> -----Original Message----- > From: Victor Rodriguez [mailto:vm.rod25@xxxxxxxxx] > Sent: Wednesday, April 5, 2017 8:09 PM > To: O Mahony, Billy <billy.o.mahony@xxxxxxxxx>; Evgeny Stupachenko > <evstupac@xxxxxxxxx> > Cc: Xi Ruoyao <ryxi@xxxxxxxxxxxxxxxxx>; gcc-help@xxxxxxxxxxx > Subject: Re: Function Multiversioning in GCC6 > > On Tue, Apr 4, 2017 at 4:18 AM, O Mahony, Billy <billy.o.mahony@xxxxxxxxx> > wrote: > > Hi Victor, > > > >> -----Original Message----- > >> From: Victor Rodriguez [mailto:vm.rod25@xxxxxxxxx] > >> Sent: Monday, April 3, 2017 10:29 PM > >> To: O Mahony, Billy <billy.o.mahony@xxxxxxxxx> > >> Cc: Xi Ruoyao <ryxi@xxxxxxxxxxxxxxxxx>; gcc-help@xxxxxxxxxxx > >> Subject: Re: Function Multiversioning in GCC6 > >> > >> On Thu, Mar 30, 2017 at 3:59 AM, O Mahony, Billy > >> <billy.o.mahony@xxxxxxxxx> wrote: > >> > Hi Victor, > >> > > >> > Thanks to you and Abraham Duenas, and Evgeny Stupachenko for your > >> work on FMV - it's a very neat feature and from my own investigations > >> so far it works with minimal overhead even when used with inlined > functions. > >> > > >> Welcome :) > >> > >> > One thing that could be stated explicitly in the article is that > >> > use of the 'target' attribute with identically named functions > >> > (i.e. the first code sample in the lwn article) is still C++-only > >> > in gcc-6 > >> > >> yes > >> > >> and if you want to do this with C you need to create your own ifunc > >> resolver and have several differently named functions. > >> > > >> > >> The idea is that you use one single funtion with just atributes , > >> instead of multiple funtions > >> > >> Why do you need old implementation of C++ FMV in C ? > > > > [[BO'M]] > > I'm working on a code base that already has different definitions of > functions optimized for specific cpu instruction sets. One definition is picked > by macro at compile time based on make flags. > > > Sounds interesting, If something is not optimized for particular target you > should file a bugzilla report. if is proper for your code, then as you mention > FMV is not the best option. However try to reevaluate if this is the best > approach for your application. Maybe is a good time to rewrite it and make > more compact and understandable. > > > In my experiments with FMV I've looked at a hashing function. There are > two definitions - a pure C definition and one that uses sse42 intrinsics. The > pure C impl is really a set of inlined functions as I pulled it out of a suite of > hashing functions (hash32, hash64, hash32_array, etc etc) for the purposes > of the experiment. > > > > I'm assuming that, sophisticated as compilers are now, it won't replace the > several layers of inlined C functions with single sse42 intrinsic call just > because I specify "sse42" as one of the list of target_clones. So I stuck with > the several definitions solution. > > Is your code open source ? so we can tak ea look ? [[BO'M]] Hi Victor, I'm looking at this wrt to Open vSwitch code https://github.com/openvswitch/ovs As an experiment I was compiling a test program with both the sse42 and default versions of hash_words_inline function which are both in lib/hash.h. Currently one implementation or the other is chosen by compile time #def. It looks like ifunc is supported by the latest version of clang. Do you know if it is supported by BSD and Windows. Thanks, Billy. > > > > > > >> > >> Regards > >> > >> > Thanks, > >> > Billy. > >> > > >> > > >> > > >> >> -----Original Message----- > >> >> From: Victor Rodriguez [mailto:vm.rod25@xxxxxxxxx] > >> >> Sent: Thursday, March 30, 2017 1:10 AM > >> >> To: O Mahony, Billy <billy.o.mahony@xxxxxxxxx> > >> >> Cc: Xi Ruoyao <ryxi@xxxxxxxxxxxxxxxxx>; gcc-help@xxxxxxxxxxx > >> >> Subject: Re: Function Multiversioning in GCC6 > >> >> > >> >> On Thu, Mar 23, 2017 at 4:45 AM, O Mahony, Billy > >> >> <billy.o.mahony@xxxxxxxxx> wrote: > >> >> > > >> >> > Hi Xi, > >> >> > > >> >> > Thanks for the clarification/confirmation. > >> >> > > >> >> > Regards, > >> >> > Billy. > >> >> > > >> >> > > -----Original Message----- > >> >> > > From: Xi Ruoyao [mailto:ryxi@xxxxxxxxxxxxxxxxx] > >> >> > > Sent: Thursday, March 23, 2017 9:21 AM > >> >> > > To: O Mahony, Billy <billy.o.mahony@xxxxxxxxx> > >> >> > > Cc: ryxi@xxxxxxxxxxxxxxxxx; gcc-help@xxxxxxxxxxx > >> >> > > Subject: Re: Function Multiversioning in GCC6 > >> >> > > > >> >> > > On 2017-03-22 10:51 +0000, O Mahony, Billy wrote: > >> >> > > > >> >> > > > However when I compile the first example in this LWN article > >> >> > > > https://lwn.net/Articles/691932/ with gcc-6 C compiler it > >> >> > > > fails with > >> >> > > > (error: redefinition of > >> >> > > > 'foo'...) . > >> >> > > > >> >> > > The document > >> >> > > <https://gcc.gnu.org/onlinedocs/gcc-6.3.0/gcc/Function- > >> >> > > Multiversioning.html> said: > >> >> > > > >> >> > > With the GNU *C++* front end, for x86 targets, you may > >> >> > > specify multiple versions of the function. > >> >> > > > >> >> > > FMV doesn't work with C FE. In C, > >> >> > > __attribute__((target("sse4.2"))) just means to compile this > >> >> > > function with -msse4.2, see > >> >> > > <https://gcc.gnu.org/onlinedocs/gcc-6.3.0/gcc/Common-Function- > >> >> > > Attributes.html>. > >> >> > > > >> >> > > -- > >> >> > > Xi Ruoyao <ryxi@xxxxxxxxxxxxxxxxx> School of Aerospace Science > >> >> > > and Technology, Xidian University > >> >> > > >> >> > >> >> You can also read this LWN article: > >> >> > >> >> https://lwn.net/Articles/691932/
