5. Pipeline and Hazards

Class outline

  1. Fibonacci sequence
  2. Transcription C code to assembler
  3. Simulation and debugging for processor without pipeline (Mips simulator).
  4. Simulation and debugging for processor with pipeline (MipsPipeS simulator).

What should I know before the class

  1. To understand the lecture about pipeline and hazards.

Program to demonstrate pitfalls of pipeline execution

.globl _start
.set noat
.set noreorder

.text
_start:

main:

    addi  $2,  $0, 10
    add   $11, $0, $2   // A : $11<-$2
    add   $12, $0, $2   // B : $12<-$2
    add   $13, $0, $2   // C : $13<-$2

    // la $5, varx  // $5 = (byte*) &varx; 
    // The macro-instruction la is compiled as two following instructions:
    lui   $5, %hi(varx) // load the upper part of address
    ori   $5, $5, %lo(varx) // append the lower part of address

    lw    $1, 0($5)     // $1 = *((int*)$5);
    add   $15, $0, $1   // D : $15<-$1
    add   $16, $0, $1   // E : $16<-$1
    add   $17, $0, $1   // F : $17<-$1
loop:
    break
    beq    $0, $0, loop
    nop

.data

varx:
	.word  1

Trace program step by step:

  1. the first, on CPU with disabled pipeline,
  2. then activate pipeline but left hazard unit switched off. Propose rules to execute program expected way.
  3. Execute program on CPU with hazard unit with and without forwarding.

Remark. Data and instruction cache are not important, both can be disabled.

Observe and analyze not only results stored in registers but even possible stall states and control signals if hazard unit is activated.

  • When are instructions A, B, C, D, E and F results computed and stored into registers and when are results correct/respect instructions program order?
  • Mow many cycles are required to execute whole program?

Number of required cycles can be read in bottom right corner of CPU window.

Question to analyze: If QtMips requires more cycles to execute program when pipeline is enabled than if executed without pipeline, does it mean that pipelined processor is generally slower?

Design enhancement: Try to modify program to better utilize pipelined execution. Is it possible to decrease number of stalls or even achieve state when it can be executed with expected results if hazard unit is switched off?

What shall we do today?

Write a code for calculation of N-th Fibonacci number (for N > 2). Fibonacci sequence is defined as follows:

F(n) = F(n-1) + F(n-2), for n > 2, and F(1) = 0, F(2) = 1.

Here is the first few numbers in the Fibonacci sequence: 0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144,…

To the calculated Fibonacci number (for instructional purposes) add 15. In your program you may use following instructions:

Possible solution in C:

t0 = 5;  //  Set value of N
s0 = 0;  //  F(0)
s1 = 1;  //  F(1)
 
for(t1 = 2; t1 <= t0; t1++)
{
	t2 = s0 + s1;
	s0 = s1;
	s1 = t2;
}
s1 += 15;
 
while(1)
	;   // Endless loop


Template:

#define t0 $8
#define t1 $9
#define t2 $10

#define s0 $16
#define s1 $17
#define s2 $18

.globl start
.set noat
.set noreorder
.ent start

start:
// Here, there is the place for your code

nop
.end start


Debug your code for Mips simulator and then make your code work in MipsPipeS simulator.

Note how the delay slots are handled. They filled automatically by compiler, who will fill in following instruction:

  1. If there is not a label set for instruction before branch, the compiler will move such instruction after the branch instruction.
  2. Otherwise the compiler will fill in the NOP instruction.

This behavior of the compiler can be turned off by following pseudoinstruction:

.set noreorder

Compile your code with this pseudoinstruction, try to execute your code in the QtMips simulator without pipeline and observe the differences. Modify your code for the pipelined version of processor with hazard unit disabled in such way, that it will produce the same value as on processor without pipeline.

Try to find out rules for the compiler, with which the compiler will produce the program without data and control hazards - program will have the same results as in Mips simulator (without pipeline).

For those with spare time

Modify your code to write the result (F(N) + 15) to memory on address 0x02 (using sw instruction) and then read the value back into a register (using lw instruction). Execute your program in MipsPipeS and MipsPipeXL simulators. Observe the execution closely, namely the sw and lw instructions.

Questions:

  • Find out how the add instruction is executed.
  • Find out how the addi instruction is executed.
  • Find out how the lw instruction is executed.
  • Find out how the sw instruction is executed.
  • How many clocks does it take to find out the branch target address? And how I will find it out? (instructions beq a bne)



courses/b35apo/en/tutorials/05/start.txt · Last modified: 2019/04/03 16:03 by pisa