On Sat, Feb 16, 2019 at 12:27:16AM +1100, Bruce Evans wrote: > On Fri, 15 Feb 2019, Konstantin Belousov wrote: > > > On Fri, Feb 15, 2019 at 07:16:04AM +0000, Alexey Dokuchaev wrote: > >> On Thu, Feb 14, 2019 at 01:53:11PM +0000, Konstantin Belousov wrote: > >>> New Revision: 344118 > >>> URL: https://svnweb.freebsd.org/changeset/base/344118 > >>> > >>> Log: > >>> Provide userspace versions of do_cpuid() and cpuid_count() on i386. > >>> > >>> Some older compilers, when generating PIC code, cannot handle inline > >>> asm that clobbers %ebx (because %ebx is used as the GOT offset > >>> register). Userspace versions avoid clobbering %ebx by saving it to > >>> stack before executing the CPUID instruction. > >>> > >>> ... > >>> +static __inline void > >>> +do_cpuid(u_int ax, u_int *p) > >>> +{ > >>> + __asm __volatile( > >>> + "pushl\t%%ebx\n\t" > >>> + "cpuid\n\t" > >>> + "movl\t%%ebx,%1\n\t" > >>> + "popl\t%%ebx" > >> > >> Is there a reason to prefer pushl+movl+popl instead of movl+xchgl? > >> > >> "movl %%ebx, %1\n\t" > >> "cpuid\n\t" > >> "xchgl %%ebx, %1" > > > > xchgl seems to be slower even in registers format (where no implicit > > lock is used). If you can demonstrate that your fragment is better in > > some microbenchmark, I can change it. But also note that its use is not > > on the critical path. > > The should have the same speed on modern x86. xchgl %reg1,%reg2 is > not slow, but it changes 2 visible registers and a needs somwhere to > hold one of the registers while changing it, so on 14 year old AthlonXP > where I know the times in cycles better, register xchgl was twice as slow > as register move (2 cycles latency instead of 1, and throughput == > latency (?)). On 2015 Haswell, register movl in a loop is in parallel > with the loop overhead (1 cycle), while xchgl and pushl/popl take 0.5 > cycles longer on average. Latency might be a problem for pushl/popl > in critical paths. There aren't many of those.
I think on modern Intels xchgl is implemented by renaming. Still it is slower than typically highly optimized push/pops. That said, what is your preference ? My version or xchgl ? My own preference is to leave it as is, since it is slightly slower, and I do not want to spend several hours again, re-testing libc changes. _______________________________________________ svn-src-all@freebsd.org mailing list https://lists.freebsd.org/mailman/listinfo/svn-src-all To unsubscribe, send any mail to "svn-src-all-unsubscr...@freebsd.org"
