Date: Fri, 31 Jul 2020 19:22:16 +0200 From: Markus Wichmann <nullplan@....net> To: musl@...ts.openwall.com Subject: Re: When to reclaim pages in __bin_chunk? On Fri, Jul 31, 2020 at 08:17:02AM +0800, Zhao Zhengyu wrote: > Hello, > > When chunks are merged, we use "(curr_size + pre_size) ^ pre_size > > pre_size" to decide whether to reclaim. I think this may be something > related to performance, but I can’t prove it. I want to know the > reason. > > Thank you! > > Zhengyu I asked that same question a while ago. For one, this was in the old malloc code, which is now usually no longer used. For two, this tries to figure out if adding current size to previous size rolls over into a new power of two. Usually, curr_size will be small and pre_size will be large. Therefore, adding the two will not change much about the high bits of pre_size, so due to the XOR operator, those bits will cancel out and the result will be smaller than pre_size. However, if the sum does roll over, then the sum has one bit set that isn't in pre_size and is larger than all the ones in pre_size. So the XOR can't cancel that bit, and the result of the XOR is greater than pre_size. Fundamentally, it is an optimized version of (a_clz(curr_size + pre_size) < a_clz(pre_size)). Ciao, Markus
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