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MIPS Simulator: Branch Predictor
Project 4 – CS 3339

Submission must be in tar format and compile on zeus per instructions to be graded.
PROBLEM STATEMENT
In this project, you will reduce the number of pipeline flushes required by adding a branch predictor to
your simulator. This predictor will predict only conditional branch instructions, i.e. beq and bne.
You should begin with a copy of your Project 3 submission. I have provided a new Makefile as well as a
class skeleton for a BranchPred class intended to be instantiated inside your existing CPU class.
You should simulate a dynamic branch predictor that uses a 64-entry direct-mapped table with no tag and
no valid bits. Each entry stores a 2-bit saturating counter (described below) used to generate a “taken” or
“not-taken” prediction, as well as a target PC (i.e., the PC the branch is predicted to jump to if it is taken).
The table should be indexed as follows: index = (PCbranch >> 2) % BPRED_SIZE. (I have defined
BPRED_SIZE for you in BranchPred.h).
All counters should be initialized to zero. Whenever a branch is taken, update the corresponding 2-bit
saturating counter by incrementing it, unless it is already at its maximum value (3). Whenever a branch is
not taken, update the counter by decrementing it, unless it is already at its minimum value (0). The
prediction should be “branch taken” if PCbranch’s count is at least 2 and “branch not taken” otherwise.
You may structure the interaction between your CPU and branch predictor however you’d like. However,
be careful: for any branch instruction, first make a prediction, then update the predictor. In other
words, you can only use past information to make a prediction. To make sure, it’s simplest to have separate
BranchPred::predict() and BranchPred::update() functions.
Up until now, your processor has flushed the pipeline behind any taken branch instruction. Modify your
code so that a flush occurs only when a branch has been mispredicted.
Here’s my recommended approach from a CPU.cpp decode of beq and bne perspective:
1.) Based on the PC, predict whether the operation will be a taken branch and to what address.
2.) Determine its actual direction (taken vs. not-taken) and target PC.
3.) Determine if there was a misprediction. A misprediction can be one of two types:
a. Direction mispredict: compare the predicted and actual taken vs. not-taken determination
b. Target mispredict: if the branch was correctly predicted taken, compare the predicted and actual
target PCs
If there was a misprediction, flush the pipeline behind the branch as you did for all taken branches
in Project 3. For a correct prediction no pipeline flush is necessary.
4.) Update the 2-bit saturating counter for the branch’s PC, based on its actual taken or not-taken
determination. If the branch was taken, also update the predictor’s target PC.
2
Your branch predictor model will calculate and report the following statistics:
• The total number of predicted branches
• The number of branches that were predicted taken vs. predicted not-taken
• The total number of mispredictions
• The number of direction mispredictions (i.e., T vs. NT decision was wrong)
• The number of target mispredictions (i.e., the branch was correctly predicted taken but the target
PC was wrong)
• The overall predictor accuracy
I have provided the following new files:
• A BranchPred.h class specification file, which you’ll need to add member variables and function
prototypes to
• A BranchPred.cpp class implementation file, which you should enhance to model the described
branch predictor and count predictions and mispredictions
• A new Makefile
• A sample output file sssp.out for you to confirm your output and check formatting.
• A debug output for qsort.mips in a separate tar file.
In addition to enhancing the BranchPred.h/.cpp skeleton, you will also need to modify CPU.h to
instantiate a BranchPred object, and CPU.cpp to call your BranchPred class functions and call
Stats::flush() only on mispredictions. You’ll also need to change CPU::printFinalStats() to match
my expected output format (see below).
ASSIGNMENT SPECIFICS
All provided source files are on TRACS cs3339_project4bp.tgz.
Begin by copying your project3 files into a new project4 directory. Then extract the additional files in
cs3339_project4bp.tgz and add them to your project4 directory. You can compile and run the simulator
program identically to previous projects, and test it using the same *.mips inputs.
Here are a few steps to get you up and running after setting up your directory.
1) Write your name in the header of BranchPred.cpp and also confirm your name is in your CPU.cpp
and Stats.cpp as well.
2) Include BranchPred.h in your CPU.h file and instantiate an object named bpred. Refer to the ALU
instantiation as an example.
3) At the bottom of CPU.cpp remove the unneeded MemOps, Branches, Taken p3 outputs and add
bpred.printFinalStats();
4) You should now be able to compile and run your project, but the branch prediction output will be
incorrect. It is highly recommended that your compile frequently as you work on your code.
3
CHECK YOUR BASELINE CODE WITHOUT BRANCH PREDICTION
Since you have not implemented the branch prediction code the output will be incorrect. Before you
implement branch prediction you should see:
CS 3339 MIPS Simulator
Branch Predictor Entries: 64
Running: ../../infiles/sssp.mips
7 1
Program finished at pc = 0x400440 (449513 instructions executed)
Cycles: 1627234
CPI: 3.62
Bubbles: 1125724
Flushes: 51990
Branches predicted: 4198352
Pred T: 0 (0.0%)
Pred NT: 4198352
Mispredictions: 4212392 (100.3%)
Mispredicted direction: 0
Mispredicted target: 1385737673
Predictor accuracy: -0.3%
Table 1 contains the expected pre-branch-predictor cycle, flush, and CPI metrics for some of the inputs.
You should not begin implementing your branch predictor until your baseline code matches these
numbers! (The other 3 inputs have un-interesting branch behavior and will not be tested).
