i. All students must work alone.
ii. Sharing of code between students is considered cheating and will receive
appropriate action in accordance with University policy. The TAs will scan
source code through various tools available to us for detecting cheating.
Source code that is flagged by these tools will be dealt severely: 0 on the
project and referral to the Office of Student Conduct for sanctions.
iii. You must do all your work in C/C++ or Java language. Exceptions must be
approved. The C language is fine to use as that is what many students are
trained in. Basic C++ extensions to the C language (classes instead of structs)
are encouraged (but by no means required) because it enables more
straightforward code reuse.
iv. Use of Linux environment is required. This is the platform where the TAs will
compile and test your simulator.
In this project, you will construct a branch predictor simulator and use it to design branch predictors well suited to the SPECint95 benchmarks.
o m is the number of low-order PC bits used to form the prediction table index. Note: discard the lowest two bits of the PC, since they are always zero, i,e., use bits m+ through 2 of the PC. o n is the number of bits in the global branch history register. Note: n<=m. Note: n may equal zero, in which case we have the simplest bimodal branch predictor.
When n=0, the gshare predictor reduces to a simple bimodal predictor. In this case, the index is based on only the branch’s PC, as shown in Figure 1.
Entry in the prediction table An entry in the prediction table contains a single 3-bit counter. All entries in the prediction table should be initialized to 4 (“weakly taken”) when the simulation begins.
Regarding branch interference Different branches may index the same entry in the prediction table. This is called “interference”. Interference is not explicitly detected or avoided: it just happens. (There is no tag array, no tag checking, and no “miss” signal for the prediction table!)
Steps: When you get a branch from the trace file, there are three steps:
(1) Determine the branch’s index into the prediction table. (2) Make a prediction. Use index to get branch’s counter from the prediction table. If the counter value is greater than or equal to 4 , then the branch is predicted taken, else it is predicted not-taken. (3) Update the branch predictor based on the branch’s actual outcome. The branch’s counter in the prediction table is incremented if the branch was taken, decremented if the branch was not taken. The counter saturates at the extremes ( 0 and 7 ), however.
When n>0, there is an n-bit global branch history register. In this case, the index is
based on both the branch’s PC and the global branch history register, as shown in
Figure 2. The global branch history register is initialized to all zeroes (00...0) at the
Steps:
When you get a branch from the trace file, there are three steps:
(1) Determine the branch’s index into the prediction table. Figure 2 shows how to
generate the index: the current n-bit global branch history register is XORed with
the lowermost n bits of the index (PC) bits.
(2) Make a prediction. Use index to get the branch’s counter from the prediction table. If the counter value is greater than or equal to 4, then the branch is predicted taken, else it is predicted not-taken. (3) Update the branch predictor based on the branch’s actual outcome. The branch’s counter in the prediction table is incremented if the branch was taken, decremented if the branch was not taken. The counter saturates at the extremes ( 0 and 7 ), however. (4) Update the global branch history register. Shift the register right by 1bit position and place the branch’s actual outcome into the most significant bit position of the register.
Model a hybrid predictor that selects between the bimodal and gshare predictors, using a chooser table of 2k 2 - bit counters. All counters in the chooser table are initialized to 1 at the beginning of the simulation. Steps: When you get a branch from the trace file, there are six top-level steps: (1) Obtain to predictions, one from gshare predictor (follow steps 1 and 2 in section 3.1.2) and one from bimodal predictor (follow steps 1 and 2 in section 3.1.1). (2) Determine the branch’s index into the chooser table. The index for the chooser table is bit k+1 to bit 2 of the branch PC (i,e., as before, discard the lowest two bits of the PC). (3) Make an overall prediction. Use index to get the branch’s chooser counter from the chooser table. If the chooser counter value is greater than or equal to 2 , then use the prediction that was obtained from the gshare predictor, otherwise use the prediction that was obtained from the bimodal predictor.
