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 15591 invoked from network); 9 Mar 2021 14:13:53 -0000 Received: from mother.openwall.net (195.42.179.200) by inbox.vuxu.org with ESMTPUTF8; 9 Mar 2021 14:13:53 -0000 Received: (qmail 11434 invoked by uid 550); 9 Mar 2021 14:13:51 -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 11415 invoked from network); 9 Mar 2021 14:13:51 -0000 Date: Tue, 9 Mar 2021 17:13:39 +0300 (MSK) From: Alexander Monakov To: musl@lists.openwall.com cc: =?ISO-8859-15?Q?=C9rico_Nogueira?= In-Reply-To: <20210309134242.GS32655@brightrain.aerifal.cx> Message-ID: References: <20210309035652.32453-1-ericonr@disroot.org> <20210309134242.GS32655@brightrain.aerifal.cx> User-Agent: Alpine 2.20.13 (LNX 116 2015-12-14) MIME-Version: 1.0 Content-Type: multipart/mixed; boundary="168458499-1519177721-1615299219=:16269" Subject: Re: [musl] [PATCH v2] add qsort_r. This message is in MIME format. The first part should be readable text, while the remaining parts are likely unreadable without MIME-aware tools. --168458499-1519177721-1615299219=:16269 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8BIT > 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.. First of all it's not necessarily a "call layer". You could change cmp call site such that NULL comparator implies that non-_r version was called and the original comparator address is in ctx: static inline int call_cmp(void *v1, void *v2, void *ctx, cmpfun cmp) { if (cmp) return cmp(v1, v2, ctx); return ((cmpfun)ctx)(v1, v2); } This is just a conditional branch at call site after trivial inlining. Second, if you make a "conventional" wrapper, then on popular architectures it is a single instruction (powerpc64 ABI demonstrates its insanity here): static int wrapper_cmp(void *v1, void *v2, void *ctx) { return ((cmpfun)ctx)(v1, v2); } Some examples: amd64: jmp %rdx i386: jmp *12(%esp) arm: bx r2 aarch64:br x2 How is this not obvious? Alexander --168458499-1519177721-1615299219=:16269--