![]() ![]() Choosing between Conversion Functions, Buffer Types, and Arithmetic Types.Introduction to the Boost.Endian library.Segmentation fault when using fscanf in c.C - Convert an uppercase letter to lowercase.conntrack delete does not stop runnig copy of big file.how to use a makefile for mingw on win32.How can adding a function call cause other symbols to become undefined when linking?. ![]() How to get CRC64 distributed calculation (use its linearity property)?.Why is the XC16 compiler removing inline function if the definition comes after use?.Set 8th bit if all lower 7 bits are set without branching.Cast char to a bit field entry of 4 bits.Find "edges" in 32 bits word bitpattern.Default structure alignment for 32 bit processor word.How do I extract bits from 32 bit number.Size of packed struct with union of bit fields less than 8 bits in C.Processor word size? 8bit processor = 8 bit word?.Bit manipulation - generate mask for first N set bits.Easiest way to print 64 bit hex data splitted by 16 bits.Reading in 16 bits from a 32 bit register.what is the most efficient way to flip all the bits from the least significant bit up to the most significant last 1 bit value?.Unset the most significant bit in a word (int32).bit manipulation:print the next smallest and largest numbers with same no of 1 bits.Return 1 if any bits in an integer equal 1 using bit operations in C.Compacting data in buffer from 16 bit per element to 12 bits.How to write a constant time function to copy the most significant bit to all bits.Efficient computation of the high order bits of a 32 bit integer multiplication.Is a bit field any more efficient (computationally) than masking bits and extracting the data by hand?.bit manipulation: clearing range of bits.Reverse the order of bits in a bit array.Bit Twiddling Hacks: interleave bits the obvious way.Fastest way to scan for bit pattern in a stream of bits.Fast way to generate pseudo-random bits with a given probability of 0 or 1 for each bit. ![]() Well, if we're doing ascii art, here's mine: 7 6 5 4 3 2 1 0 X = ((x & 0x0f0f0f0f) > 4) // swap _Īn example of the first 3 assignment, with a uint8_t example: b7 b6 b5 b4 b3 b2 b1 b0 the o(log n) way is (for up to 32 bits): uint32_t reverse(uint32_t x, int bits) It's actually called "bit reversal", and is commonly done in fft scrambling. Rbit has single-cycle latency, and does a whole 32-bit integer in one instruction. (round trip latency = 3 cycles on an intel cpu like haswell.) arm: ![]() that only costs 2 extra movd instructions to get an integer from an integer register into xmm and back. It's still good for a single 32-bit int because x86 has very efficient round-trip between integer and vector regs: int bitrev = _mm_cvtsi128_si32 ( rbit32( _mm_cvtsi32_si128(input) ) ). the same code ported to 128-bit vectors would compile just fine with avx. it's basically the same problem as for pshufb-based popcount.Īvx2 register bits reverse shows how to do this for a packed vector of 32-bit elements. you only need 8 lookups for the 8 nibbles in a single 32-bit integer, but the real problem is splitting the input bytes into separate nibbles (with their upper half zeroed). On x86 with ssse3 (core2 and later, bulldozer and later), pshufb ( _mm_shuffle_epi8) can be used as a nibble lut to do 16 lookups in parallel. Nearly a duplicate of most efficient algorithm for bit reversal ( from msb->lsb to lsb->msb) in c (which has a lot of answers, including one avx2 answer for reversing every 8-bit char in an array). (see it on the godbolt compiler explorer) this loop is good for small code size (no large masks), but otherwise pretty much no advantage.Ĭompilers unfortunately don't recognize this loop as a bit-reverse and optimize it to arm rbit or whatever. of course, there are much faster ways to bit-reverse see the other answers. The += instead of |= makes gcc compile it more efficiently for x86 (using x86's shift-and-add instruction, lea). the fixed-width types are a good idea here. #include įor other types, the number of bits of storage is sizeof(input) * char_bit, but that includes potential padding bits that aren't part of the value. The naive / slow / simple way is to extract the low bit of the input and shift it into another variable that accumulates a return value. ![]()
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