CSC3050 Project 3: RISC-V Simulator
1 Background
Efficient execution of instructions in a RISC-V pipeline relies on avoiding data hazards, where an instruction
depends on the result of a previous instruction that has not yet completed. Data hazards can cause stalls,
reducing the efficiency of the processor. To mitigate these hazards, instruction reordering and specialized fused
operations like fmadd (fused multiply-add) can be utilized.
This assignment has two parts:
• Implementing the fmadd Instruction In this part, you will implement the fused multiply-add (fmadd)
instruction, which performs a multiplication followed by an addition in a single step. This reduces
the number of instructions executed and can eliminate certain data hazards, leading to more efficient
computation.
• Reordering Instructions to Avoid Data Hazards You will be given a sequence of RISC-V instruc tions (add/mul) that suffer from data hazards. Your task will be to rearrange them while maintaining
correctness. This exercise will help you understand the importance of instruction scheduling in hazard
mitigation and performance optimization.
By completing this assignment, you will gain some basic hands-on experience with hazard avoidance
strategies in RISC-V, while learning how fmadd can be used to optimize multiplication-addition sequences and
how instruction reordering can improve pipeline execution efficiency.
2 RISC-V GNU Toolchain
RISC-V GNU Toolchain is already in your Docker, so you do not need to download it from the official link.
But we highly suggest that you open the official link and read the README.
To set up the RISC-V development environment, you need to compile and install the RISC-V GNU toolchain.
This toolchain supports the RISC-V 32I instruction set with M extension (integer multiplication and division),
based on the RISC-V Specification 2.2. Follow these steps to configure and compile the toolchain:
Create a build directory, configure the toolchain, and compile it with the following commands:
mkdir build; cd build
../configure --with-arch=rv32im --enable-multilib --prefix=/path/to/riscv32i
make -j$(nproc)
3 A Simple RISC-V64I Simulator
We use a modified version of Hao He‘s simulator. You can find the modified repository:
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It is a simple RISC-V Emulator suppprting user mode RV64I instruction set, from PKU Computer Architecture
Labs, Spring 2019.
3.1 Compile
mkdir build
cd build
cmake ..
make
3.2 Usage
./Simulator riscv-elf-file-name [-v] [-s] [-d] [-b strategy]
3.3 Parameters
• -v for verbose output, can redirect output to file for further analysis.
• -s for single step execution, often used in combination with -v.
• -d for creating memory and register history dump in dump.txt.
• -b for branch prediction strategy (default BTFNT), accepted parameters are AT, NT, BTFNT, and BPB.
– AT: Always Taken
– NT: Always Not Taken
– BTFNT: Back Taken Forward Not Taken
– BPB: Branch Prediction Buffer (2-bit history information)
4 Part I: RISC-V32I Simulator
The first task in this assignment is to change the RISC-V64I simulator to be RISC-V32I simulator. This is an
easy job, but we suggest that you carefully read the code and know the logical structure of the simulator.
You can re-compile the sample test cases to test your RISC-V32I simulator. Take quicksort as an example:
riscv32-unknown-elf-gcc -march=rv32i \
test/quicksort.c test/lib.c -o riscv-elf/quicksort.riscv
You can change -march=rv32i to -march=rv32imf for the remain part of the assignment.
5 Part I: Fused Instructions
The fused instruction is part of the RISC-V ISA’s F (single-precision floating-point) and D (double-precision
floating-point) extensions. These extensions provide support for floating-point arithmetic operations. In this
project, you only need to implement the integer version.
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Take fmadd.s instruction as an example.
This instruction performs a fused multiply-add operation for floating-point numbers, which means it computes
the product of two floating-point numbers and then adds a third floating-point number to the result, all in a
single instruction. Obviously, this operation is beneficial for both performance and precision, as it reduces the
number of rounding errors compared to performing the multiplication and addition separately.
In this assignment, you are required to implement the fused instruction for integer type. We used the same
format as the standard RISCV R4 instruction. We used the reserved custom opcode 0x0B as our opcode.
