On Tue, Jun 02, 2015 at 06:21:33PM -0400, Joshua Kinard wrote: > From: Joshua Kinard <kumba@xxxxxxxxxx> > > The R12000 added a new feature to enhance branch prediction called > "global history". Per the Vr10000 Series User Manual (U10278EJ4V0UM), > Coprocessor 0, Diagnostic Register (22): > > """ > If bit 26 is set, branch prediction uses all eight bits of the global > history register. If bit 26 is not set, then bits 25:23 specify a count > of the number of bits of global history to be used. Thus if bits 26:23 > are all zero, global history is disabled. > > The global history contains a record of the taken/not-taken status of > recently executed branches, and when used is XOR'ed with the PC of a > branch being predicted to produce a hashed value for indexing the BPT. > Some programs with small "working set of conditional branches" benefit > significantly from the use of such hashing, some see slight performance > degradation. > """ > > This patch enables global history on R12000 CPUs and up by setting bit > 26 in the branch prediction diagnostic register (CP0 $22) to '1'. Bits > 25:23 are left alone so that all eight bits of the global history > register are available for branch prediction. Will apply but could you also submit a patch to set cpu_has_bp_ghist to 0/1 as applicable in all cpu-feature-overrides.h? Also the manual suggests this CPU feature may not always be neneficial for performance so I'm wondering if we should add a way to modify it at runtime. I'm curious, have you checked the default setting of the global history on kernel entry? Ralf
