Date: Tue, 9 Mar 2021 08:42:43 -0500 From: Rich Felker <dalias@...c.org> To: Alexander Monakov <amonakov@...ras.ru> Cc: musl@...ts.openwall.com, Érico Nogueira <ericonr@...root.org> Subject: Re: [PATCH v2] add qsort_r. On Tue, Mar 09, 2021 at 12:11:37PM +0300, Alexander Monakov wrote: > On Tue, 9 Mar 2021, Érico Nogueira wrote: > > > since most discussion around the addition of this function has centered > > around the possible code duplication it requires or that qsort would > > become much slower if implemented as a wrapper around qsort_r > > How much is "much slower", did anyone provide figures to support this claim? > The extra cost that a wrapper brings is either one indirect jump instruction, > or one trivially-predictable conditional branch per one comparator invocation. Quite a bit I'd expect. Each call to cmp would involve an extra level of call wrapper. With full IPA/inlining it could be optimized out, but only by making a non-_r copy of all the qsort code in the process at optimize time. > Constant factor in musl qsort is quite high, I'd be surprised if the extra > overhead from one additional branch is even possible to measure. I don't think it's just a branch. It's a call layer. qsort_r internals with cmp=wrapper_cmp, ctx=real_cmp -> wrapper_cmp(x, y, real_cmp) -> real_cmp(x, y). But I'm not opposed to looking at some numbers if you think it might not matter. Maybe because it's a tail call it does collapse to essentially just a branch in terms of cost.. Rich
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