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Also helpful: being able to represent integer types with binary. Since I'm not a genius and I write code to be maintained by other not-geniuses, `mask = 0b_1010_1010` is clearer than `mask = 0xAA`.
Earlier quoted context omitted.
I’ve had that question twice in google phone screen interviews, and iirc, the best answer has changed from the hash table thing to just use the hardware instruction.
I don't believe companies want to hire for knowing that popcnt exists. The interviewer should follow up with the hypothetical situation when either the language or the CPU doesn't have the instruction. Nobody should ask this anyway, this has become the fizzbuzz type of question.
What's funny is that 15 years later, I actually had a use for popcnt, put it together with something that seemed expensive at the time, and wound up with a C program that exhaustively searched a problem space in .12 sec. On a single core, in a vm, on a laptop. So much for me trying GPU programming on that one. (as in , feckit, there aren't that many possibilities, we'll just count them all)
At my last job interview I had to implement popcount and explain how to optimise it (amongst other things). I was able to jot down a naïve implementation and the obvious optimisation based on a lookup tables. I only vaguely remembered the Bit Twiddling treatment of the subject but, with a bit of nudging from the interviewer, I managed to implement and explain the variant that runs in O(set bits) (“Brian Kernighan's w…
I mean, it wouldn't hurt to know how to do all of this, plus all the data structures & algorithms stuff, plus all major and minor details about your language and environment of choice, plus anything else that someone deems important, but realistically most of us need a job before you can learn all of that. Even if you do learn it all, people tend to lose knowledge they don't use, so it may have an expiration date dep…
Earlier quoted context omitted.
I mean, it wouldn't hurt to know how to do all of this, plus all the data structures & algorithms stuff, plus all major and minor details about your language and environment of choice, plus anything else that someone deems important, but realistically most of us need a job before you can learn all of that. Even if you do learn it all, people tend to lose knowledge they don't use, so it may have an expiration date dep…
DSA = data structures & algorithms
Earlier quoted context omitted.
I mean, it wouldn't hurt to know how to do all of this, plus all the data structures & algorithms stuff, plus all major and minor details about your language and environment of choice, plus anything else that someone deems important, but realistically most of us need a job before you can learn all of that. Even if you do learn it all, people tend to lose knowledge they don't use, so it may have an expiration date dep…
DSA = data structures & algorithms
At my last job interview I had to implement popcount and explain how to optimise it (amongst other things). I was able to jot down a naïve implementation and the obvious optimisation based on a lookup tables. I only vaguely remembered the Bit Twiddling treatment of the subject but, with a bit of nudging from the interviewer, I managed to implement and explain the variant that runs in O(set bits) (“Brian Kernighan's w…
I mean, it wouldn't hurt to know how to do all of this, plus all the data structures & algorithms stuff, plus all major and minor details about your language and environment of choice, plus anything else that someone deems important, but realistically most of us need a job before you can learn all of that. Even if you do learn it all, people tend to lose knowledge they don't use, so it may have an expiration date dep…
The interview was for a senior position.
> Even if you do learn it all, people tend to lose knowledge they don't use
Let me emphasise that my interview was not a knowledge test (and at any rate I don’t study for interviews). I wasn’t expected to know by heart how to implement popcount. The interviewer was trying to see me work. Successfully, I might add. — Another question I got concerned something I had no knowledge of, and I had to derive a solution myself. In fact, I failed to do so, but that didn’t prevent me from getting the job since the interviewer was satisfied with what they observed about my thought process.
template int sgn(T val) {
return (T(0) My favorite thing about these is how they come full circle - programmers find convoluted ways to outsmart the compiler in some primitive operation, but then CPU manufacturers add dedicated instructions for those operations, and eventually the compilers outsmart the programmer by deleting their smartass method and emitting a single instruction instead https://godbolt.org/z/nPdrcz
This of course refers to the POPCNT instruction, that is still quite tricky to get compilers to generate reliably, and which RISC-V specifications and all implementations to date mysteriously lack. Compilers failing to generate POPCNT cause sometimes 10x slowdowns in key operations.
Earlier quoted context omitted.
This of course refers to the POPCNT instruction, that is still quite tricky to get compilers to generate reliably, and which RISC-V specifications and all implementations to date mysteriously lack. Compilers failing to generate POPCNT cause sometimes 10x slowdowns in key operations.
Indeed it's better to use intrinsics in new code rather than rely on compiler magic https://godbolt.org/z/n3z8nz Portability is an issue though, that works on GCC/Clang but you need a different intrinsic for MSVC