Telling gcc to align the array on a 16-byte boundary make the address calculation cheaper, but the total instruction count is 18, and some of them are multi-cycle instructions. Or if your array is 256-byte aligned, you could just set the upper byte of a pointer register and put your lookup value in the low byte. If your array is in the low 256 bytes of address space, this could be cheaper. So the 16-bit array address has to be added to the 4-bit nibble values. The only choice of addressing mode is post-increment or pre-decrement, no immediate displacements. (But it can be rolled up into a small loop).Īrray indexing is not cheap in AVR. So this is slightly better than answer, which needs 16 single-cycle instructions not including a ret. reverse_byte_alu:ġ6 instructions, 2 bytes each, all of them 1-cycle. X = ((x > 4)) // 4-bit rotate is recognized as SWAPĬompiles into this asm with gcc4.6 -O3 (Godbolt compiler explorer). X = (xeven > 2) // swap adjacent 2-bit chunks X = (xeven > 1) // swap adjacent bit pairs Utility for byte swapping of all java data types / (C) 2004 - Geotechnical Software Services This code is free software you can redistribute it and/or modify it under the terms of the GNU Lesser General Public License as published by the Free Software Foundation either version 2.1 of the License, or (at your option) any later version. Uint8_t xeven = x & 0x55, xodd = x & 0xaa Shifts are only available with one count per instruction.) #include (AVR doesn't have a multiple-count rotate instruction, and normal rotates are rotate-through-carry. Ironically it does optimize x>4 into a rotate using the SWAP instruction. gcc4.6 has surprisingly terrible missed-optimizations when compiling Brett's code (actually promoting to int and not taking full advantage of truncating the result to uint8_t). But that would cost more in indexing (using a branch on bit 4 of index to conditionally SWAP before masking) than it saves in table size (8 bytes = 4 instructions). It would be possible to pack the LUT, using the high and low half of each byte. To convert it to a byte array, we translate the sequence of characters into a sequence of bytes. A String is stored as an array of Unicode characters in Java. Then we'll look at similar operations in reverse. (It turns out not to be worth it for speed or size, except maybe if you can align your array by 256 bytes to make indexing much cheaper than what gcc does.) If the underlying byte array is exactly 1 byte / 8 bit long, returns signed two-complement representation for a Java byte value. First, we'll look at various ways to convert a String to a byte array. We can go through each index and bit-or the corresponding index (at reversed location).A 4-bit (16 entry) lookup table for the two halves of the byte looks like a good tradeoff (as points out in a comment).ĪVR instructions are 2 bytes each, so a 16-byte table costs the same space as 8 instructions. The input must be a binary string of length 32 If this function is called many times, how would you optimize it? Therefore, in Example 2 above, the input represents the signed integer -3 and the output represents the signed integer -1073741825. In Java, the compiler represents the signed integers using 2's complement notation. They should not affect your implementation, as the integer's internal binary representation is the same, whether it is signed or unsigned. In this case, both input and output will be given as a signed integer type. Note that in some languages such as Java, there is no unsigned integer type. Reverse bits of a given 32 bits unsigned integer.
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