5/13/99 Ashish Sabharwal1 Pipelining and Hazards n Hazards occur because –Don’t have enough resources (ALU’s, memory,…) Structural Hazard –Need a value that is not yet there Data Hazard Why would this happen? - Program specifies instructions in the correct order - We compute things in the correct order - But we write to memory/registers in a later cycle than when values are read for computation –Can’t compute address of the next instruction correctly Control Hazard n In essence, all three problems arise because of the way our pipeline is designed and the order in which instructions are presented. n Can therefore get rid of (or reduce the penalty of) some of the hazards by –restructuring our pipeline Hardware solutions –presenting instructions to the hardware in a better manner Compiler Solutions

5/13/99 Ashish Sabharwal2 Hazards vs. dependencies n Dependencies are a property of the code –Instructions do need to use values computed by previous instructions! –Compiler can sometimes help give us a better code, but we may still have dependencies n Hazards are a property of code + implementation –No hazards on single cycle / multi cycle implementations n Three kinds of dependencies –Read after write (RAW) - don’t want to read a value that has not been written yet - the one we have been looking at - the most natural dependency –Write after read (WAR) - don’t want to overwrite a value that had to be read by some intermediate instruction - required for correctness - Why doesn’t this cause trouble in our pipeline ? –Write after write (WAW) - if a register/memory location is being written twice, I want to preserve the order in which they are written - required for correctness - why doesn’t this cause trouble in our pipeline ? n What about Read after Read ?

5/13/99 Ashish Sabharwal3 The assignment - 1 n 6.9. “… determine as much as you can about the five instructions in the five pipelining stages”, given values of control signals and some of the instruction fields. –Easy –Just look carefully at each field/control value –Difficult part: Need to determine something about all 5 instructions given a snapshot at some cycle. n Given a piece of assembly code, “explain what the forwarding unit is doing during the fifth cycle of execution.” Mention any comparisons that are being made. –Fill the pipeline with 5 instructions –You know all register specifiers at this stage and also know what registers need to be compared Given a sequence of 100 lw instructions, each dependent on the previous, how long will it take to execute them? –Every instruction is dependent on the previous one –Compute the number of cycles you need to stall between every two instructions –Remember the time needed to fill the pipeline

5/13/99 Ashish Sabharwal4 The assignment (2) Situation 1: add $2, _, _ add _, $2, _ Situation 2: sw _, _($2) lw _, _($2) How do these situations differ in terms of hazards on our pipeline? –(We already talked about hazards given dependencies) Memory to memory copy: lw $2, 100($5) sw $2, 200($6) What additional forwarding hardware can be used to improve performance for such a sequence? –We already have a mechanism for better performance for load followed by its use in an ALU op –Do something similar for load followed by store. –Easy!

5/13/99 Ashish Sabharwal5 The assignment (3) Is there a code sequence for which, at some cycle, we generate signals to flush a pipeline as well as to stall? beq $1, $2, TARGET # assume “taken” lw $3, 40($4) add $3, $3, $3 sw $3, 40($4) TARGET: or $10, $11, $12 Do some of the resulting actions cooperate? Do some of the resulting actions conflict? If yes, how can you modify the pipeline to take care of this? n Loop unrolling Given a loop, unroll it to make 3 copies for each iteration. (Note: number of iterations may not be a multiple of 3 !) Now reschedule the code to avoid hazards as far as possible. How much faster is the new code (include stalls)?

5/13/99 Ashish Sabharwal6 The assignment (4) –Loop unrolling: Original - Loop:lw $t0, 0($s1) addu $t0, $t0, $s2 sw $t0, 0($s1) addi $s1, $s1, -4 bne $s1, $zero, Loop Unrolled once to make 2 copies - Loop:lw $t0, 0($s1) addu $t0, $t0, $s2 sw $t0, 0($s1) lw $t0, -4($s1) addu $t0, $t0, $s2 sw $t0, -4($s1) addi $s1, $s1, -8 bgt $s1, 4, Loop beq$s1, $zero, Done Done: –Code scheduling: Try to “space-out” producer and consumer by adding independent instructions in-between.

5/13/99 Ashish Sabharwal7 The assignment (5) n What happens if we change the pipeline to have the following stages ? IF1 IF2 ID EX MEM1 MEM2 WB What hazards are possible? What needs to be forwarded to which stage to avoid these hazards? –Instruction fetch takes 2 cycles instead of 1 –Memory access takes 2 cycles instead of 1