Date: Thu, 17 Sep 2020 18:25:55 +0100 From: Jessica Clarke <jrtc27@freebsd.org> To: rgrimes@freebsd.org Cc: Warner Losh <imp@bsdimp.com>, Alex Richardson <arichardson@freebsd.org>, src-committers <src-committers@freebsd.org>, svn-src-all <svn-src-all@freebsd.org>, svn-src-head <svn-src-head@freebsd.org> Subject: Re: svn commit: r365836 - head/share/mk Message-ID: <307760E0-1208-4F4C-AD7D-9E0A3C1B3A3B@freebsd.org> In-Reply-To: <C1895D1E-B3FF-4CE0-8CF1-D8151FEA0D59@freebsd.org> References: <202009171705.08HH5CtE014644@gndrsh.dnsmgr.net> <C1895D1E-B3FF-4CE0-8CF1-D8151FEA0D59@freebsd.org>
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> On 17 Sep 2020, at 18:23, Jessica Clarke <jrtc27@freebsd.org> wrote: >=20 >> On 17 Sep 2020, at 18:05, Rodney W. Grimes = <freebsd@gndrsh.dnsmgr.net> wrote: >>=20 >>> On Thu, Sep 17, 2020 at 9:39 AM Steffen Nurpmeso = <steffen@sdaoden.eu> wrote: >>>=20 >>>> Alex Richardson wrote in >>>> <202009171507.08HF7Qns080555@repo.freebsd.org>: >>>> |Author: arichardson >>>> |Date: Thu Sep 17 15:07:25 2020 >>>> |New Revision: 365836 >>>> |URL: https://svnweb.freebsd.org/changeset/base/365836 >>>> | >>>> |Log: >>>> | Stop using lorder and ranlib when building libraries >>>> | >>>> | Use of ranlib or lorder is no longer necessary with current = linkers >>>> | (probably anything newer than ~1990) and ar's ability to create = an >>>> object >>>> | index and symbol table in the archive. >>>> | Currently the build system uses lorder+tsort to sort the .o = files in >>>> | dependency order so that a single-pass linker can use them. = However, >>>> | we can use the -s flag to ar to add an index to the .a file = which makes >>>> | lorder unnecessary. >>>> | Running ar -s is equivalent to running ranlib afterwards, so we = can >>>> also >>>> | skip the ranlib invocation. >>>>=20 >>>> That ranlib thing yes (for long indeed), but i have vague memories >>>> that the tsort/lorder ordering was also meant to keep the things >>>> which heavily interdepend nearby each other. (Luckily Linux >>>> always had at least tsort available.) >>>> This no longer matters for all the platforms FreeBSD supports? >>>>=20 >>>=20 >>> tsort has no notion of how dependent the modules are, just an order = that >>> allows a single pass through the .a file (otherwise you'd need to = list the >>> .a file multiple times on the command line absent ranlib). That's = the >>> original purpose of tsort. tsort, lsort, and ranlib all arrived in = 7th >>> edition unix on a PDP-11, where size was more important than = proximity to >>> locations (modulo overlays, which this doesn't affect at all). >>>=20 >>> There were some issues of long vs short jumps on earlier = architectures that >>> this helped (since you could only jump 16MB, for example). However, = there >>> were workarounds for this issue on those platforms too. And if you = have a >>> program that this does make a difference, then you can still use >>> tsort/lorder. They are still in the system. >>>=20 >>> I doubt you could measure a difference here today. I doubt, = honestly, that >>> anybody will notice at all. >>=20 >> The x86 archicture has relative jmps of differning lengths, even in = long mode >> there is support for rel8 and rel32. >=20 > That's irrelevant though for several reasons: >=20 > 1. The compiler has already decided on what jump instructions to use = based on > the requested code model (unless you're on RISC-V and using GNU bfd = ld as > that supports linker relaxations that actually delete instruction = bytes). >=20 > 2. The linker is still free to reorder input sections however it = likes, it > doesn't have to follow the order of the input files (and the files = within > any archive). Hm actually that's only true for archives; it needs to respect the order = of files on the command line for things like crti.o to work. But = regardless, the other points (and this one, partially) still hold. > 3. If you care about those kinds of optimisations you should use = link-time > optimisation which will likely do far more useful things than just = optimise > branches, but again isn't constrained by the order of the input = files, it > can lay out the code exactly how it wants. >=20 > Not to mention that this is just a topological sort, not a clustering = sort. >=20 > Jess
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