From mboxrd@z Thu Jan 1 00:00:00 1970 X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on inbox.vuxu.org X-Spam-Level: X-Spam-Status: No, score=-3.3 required=5.0 tests=MAILING_LIST_MULTI, RCVD_IN_DNSWL_MED,RCVD_IN_MSPIKE_H3,RCVD_IN_MSPIKE_WL autolearn=ham autolearn_force=no version=3.4.4 Received: (qmail 11933 invoked from network); 9 Mar 2021 13:42:58 -0000 Received: from mother.openwall.net (195.42.179.200) by inbox.vuxu.org with ESMTPUTF8; 9 Mar 2021 13:42:58 -0000 Received: (qmail 30384 invoked by uid 550); 9 Mar 2021 13:42:56 -0000 Mailing-List: contact musl-help@lists.openwall.com; run by ezmlm Precedence: bulk List-Post: List-Help: List-Unsubscribe: List-Subscribe: List-ID: Reply-To: musl@lists.openwall.com Received: (qmail 30357 invoked from network); 9 Mar 2021 13:42:55 -0000 Date: Tue, 9 Mar 2021 08:42:43 -0500 From: Rich Felker To: Alexander Monakov Cc: musl@lists.openwall.com, =?utf-8?B?w4lyaWNv?= Nogueira Message-ID: <20210309134242.GS32655@brightrain.aerifal.cx> References: <20210309035652.32453-1-ericonr@disroot.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) Subject: Re: [musl] [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