Date: Tue, 31 Mar 2015 03:32:39 +0300 From: Konstantin Belousov <kostikbel@gmail.com> To: John Baldwin <jhb@freebsd.org> Cc: svn-src-head@freebsd.org, svn-src-all@freebsd.org, src-committers@freebsd.org, Bruce Evans <brde@optusnet.com.au> Subject: Re: svn commit: r280279 - head/sys/sys Message-ID: <20150331003239.GK2379@kib.kiev.ua> In-Reply-To: <1853187.nsx00mfzo7@ralph.baldwin.cx> References: <201503201027.t2KAR6Ze053047@svn.freebsd.org> <2526359.g5B2nXdKeQ@ralph.baldwin.cx> <20150330172434.GG2379@kib.kiev.ua> <1853187.nsx00mfzo7@ralph.baldwin.cx>
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On Mon, Mar 30, 2015 at 04:37:10PM -0400, John Baldwin wrote: > On Monday, March 30, 2015 08:24:34 PM Konstantin Belousov wrote: > > > That is not what Bruce said. He suggested using bitcount64() for the fallback > > > if the cpuid check fails. He did not say to remove the runtime check to use > > > popcnt if it is available: > > > > > > "Always using [bitcount64] would lose the micro-optimization... [to] keep > > > [it], it seems best to keep the inline asm but replace popcnt_pc_map_elem(elem) > > > by [bitcount64(elem)]." > > Ok, thank you for the clarification. > > > > I updated the pmap patch, see the end of the message. > > I think the pmap change looks fine. If we know which compilers include a > workaround we might also consider specifying "-mno-popcount" for everything > except known-ok compilers in at least kern.mk. Right now the compilers which implement the workaround are gcc 4.9.2 and gcc trunk, to be released as gcc 5.0. In-tree clang 3.6.0 does not try to eliminate the false dependency. > > > > This looks fine to me. It seems to be manually converting certain symbols > > > to use a dynamic lookup that must be explicitly resolved before first > > > use? > > I am not sure what do you mean by dynamic lookup, but possibly it > > was mentioned. I can emulate the ifuncs more sincerely, by requiring > > a resolver function, which is called on the first real function > > invocation. I did not see it as very useful, but it is definitely > > doable. > > I just mean that the effect at runtime is similar to that of dynamic > symbols once they are resolved (a call into a PLT entry (or is it GOT? > I keep getting those confused) that does a jump to the resolved symbol). PLT is slightly more expensive, since after the resolution it gives call symbol@plt jmp *symbol@gotpcrel while this code results in call *symbol_selector
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