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Arthur Perais 497cc2d373 cpu: disallow speculative update of branch predictor tables (o3) 
The Minor and o3 cpu models share the branch prediction
code. Minor relies on the BPredUnit::squash() function
to update the branch predictor tables on a branch mispre-
diction. This is fine because Minor executes in-order, so
the update is on the correct path. However, this causes the
branch predictor to be updated on out-of-order branch
mispredictions when using the o3 model, which should not
be the case.

This patch guards against speculative update of the branch
prediction tables. On a branch misprediction, BPredUnit::squash()
calls BpredUnit::update(..., squashed = true). The underlying
branch predictor tests against the value of squashed. If it is
true, it restores any speculatively updated internal state
it might have (e.g., global/local branch history), then returns.
If false, it updates its prediction tables. Previously, exist-
ing predictors did not test against the "squashed" parameter.

To accomodate for this change, the Minor model must now call
BPredUnit::squash() then BPredUnit::update(..., squashed = false)
on branch mispredictions. Before, calling BpredUnit::squash()
performed the prediction tables update.

The effect is a slight MPKI improvement when using the o3
model. A further patch should perform the same modifications
for the indirect target predictor and BTB (less critical).

Signed-off-by: Jason Lowe-Power <jason@lowepower.com>
2016-12-21 15:07:16 -06:00
build_opts riscv: [Patch 5/5] Added missing support for timing CPU models 2016-11-30 17:10:28 -05:00
configs python: Export periodicStatDump 2016-12-19 16:25:39 +00:00
ext arch: [Patch 1/5] Added RISC-V base instruction set RV64I 2016-11-30 17:10:28 -05:00
src cpu: disallow speculative update of branch predictor tables (o3) 2016-12-21 15:07:16 -06:00
system arm, config: Add an example ARM big.LITTLE(tm) configuration script 2016-07-21 17:19:16 +01:00
tests stats: update references 2016-12-19 11:03:28 -06:00
util dist, dev: fix etherswitch upgrade script 2016-12-19 12:12:28 -06:00
.gitignore misc: Add a gitignore file based on the current hgignore 2016-03-16 17:20:00 +00:00
.hgignore misc: ignore object files and static libs in util/m5 2015-11-13 17:03:48 -05:00
.hgtags Added tag stable_2015_09_03 for changeset 60eb3fef9c2d 2015-09-03 15:38:46 -05:00
COPYING copyright: Add code for finding all copyright blocks and create a COPYING file 2011-06-02 17:36:07 -07:00
LICENSE copyright: Add code for finding all copyright blocks and create a COPYING file 2011-06-02 17:36:07 -07:00
README misc: README direct to website for dependencies 2014-08-26 10:12:04 -04:00
SConstruct scons: Bump minimum gcc version to 4.8 2016-05-30 02:10:48 -04:00

README 

This is the gem5 simulator.

The main website can be found at http://www.gem5.org

A good starting point is http://www.gem5.org/Introduction, and for
more information about building the simulator and getting started
please see http://www.gem5.org/Documentation and
http://www.gem5.org/Tutorials.

To build gem5, you will need the following software: g++ or clang,
Python (gem5 links in the Python interpreter), SCons, SWIG, zlib, m4,
and lastly protobuf if you want trace capture and playback
support. Please see http://www.gem5.org/Dependencies for more details
concerning the minimum versions of the aforementioned tools.

Once you have all dependencies resolved, type 'scons
build/<ARCH>/gem5.opt' where ARCH is one of ALPHA, ARM, NULL, MIPS,
POWER, SPARC, or X86. This will build an optimized version of the gem5
binary (gem5.opt) for the the specified architecture. See
http://www.gem5.org/Build_System for more details and options.

With the simulator built, have a look at
http://www.gem5.org/Running_gem5 for more information on how to use
gem5.

The basic source release includes these subdirectories:
   - configs: example simulation configuration scripts
   - ext: less-common external packages needed to build gem5
   - src: source code of the gem5 simulator
   - system: source for some optional system software for simulated systems
   - tests: regression tests
   - util: useful utility programs and files

To run full-system simulations, you will need compiled system firmware
(console and PALcode for Alpha), kernel binaries and one or more disk
images. Please see the gem5 download page for these items at
http://www.gem5.org/Download

If you have questions, please send mail to gem5-users@gem5.org

Enjoy using gem5 and please share your modifications and extensions.