Input Cycle Count Flushes CPI
hash.mips 134904 6526 3.45
nqueens.mips 629285095 24671130 3.03
prime.mips 988775 54450 3.60
qsort.mips 167458 8334 3.64
sssp.mips 1627234 51990 3.62
Table 1: Initial values before Branch Prediction
Note prime was updated 4/2/2019
This is before
branch prediction
– not final output
4
IMPLEMENT BRANCH PREDICTION
The following is the expected output for sssp.mips after branch prediction has been implemented. Your
output must match this format verbatim; you should check it by using diff and the provided sssp.out file.
Misspellings and capitalization will throw off the grading scripts.
CS 3339 MIPS Simulator
Branch Predictor Entries: 64
Running: sssp.mips
7 1
Program finished at pc = 0x400440 (449513 instructions executed)
Cycles: 1586032
CPI: 3.53
Bubbles: 1125724
Flushes: 10788
Branches predicted: 42954
Pred T: 25851 (60.2%)
Pred NT: 17103
Mispredictions: 5393 (12.6%)
Mispredicted direction: 2803
Mispredicted target: 2590
Predictor accuracy: 87.4%
Table 2 shows the expected cycle count, flush count, CPI, and predictor accuracy after the inclusion of the
simulated branch predictor.
Additional Requirements:
• Your code must compile with the given Makefile and run on zeus.cs.txstate.edu
• Your code must be well-commented, sufficient to prove you understand its operation
• Make sure your code doesn’t produce unwanted output such as debugging messages. (You can
accomplish this by using the D(x) macro defined in Debug.h)
• Make sure your code’s runtime is not excessive
• Make sure your code is correctly indented and uses a consistent coding style
• Clean up your code before submitting i.e., make sure there are no unused variables, unreachable
code, etc.
Input Cycle Count Flushes CPI Predictor
Accuracy
hash.mips 128836 458 3.29 98.0%
nqueens.mips 611789985 7176020 2.95 92.8%
prime.mips 940807 6482 3.42 91.6%
qsort.mips 160130 1006 3.49 95.6%
sssp.mips 1586032 10788 3.53 87.4%
Table 2: Expected values with Branch Prediction
5
DEBUGGING
Most importantly make sure your initial outputs match the values in Table 1. There is a D();
output in the provided BranchPred.cpp file which will be enabled when debug outputs are enabled
in Debug.h. You can use this to trace the operation of your branch predictor; a debug output file is
provided for qsort.mips. Remember the debug output files are VERY LARGE be sure to delete
them and never run with the nqueen.mips input. Turn off debug outputs before submission.
To redirect your output to a file:
$ ./simulator qsort.mips > temp.txt
Or if you want to see the output on the screen at the same time use:
$ ./simulator qsort.mips | tee temp.txt
The following command will show you just the branch instructions:
$ grep -B1 -A10 ‘BPRED’ temp.txt | more
The grep command extracts all lines containing ‘BPRED’, the -B1 option displays one line before
the match and the -A10 option displays 10 lines after the match so you can see the next instruction
and tell whether the branch was taken. 10 points extra-credit to the first person who emails me a
simple way to remove the register dump from this output.
6
SUBMISSION INSTRUCTIONS
Submit all of the code necessary to compile your simulator (all of the .cpp and .h files as well as the
Makefile) as a compressed tarball. You can do this using the following Linux command:
$ tar czvf yourNetID_project4.tgz *.cpp *.h Makefile
Do not submit the executables (*.mips files), outputs (*.out) or any other files. Do not rename any of
the files inside of the tar file. Do not leave Debug outputs enabled. Do not submit a zip or OSX compressed
archive; only tar files generated with above command will work. Any special instructions or comments to
the grader, including notes about features you know do not work, should be included in a separate text file
(not inside the tarball) named yourNetID_README.txt. Check your submission with the submit_test
script that is provided on TRACS – instructions are in the script file itself. Files that do not untar, compile,
and run without debug outputs on zeus will likely receive zero points.
All project files are to be submitted using TRACS. You should not assume your submission has been made
until you have a confirmation email from TRACS.
You may submit your file(s) as many times as you’d like before the deadline. Only the last submission will
be graded. TRACS will not allow submission after the deadline, so I strongly recommend that you don’t
come down to the final seconds of the assignment window. Late assignments will not be accepted.
ACADEMIC HONESTY (from course syllabus)
You are expected to adhere to the Texas State University Honor Code which is described here
http://www.txstate.edu/honorcodecouncil/Academic-Integrity.html
All assignments and exams must be done individually, unless otherwise noted in the assignment
handout. Turning in an exam or assignment that is not entirely your own work is cheating and will not be
tolerated.
Group discussion about course content is NOT cheating and is in fact strongly encouraged! You are
welcome to study together and to discuss information and concepts covered in class, as well as to offer (or
receive) help with debugging or understanding course concepts to (or from) other students. However, this
cooperation should never involve students possessing a copy of work done by another student, including
solutions from previous semesters, other course sections, the Internet, or any other sources.
Turning in an assignment any part of which is copied from the Internet, another student’s work, or any
other non-approved source will result in a 0 grade on the assignment and will be reported to the Texas
State Honor Code Council. Should one student copy from another, both the student who copied work and
the student who gave material to be copied will receive 0s and be reported to the Honor Code Council. You
should never grant anyone access to your files or email your programs to anyone (other than the
instructor)!

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