Re: [PATCH v3] MIPS: R12000: Enable branch prediction global history

[Ralf Baechle <ralf@linux-mips.org>]

On Tue, Jun 02, 2015 at 06:21:33PM -0400, Joshua Kinard wrote:

> From: Joshua Kinard <kumba@gentoo.org>
> 
> 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