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101. There is a great trick to calculate the absolute value of a 2s-complement integer without using an if statement. The theory goes, if the value is negative you want to toggle the bits and add one, otherwise you want to pass the bits through as is. A XOR 1 happens to toggle A and A XOR 0 happens to leave A intact.
The fastest way would probably involve manually changing the sign bit of the floating point value. I'm not going to benchmark this for you since it is 3am for me right now, but it would probably involve creating a function marked as unsafe, reinterpreting it as an int with something like int bits = *(int*)(&val);, changing the sign bit with something like bits &= ~(1 << 31); and then ...
Which is the fastest way to get the absolute value of a number (16 answers) Closed 8 years ago . What is the fastest way to compute absolute value of integer of long long int type in C++ ?
max(max(a),-min(a)) It's the fastest for now, since no intermediate list is created (for 100 000 values): In [200]: %timeit max(max(a),-min(a)) 100 loops, best of 3: 8.82 ms per loop In [201]: %timeit abs(max(a,key=abs)) 100 loops, best of 3: 13.8 ms per loop In [202]: %timeit max(map(abs,a)) 100 loops, best of 3: 13.2 ms per loop In [203]: %timeit max(abs(n) for n in a) 10 loops, best of 3: ...
I want to calculate the absolute values of the elements of a complex array in C or C++. The easiest way would be . for(int i = 0; i < N; i++) { b[i] = cabs(a[i]); } But for large vectors that will be slow. Is there a way to speed that up (by using parallelization, for example)? Language can be either C or C++.
If latency isn't an issue (the function is only used as part of short non-loop-carried dep chains, e.g. arr[i] = abs(2.0 + arr[i]);), and you want to avoid the constant in memory: 4, because it's only 2 uops. If abs comes at the start or end of a dep chain, there won't be a bypass delay from a load or to a store.
1. What is the fastest way to take the absolute value of a standard 32 bit float on x86-64 architectures in C99? The builtin functions fabsf and fabs are not fast enough. My current approach is bit twiddling: unsigned int tmp = *((unsigned int *)&f) & 0x7fffffff; float abs = *((float *)&tmp); It works but is ugly.
0. For assembly the most efficient would be to initialize a value to 0, substract the integer, and then take the max: pxor mm1, mm1 ; set mm1 to all zeros. psubw mm1, mm0 ; make each mm1 word contain the negative of each mm0 word. pmaxswmm1, mm0 ; mm1 will contain only the positive (larger) values - the absolute value.
7. For what it's worth, absolute value of a 32-bit signed, 2's complement format int is usually implemented like this: abs (x) = (x^ (x>>31))- (x>>31) Ah I doubt that. It's definitely not implemented that way in Java and since in .NET the abs () class throws an exception for -2^31, that trick also won't work.
x = x*x. return x. When I time it, this is what I get: The time it takes for x**2: 101230500. The time it takes for x*x: 201469200. I have tried it many many times, they are either equal, or x ** 2 is faster than x * x. But x*x is never faster than x**2. So I disassembled the code: For x**2: