Lolling at programmers, how many ways are there to create a bitmask? ;) :)
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From: Andy 'Krazy' Glew on 25 May 2010 11:51 On 5/25/2010 6:09 AM, Skybuck Flying wrote: > 1. (Skybuck) Mask := not word(65535 shl BitCount); // not 1111000 = > 0000111 Many instruction sets do not handle constants greater than some size well. Above you have a 16 bit constant. Even though x86 handles most constants in the instruction set, larger constants cost instruction bytes. > 2. (Michael Vinther) Mask := (1 shl BitCount)-1; // 10000-1 = 09999 = 01111 > ;) :) This is what I would usually recommend. Although, on some machines, shifts of any form are slow. > 3. (Skamradt) (Mask := ($FFFF shl BitCount) xor $FFFF; // 1111000 xor > 1111111 = 0000111 Constant size. > 4. (MitchAlsup) Mask := not word((not 0) shl BitCount); // not((not 0 = > 1111111) shl 3 = 1111000) = 0000111 Trades off constant size for instruction count. --- When shifts are slow, instruction sets are advised to have instructions to generate bitmasks. dest := bitmask(frombit,tobit) but even these have 2 6 bit constants (on a 64 bit machine). dest := bitmask(bitpos6,len) not much better, except for small masks. |