In this commit, we add License for XiangShan project. 

* difftest: use DPI-C to refactor difftest

In this commit, difftest is refactored with DPI-C calls.
There're a few reasons:
(1) From Verilator's manual, DPI-C calls should be more efficient than accessing from dut_ptr.
(2) DPI-C is cross-platform (Verilator, VCS, ...)
(3) difftest APIs are splited from emu.cpp to possibly support more backend platforms
(NEMU, Spike, ...)

The performance at this commit is quite slower than the original emu.
Performance issues will be fixed later.

* [WIP] SimTop: try to use 'XSTop' as soc

* CircularQueuePtr: ues F-bounded polymorphis instead implict helper

* Refactor parameters & Clean up code

* difftest: support basic difftest

* Support diffetst in new sim top

* Difftest; convert recode fmt to ieee754 when comparing fp regs

* Difftest: pass sign-ext pc to dpic functions && fix exception pc

* Debug: add int/exc inst wb to debug queue

* Difftest: pass sign-ext pc to dpic functions && fix exception pc

* Difftest: fix naive commit num limit
Co-authored-by: NYinan Xu <xuyinan1997@gmail.com>
Co-authored-by: NWilliam Wang <zeweiwang@outlook.com>

XSPerfAccumulate: sum up performance values.
XSPerfHistogram: count the occurrence of performance values, split them
into bins, so that we can estimate their distribution.
XSPerfMax: get max of performance values.

previously the biggest problem was using '+' instead of '+&' to
do sums

* core: enable sc

* sc: calculate sum again on update

* sc: clean ups

* sc: add some debug info

* sc, tage, bim: fix wrbypass logic, add wrbypass for SC

* sc: restrict threshold update conditions and prevent overflow problem

* sc: use seperative thresholds for each bank

* sc: update debug info

* sc: use adaptive threshold algorithm from the original O-GEHL

* tage, bim, sc: optimize wrbypass logic

* sc: initialize threshold to 60

* loop: remove unuseful RegNext on redirect

* ifu: add perf counters

* Perf: Add loopPredictor perf counters

* sc: fix perf logics
Co-authored-by: Njinyue110 <jinyue161@mails.ucas.ac.cn>
Co-authored-by: Nzoujr <18870680299@163.com>

* core: enable sc

* sc: calculate sum again on update

* sc: clean ups

* sc: add some debug info

* sc, tage, bim: fix wrbypass logic, add wrbypass for SC

* sc: restrict threshold update conditions and prevent overflow problem

* sc: use seperative thresholds for each bank

* sc: update debug info

* sc: use adaptive threshold algorithm from the original O-GEHL

* tage, bim, sc: optimize wrbypass logic

* sc: initialize threshold to 60

* loop: remove unuseful RegNext on redirect

* Replacement: change state in way method.

* State change is also needed when miss occurs, otherwise we will choose
a way that has been just refilled into cache as the victim.

* Optimize ctrlblock timing (#620)

* CtrlBlock: delay exception flush for 1 cycle

* CtrlBlock: delay load replay for 1 cycle

* roq: delay wb from exu for one clock cycle to meet timing

* CtrlBlock: fix pipeline bug between decode and rename
Co-authored-by: NYinan Xu <xuyinan1997@gmail.com>

* L1plusCache: use plru replacement policy.

* ICache: fix mmio bugs

1. MMIO cut helper uses packet align logic
2. still send req to uncache when flush

* ICache: change packet from mmio

use packet align as the mem

* IntrUncache: fix state bug

state will change into s_invalid and get stuck

* fix Registers that not being initiated

* core: enable sc

* sc: calculate sum again on update

* sc: clean ups

* sc: add some debug info

* sc, tage, bim: fix wrbypass logic, add wrbypass for SC

* core: disable sc by default
Co-authored-by: Njinyue110 <jinyue161@mails.ucas.ac.cn>

* csr: add sbpctrl to control branch predictors

* bpu: add dynamic switch to each predictor

* csr: change spfctl and sbpctl address

* bpu: fix s3 connections
Co-authored-by: NYinan Xu <xuyinan1997@gmail.com>

     for bim, btb, tage and ubtb