Daniel Micay on Twitter: "@volatile_void @RichFelker @johnregehr In https://t.co/XgCPXciXM6, I use libdivide for mapping the address of a slab allocation to the index of the slab metadata (https://t.co/U8wLFD1v5Z) and to find the slot of the slab allocation within the slab (https://t.co/547sJ5FItC and https://t.co/3kuSZ2xwkx) for the bitmaps." / Twitter

On the contrary, this aims at obtaining from GCC the constants that allow to implement these divisions as multiplications… in a context where the numbers to divide could have been represented as BCD, and then division would be trivial, but they weren't.

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There's a great blog post on generating them yourself. We explored doing it in musl ldso to perform the % operation for hash lookups. I'll try to find link if you're interested.

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Here it is: ridiculousfish.com/blog/posts/lab

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From the author of libdivide, unsurprisingly …

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There's an even faster way to do 32-bit mod, for cases like that hash example: arxiv.org/pdf/1902.01961 github.com/lemire/fastmod

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In github.com/GrapheneOS/har, I use libdivide for mapping the address of a slab allocation to the index of the slab metadata (github.com/GrapheneOS/har) and to find the slot of the slab allocation within the slab (github.com/GrapheneOS/har and github.com/GrapheneOS/har) for the bitmaps.

10:22 PM · May 5, 2019Twitter Web Client

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There's also the fancy approach to generating random numbers uniformly within a range without using a mod operation: arxiv.org/pdf/1805.10941 I use that in github.com/GrapheneOS/har and it was a nice performance boost. I'll probably use these approaches elsewhere now too.