render ops via state, etc.
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One thing to keep in mind is that accessing bit fields will require the compiler to produce additional bit masking instructions, which will *increase* the memory footprint of your instruction code. If you try to use bit fields as a general rule instead of a situational optimisation, you'll end up with slower code.

Also, the alignment issues are subtle. Most types want to align on 4 byte boundaries, so AFAIK if you have a structure containing, say, an integer, and any less than 5 flags, then converting these flags to single bits won't save any space at all.
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