Inst Name funct2 funct3 Description
fmadd.i Fused Mul-Add 0x0 0x0 rd = rs1 * rs2 + rs3
fmadd.u Unsigned Fused Mul-Add 0x1 0x0 rd = rs1 * rs2 + rs3
fmsub.i Fused Mul-Sub 0x2 0x0 rd = rs1 * rs2 - rs3
fmsub.u Unsigned Fused Mul-Sub 0x3 0x0 rd = rs1 * rs2 - rs3
fmnadd.i Fused Neg Mul-Add 0x0 0x1 rd = -rs1 * rs2 + rs3
fnmsub.i Fused Neg Mul-Sub 0x1 0x1 rd = -rs1 * rs2 - rs3
5.1 R4 Instruction
R4 instructions, as in Figure 1, involve four registers (rs1, rs2, rs3, rd), which is different from those you are
familiar with. To use standard R4 format, you need to add F-extension when compiling. In other words you
should use -march=rv32if but NOT change the compiling commands of RISC-V GNU Toolchain. (Again,
we are not using floating-points.)
Figure 1: R4 format
5.2 Cycle counts
The fused instruction needs more cycles to process, so we define that our fused instruction needs 3 more
cycles to execute. Specifically, the number of cycles required to complete this instruction is +3 compared to
standard instructions. The mul instruction also incurs an additional 3 cycles, making fmadd more efficient in
terms of cycle count.
Suppose add instruction takes 5 cycles to complete, then we have:
add(1) + mul(3) = 4 > fmadd(3)
5.3 Other Important Information
We also provide some basic test cases for reference. Please refer to the README and /test-fused under
the root of the project.
Also, you can try to compare the number of cycles between fused instructions and basic mul and add instruc tions.
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6 Part I: Disable Data Forwarding
Add an option -x to disable data forwarding.
You can modify the logical of parsing arguments in the method parseParmeters in MainCache.cpp.
7 Part II: Introduction
In this part of the assignment, you will analyze a given sequence of RISC-V instructions that suffer from data
hazards. (With forwarding turned off) Your task is to rearrange these instructions while maintaining correct ness, ensuring that the processor pipeline executes efficiently. Then, you should be able to further optimize it
by substituting add/mul operations with fmadd operations. By strategically reordering instructions, you will
learn how to reduce stalls, improve instruction throughput, and optimize execution flow in a pipelined RISC-V
architecture.
8 Part II: Rearrange
In the part2.s file, you will find a RISC-V program that contains several data hazards affecting pipeline
efficiency. Your task is to rearrange the instructions to minimize stalls while ensuring the program produces
the same output as the original. You should start by reviewing and running part2.s in the simulator to
understand its functionality and identify potential improvements. Your optimized version should preserve
correctness while reducing the number of stalled cycles. Grading will be based on both correctness and
execution efficiency (fewer cycles due to reduced hazards). Name your result with part2 p2.s
9 Part II: Using fmadd.i
After optimizing part2.s, you may identify opportunities to replace certain instruction sequences with the more
efficient fmadd.i instruction (based on either part2.s or part2 p2.s). The final optimized file, part2 p3.s,
should produce the same output as both part2.s and part2 p2.s while improving execution efficiency. Since
fmadd.i combines multiplication and addition into a single operation, the total cycle count should be further
reduced. Grading will be based on both correctness and execution efficiency. Name your optimized file
part2 p3.s before submission.
10 Grading Criteria
The maximum score you can get for this lab is 100 points, and it is composed by the following components:
• Part 1 correctness of implementation 55 pts
• Part 2 correctness of part2 p2.s 20 pts
• Part 2 efficiency of part2 p2.s 20 pts
• A short report about anything you have learn in this project 5 pts
• Part 2 correctness of part2 p3.s Extra Credit 1 pts
• Part 2 efficiency of part2 p3.s Extra Credit 1 pts
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11 Submission
You should make sure your code compiles and runs. Then, it should be compressed into a .zip file and
submitted to BlackBoard. Any necessary instructions to compile and run your code should also be doc umented and included. Finally, you are also required to include a report containing the results of your test
case execution.
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