Don’t store your structure lengths at pointer-4. Thank you.
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I got no work done for the rest of the day after I discovered that macro. I was just browsing kernel code and looking at a vmcore in crash as though I had stumbled upon evidence of a murder.
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Are there any malloc implementations that are NOT cursed?
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Well, I made github.com/GrapheneOS/har, so it's hard for me to consider it cursed. It's not a performance-oriented allocator though. If it was it would have small thread caches, free lists in slabs instead of bitmaps and wouldn't have all the sanity checks and security features.
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To be fair to jemalloc, they have the super cursed red-black tree implementation for real reasons. It's blazing fast and with RB_COMPACT the nodes are only 2 words, with the color bit packed into a pointer. It's intrusive so no extra allocation and multiple can go in an object.
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It goes out of the way to avoid recursion in any of the hot paths to avoid the cost of creating stack frames which makes it a lot messier. I definitely do think jemalloc is very over-engineered and that a simpler approach from scratch can be better all around but it's not easy.