Assignment: Optimizing the Performance of a Pipelined Processor

1 Introduction

In this lab, you will learn about the design and implementation of a pipelined Y86-64 processor, optimizing both it and a benchmark program to maximize performance. You are allowed to make any semantics- preserving transformation to the benchmark program, or to make enhancements to the pipelined processor, or both. When you have completed the lab, you will have a keen appreciation for the interactions between code and hardware that affect the performance of your programs.

The lab is organized into three parts, each with its own handin. In Part A you will write some simple Y86-64 programs and become familiar with the Y86-64 tools. In Part B, you will extend the SEQ simulator with a new instruction. These two parts will prepare you for Part C, the heart of the lab, where you will optimize the Y86-64 benchmark program and the processor design.

2 Logistics

You will work on this lab alone.

Any clarifications and revisions to the assignment will be posted on the course Web page.

3 Handout Instructions

SITE-SPECIFIC: Insert a paragraph here that explains how students should download the archlab-handout.tar file.

1. Start by copying the file archlab-handout.tar to a (protected) directory in which you plan to do your work.

2. Then give the command: tar xvf archlab-handout.tar. This will cause the following files to be unpacked into the directory: README, Makefile, sim.tar, archlab.pdf, and simguide.pdf.

3. Next, give the command tar xvf sim.tar. This will create the directory sim, which contains your personal copy of the Y86-64 tools. You will be doing all of your work inside this directory.

4. Finally, change to the sim directory and build the Y86-64 tools:

unix> cd sim
unix> make clean; make

4 Part A

You will be working in directory sim/misc in this part.

Your task is to write and simulate the following three Y86-64 programs. The required behavior of these programs is defined by the example C functions in examples.c. Be sure to put your name and ID in a comment at the beginning of each program. You can test your programs by first assemblying them with the program YAS and then running them with the instruction set simulator YIS.

In all of your Y86-64 functions, you should follow the x86-64 conventions for passing function arguments, using registers, and using the stack. This includes saving and restoring any callee-save registers that you use.

sum.ys: Iteratively sum linked list elements

Write a Y86-64 program sum.ys that iteratively sums the elements of a linked list. Your program should consist of some code that sets up the stack structure, invokes a function, and then halts. In this case, the function should be Y86-64 code for a function (sum list) that is functionally equivalent to the C

rsum.ys: Recursively sum linked list elements

Write a Y86-64 program rsum.ys that recursively sums the elements of a linked list. This code should be similar to the code in sum.ys, except that it should use a function rsum list that recursively sums a list of numbers, as shown with the C function rsum list in Figure 1. Test your program using the same three-element list you used for testing list.ys.

copy.ys: Copy a source block to a destination block

Write a program (copy.ys) that copies a block of words from one part of memory to another (non- overlapping area) area of memory, computing the checksum (Xor) of all the words copied.

Your program should consist of code that sets up a stack frame, invokes a function copy block, and then halts. The function should be functionally equivalent to the C function copy block shown in Figure Figure 1. Test your program using the following three-element source and destination blocks:

5 Part B

You will be working in directory sim/seq in this part.

Your task in Part B is to extend the SEQ processor to support the iaddq, described in Homework problems and 4.52. To add this instructions, you will modify the file seq-full.hcl, which implements the version of SEQ described in the CS:APP3e textbook. In addition, it contains declarations of some constants that you will need for your solution.

Your HCL file must begin with a header comment containing the following information:

• Your name and ID.
• A description of the computations required for the iaddq instruction. Use the descriptions of irmovq and OPq in Figure 4.18 in the CS:APP3e text as a guide.

Building and Testing Your Solution

Once you have finished modifying the seq-full.hcl file, then you will need to build a new instance of the SEQ simulator (ssim) based on this HCL file, and then test it:

• Building a new simulator. You can use make to build a new SEQ simulator:

unix> make VERSION=full

This builds a version of ssim that uses the control logic you specified in seq-full.hcl. To save typing, you can assign VERSION=full in the Makefile.

• Testing your solution on a simple Y86-64 program. For your initial testing, we recommend running simple programs such as asumi.yo (testing iaddq) in TTY mode, comparing the results against the ISA simulation:

unix> ./ssim -t ../y86-code/asumi.yo

If the ISA test fails, then you should debug your implementation by single stepping the simulator in GUI mode:

unix> ./ssim -g ../y86-code/asumi.yo

• Retesting your solution using the benchmark programs. Once your simulator is able to correctly execute small programs, then you can automatically test it on the Y86-64 benchmark programs in

../y86-code:

unix> (cd ../y86-code; make testssim)

This will run ssim on the benchmark programs and check for correctness by comparing the resulting processor state with the state from a high-level ISA simulation. Note that none of these programs test the added instructions. You are simply making sure that your solution did not inject errors for the original instructions. See file ../y86-code/README file for more details.

• Performing regression tests. Once you can execute the benchmark programs correctly, then you should run the extensive set of regression tests in ../ptest. To test everything except iaddq and leave:

For more information on the SEQ simulator refer to the handout CS:APP3e Guide to Y86-64 Processor Simulators (simguide.pdf).