(4) Update the selected branch predictor based on branch’s actual outcome. Only the
branch predictor that was selected in step 3, above, is updated (if gshare was
selected, follow step 3 in Section 3.1.2, otherwise follow step 3 in section 3.1.1).
(5) Note that the gshare ’s global branch history register must always be updated, even
if bimodal was selected (follow step 4 in section 3.1.2).
(6) Update the branch’s chooser counter using the following rule:
The simulator reads a trace file in the following format:
<hex branch PC> t|n
<hex branch PC> t|n
...
Where
o <hex branch PC> is the address of the branch instruction in memory. This
field is used to index the predictors.
o “t” indicates the branch is actually taken (Note! Not that it is predicted
taken!). Similarly, “n” indicates the branch is actually not-taken.
Example:
00a3b5fc t
00a3b604 t
00a3b60c n
...
The simulator outputs the following measurements after completion of the run:
a. Number of accesses to the predictor (i,e., number of branches)
b. Number of branch mispredictions (predicted taken when not-taken, or predicted
not-taken when taken)
c. Branch predictions rate (# of mispredictions / # of branches)
Sample simulation outputs will be provided on the webcourses. These are called “validation runs”. You must run your simulator and debug it until it matches the validation runs.
Each validation run includes:
- The branch predictor configuration
- The final contents of the branch predictor
- All measurements described in Section 5.
Your simulator must print outputs to the console (i,e., to the screen). (Also see Section 6. about the requirement.)
Your output must match both numerically and in formatting, because he TAs will literally “diff” your output with the correct output. You must confirm correctness of your simulator by following these two steps for each validation run:
- Redirect the console output of your simulator to a temporary file. This can be achieved by placing > your_output_file after the simulator command.
- Test whether or not your outputs match properly, by running this unix command: diff -iw <your_output_file> <posted_output_file> The -iw tags tell “diff” to treat upper-case and lower-case as equivalent and to ignore the amount of whitespaces between words. Therefore, you do not need to worry about the exact number of spaces or tabs as long as there is some whitespace where the validation runs have whitespace.
You will hand in source code and the Tas will compile and run your simulator. As such, you must meet the following strict requirements. Failure to meet these requirements will result in point deductions (see section “Grading”).
- You must be able to compile and run your simulator on Linux machine. This is required so that the Tas can compile and run your simulator.
- Along with your source code, you must provide a Makefile that automatically compiles the simulator. This Makefile must create a simulator named “sim”. The TAs should be able to type only “make” and the simulator will successfully compile. The TAs should be able to type only “make clean” to automatically remove object files and the simulator executable. An example Makefile is posted in the MachineProblem2.zip folder, which you can copy and modify for your needs.
- Your simulator must accept command-line arguments as follows:
To simulate a bimodal predictor: sim bimodal <M2> <tracefile>, where M
is the number of PC bits used to index the bimodal table.
To simulate a gshare predictor: sim gshare <M1> <N> <tracefile>, where
M1 and N are the number of PC bits and global branch history register bits used to
index the gshare table, respectively.
To simulate a hybrid predictor: sim hybrid <K> <M1> <N> <M2>
<tracefile>, where K is the number of PC bits used to index the chooser table, M
and N are the number of PC bits and global branch history register bits used to index
the gshare table (respectively), and M2 is the number of PC bits used to index the
bimodal table.
(<tracefile> is the filename of the input trace.)
- Your simulator must print outputs to the console (i.e., to the screen). This way, when a TA runs your simulator, he/she can simply redirect the output of your simulator to a filename of his/her choosing for validating the results.
Correctness of your simulator is of paramount importance. That said, making your simulator efficient is also important for a couple of reasons.
First, the TAs need to test every student’s simulator. Therefore, we are placing the constraint that your simulator must finish a single run in 2 minutes or less. If your simulator takes longer than 2 minutes to finish a single run, please see the TAs as they may be able to help you speed up your simulator.
Second, you will be running many experiments: many branch predictor configurations and multiple traces. Therefore, you will benefit from implementing a simulator that is reasonably fast.
[The following applies to students working with C.]
