I love libdivide. Moving to libdivide-2.0 in hardened_malloc is an easy win.
16 byte malloc microbenchmark on Broadwell-E:
Hardware division: 1s
libdivide-1.1: 0.74s
libdivide-2.0: 0.71s
In a lightweight build:
Hardware division: 1s
libdivide-1.1: 0.62s
libdivide-2.0: 0.59s
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this is amazing. I guess divide isn't a bottleneck often enough for Intel to give that unit a lot of attention?
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I never used to think about it but I'm come to realize that integer division is ridiculously slow and it stands out to me in code that's supposed to perform well. The trick with libdivide is that it's doing the same kind of division by a constant optimizations as compilers.
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So it's doing a fair bit of work to figure out the proper shifts / multiplications when you set up the divisor and then you reuse it many times. In hardened_malloc, it sets up a slab size divisor and size divisor for each size class, and then uses those to find the metadata.
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ok, this answers my question "why doesn't the compiler just emit the right thing in the first place"
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fwiw the same “slightly too sophisticated for compilers” problem comes up when implementing a decimal floating-point library. Ideally you'd like to write:
uint64_t power_of_ten[17]={1,10,100,1000…};
and then let the compiler optimize “e/power_of_ten[k]” but current compilers…
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