6 Part C

You will be working in directory sim/pipe in this part.

The ncopy function in Figure 2 copies a len-element integer array src to a non-overlapping dst, re- turning a count of the number of positive integers contained in src. Figure 3 shows the baseline Y86-64 version of ncopy. The file pipe-full.hcl contains a copy of the HCL code for PIPE, along with a declaration of the constant value IIADDQ.

Your task in Part C is to modify ncopy.ys and pipe-full.hcl with the goal of making ncopy.ys run as fast as possible.

You will be handing in two files: pipe-full.hcl and ncopy.ys. Each file should begin with a header comment with the following information:

• Your name and ID.
• A high-level description of your code. In each case, describe how and why you modified your code.

Coding Rules

You are free to make any modifications you wish, with the following constraints:

• Your ncopy.ys function must work for arbitrary array sizes. You might be tempted to hardwire your solution for 64-element arrays by simply coding 64 copy instructions, but this would be a bad idea because we will be grading your solution based on its performance on arbitrary arrays.

• Your ncopy.ys function must run correctly with YIS. By correctly, we mean that it must correctly copy the src block and return (in %rax) the correct number of positive integers.

• The assembled version of your ncopy file must not be more than 1000 bytes long. You can check the length of any program with the ncopy function embedded using the provided script check-len.pl:

unix> ./check-len.pl < ncopy.yo

• Your pipe-full.hcl implementation must pass the regression tests in ../y86-code and ../ptest (without the -i flag that tests iaddq).

Other than that, you are free to implement the iaddq instruction if you think that will help. You may make any semantics preserving transformations to the ncopy.ys function, such as reordering instruc- tions, replacing groups of instructions with single instructions, deleting some instructions, and adding other instructions. You may find it useful to read about loop unrolling in Section 5.8 of CS:APP3e.

Building and Running Your Solution

In order to test your solution, you will need to build a driver program that calls your ncopy function. We have provided you with the gen-driver.pl program that generates a driver program for arbitrary sized input arrays. For example, typing

unix> make drivers

will construct the following two useful driver programs:

• sdriver.yo: A small driver program that tests an ncopy function on small arrays with 4 elements. If your solution is correct, then this program will halt with a value of 2 in register %rax after copying the src array.

• ldriver.yo: A large driver program that tests an ncopy function on larger arrays with 63 ele- ments. If your solution is correct, then this program will halt with a value of 31 (0x1f) in register %rax after copying the src array.

Each time you modify your ncopy.ys program, you can rebuild the driver programs by typing

unix> make drivers

Each time you modify your pipe-full.hcl file, you can rebuild the simulator by typing

unix> make psim VERSION=full

If you want to rebuild the simulator and the driver programs, type

unix> make VERSION=full

To test your solution in GUI mode on a small 4-element array, type

unix> ./psim -g sdriver.yo

To test your solution on a larger 63-element array, type

unix> ./psim -g ldriver.yo

Once your simulator correctly runs your version of ncopy.ys on these two block lengths, you will want to perform the following additional tests:

• Testing your driver files on the ISA simulator. Make sure that your ncopy.ys function works prop- erly with YIS:

unix> make drivers
unix> ../misc/yis sdriver.yo

• Testing your code on a range of block lengths with the ISA simulator. The Perl script correctness.pl generates driver files with block lengths from 0 up to some limit (default 65), plus some larger sizes.

It simulates them (by default with YIS), and checks the results. It generates a report showing the status for each block length:

unix> ./correctness.pl

This script generates test programs where the result count varies randomly from one run to another, and so it provides a more stringent test than the standard drivers.

If you get incorrect results for some length K, you can generate a driver file for that length that includes checking code, and where the result varies randomly:

unix> ./gen-driver.pl -f ncopy.ys -n K -rc > driver.ys
unix> make driver.yo
unix> ../misc/yis driver.yo

The program will end with register %rax having the following value:
0xaaaa : All tests pass.
0xbbbb : Incorrect count
0xcccc : Function ncopy is more than 1000 bytes long.
0xdddd : Some of the source data was not copied to its destination.
0xeeee : Some word just before or just after the destination region was corrupted.

• Testing your pipeline simulator on the benchmark programs. Once your simulator is able to cor- rectly execute sdriver.ys and ldriver.ys, you should test it against the Y86-64 benchmark programs in ../y86-code:

unix> (cd ../y86-code; make testpsim)

This will run psim on the benchmark programs and compare results with YIS.

• Testing your pipeline simulator with extensive regression tests. Once you can execute the benchmark programs correctly, then you should check it with the regression tests in ../ptest. For example, if your solution implements the iaddq instruction, then

unix> (cd ../ptest; make SIM=../pipe/psim TFLAGS=-i)

• Testing your code on a range of block lengths with the pipeline simulator. Finally, you can run the same code tests on the pipeline simulator that you did earlier with the ISA simulator

unix> ./correctness.pl -p

Attachment:- Optimizing the Performance of a Pipelined Processor.pdf