One simple thing you can do to make your simulator run faster is to compile it with a high optimization level. The example Makefile posted on the MachineProblem2.zip folder includes the -O3 optimization flag.
Note that, when you are debugging your simulator in a debugger (such as gdb), it is recommended that you compile without -O3 and with - g. Optimization includes register allocation. Often, register-allocated variables are not displayed properly in debuggers, which is why you want to disable optimization when using a debugger. The -g flag tells the compiler to include symbols (variable names, etc.) in the compiled binary. The debugger needs this information to recognize variable names, function names, line numbers in the source code, etc. When you are done debugging, recompile with -O3 and without -g, to get the most efficient simulator again.
(a) Match the three validation runs “val_bimodal_1.txt”, “val_bimodal_2.txt” and “val_bimodal_3.txt”, posted on the Webcourses for the BIMODAL PREDICTOR. You must match all validation runs to get credit for the experiments with the bimodal
(b) Simulate BIMODAL PREDICTOR configurations for 7 <= m<=12. Use the traces in
Graphs: produce one graph for each benchmark. Graph title: “<benchmark>,
bimodal”. Y-axis: branch misprediction rate. X-axis: m. Per graph, there should be
only one curve consisting of 6 datapoints (connect the datapoints with a line).
(a) Match the three validation runs “val_gshare_1.txt”, “val_gshare_2.txt” and “val_gshare_ 3 .txt” posted on the webcourses for the GSHARE PREDICTOR. You must match all validation runs to get credit for the experiments with the gshare
(b) Simulate GSHARE PREDICTOR configurations for 7 <= m<=12 and 2<=n<=m, n is
Graphs: produce one graph for each benchmark. Graph title: “<benchmark>, gshare”.
Y-axis: branch misprediction rate. X-axis: m. Per graph, there should be a total 27
datapoints plotted as a family of 6 curves. Datapoints having the same value of n are
connected with a line, i.e., one curve for each value of n. Note that not all curves have
the same number of datapoints.
Match the two validation runs “val_hybrid_1.txt” and “val_hybrid_2.txt” posted on
the webcourses for the HYBRID PREDICTOR. The TAs will also check that your
simulator matches two mystery validation runs. Each validation run is worth ¼ of the
points for this part (5 points each).
Item Points
Substantial simulator turned in 30
Item Points
val_bimodal_1.txt 5
val_bimodal_2.txt 5
val_bimodal_3.txt 5
Mystery run A 5
val_gshare_1.txt 5
val_gshare_2.txt 5
val_gshare_3.txt 5
Mystery run B 5
val_hybrid_1.txt 5
Mystery run C 5
Item Points
Graph for Bimodal Predictor with configurations for
7 <= m<=
Graph for gshare predictor configurations for 7 <= m<=
and 2<=n<=m, n is even
You must hand in a single zip files called project2.zip. Below is an example showing how to create project2.zip from an EOS Linux machine. Suppose you have a bunch of source code files (*.cc, *.h), the Makefile, and your project report (report.doc).
zip project2 *.cc *.h Makefile report.doc
project2.zip must contain the following (any deviation from the following requirements may delay grading your project and may result in point deductions, late penalties, etc.
- Project report. This must be a single PDF document named report.pdf. (No Word document please.) The report must include the following: o A cover page with the project title, the Honor Pledge, and your full name as electronic signature of the Honor Pledge. A sample cover page is posted in the project directory. o See Section 7 for the required content of the report.
- Source code. You must include the commented source code for the simulator program itself. You may use any number of .cc/.h files, .c/.h files etc.
- Makefile: See Section 6.2, item #2, for strict requirements. If you fail to meet these requirements, it may delay grading your project and may result in point deductions.
Various deductions (out of 100 points):
Up to -10 points: for not complying with specific procedures. Follow all procedures very carefully to avoid penalties.
Cheating: Source code that is flagged by tools available to us will be dealt with according to University Policy. This includes a ‘0’ for the project and other disciplinary actions.