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. _______________________________________________ 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"
