1 Lecture 4: Advanced Pipelines • Data hazards, control hazards, multi-cycle in-order pipelines (Appendix A.4-A.10)2 A 5-Stage Pipeline3 Conflicts/Problems • I-cache and D-cache are accessed in the same cycle – it helps to implement them separately • Registers are read and written in the same cycle – easy to deal with if register read/write time equals cycle time/2 (else, use bypassing) • Branch target changes only at the end of the second stage -- what do you do in the meantime? • Data between stages get latched into registers (overhead that increases latency per instruction)4 Hazards • Structural hazards: different instructions in different stages (or the same stage) conflicting for the same resource • Data hazards: an instruction cannot continue because it needs a value that has not yet been generated by an earlier instruction • Control hazard: fetch cannot continue because it does not know the outcome of an earlier branch – special case of a data hazard – separate category because they are treated in different ways5 Structural Hazards • Example: a unified instruction and data cache stage 4 (MEM) and stage 1 (IF) can never coincide • The later instruction and all its successors are delayed until a cycle is found when the resource is free these are pipeline bubbles • Structural hazards are easy to eliminate – increase the number of resources (for example, implement a separate instruction and data cache)6 Data Hazards SUB R2 R1, R3 Uses R2 Uses R2 Uses R2 Uses R27 Bypassing • Some data hazard stalls can be eliminated: bypassing8 Example add R1, R2, R3 lw R4, 8(R1)9 Example lw R1, 8(R2) lw R4, 8(R1)10 Example lw R1, 8(R2) sw R1, 8(R3)11 Summary • For the 5-stage pipeline, bypassing can eliminate delays between the following example pairs of instructions: add/sub R1, R2, R3 add/sub/lw/sw R4, R1, R5 lw R1, 8(R2) sw R1, 4(R3) • The following pairs of instructions will have intermediate stalls: lw R1, 8(R2) add/sub/lw R3, R1, R4 or sw R3, 8(R1) fmul F1, F2, F3 fadd F5, F1, F412 Control Hazards • Simple techniques to handle control hazard stalls: for every branch, introduce a stall cycle (note: every 6th instruction is a branch!) assume the branch is not taken and start fetching the next instruction – if the branch is taken, need hardware to cancel the effect of the wrong-path instruction fetch the next instruction (branch delay slot) and execute it anyway – if the instruction turns out to be on the correct path, useful work was done – if the instruction turns out to be on the wrong path, hopefully program state is not lost13 Branch Delay Slots14 Slowdowns from Stalls • Perfect pipelining with no hazards an instruction completes every cycle (total cycles ~ num instructions) speedup = increase in clock speed = num pipeline stages • With hazards and stalls, some cycles (= stall time) go by during which no instruction completes, and then the stalled instruction completes • Total cycles = number of instructions + stall cycles • Slowdown because of stalls = 1/ (1 + stall cycles per instr)15 Pipelining Limits A B C A B C A B C D E F A B C D E F Assume that there is a dependence where the final result of the first instruction is required before starting the second instruction Gap between indep instrs: T Gap between dep instrs: T Gap between indep instrs: T/3 + Tovh Gap between dep instrs: T + 2Tovh Gap between indep instrs: T/6 + Tovh Gap between dep instrs: T + 5Tovh16 Pipeline Implementation • Signals for the muxes have to be generated – some of this can happen during ID • Need look-up tables to identify situations that merit bypassing/stalling – the number of inputs to the muxes goes up17 Situation Example code Action No dependence LD R1, 45(R2) DADD R5, R6, R7 DSUB R8, R6, R7 OR R9, R6, R7 No hazards Dependence requiring stall LD R1, 45(R2) DADD R5, R1, R7 DSUB R8, R6, R7 OR R9, R6, R7 Detect use of R1 during ID of DADD and stall Dependence overcome by forwarding LD R1, 45(R2) DADD R5, R6, R7 DSUB R8, R1, R7 OR R9, R6, R7 Detect use of R1 during ID of DSUB and set mux control signal that accepts result from bypass path Dependence with accesses in order LD R1, 45(R2) DADD R5, R6, R7 DSUB R8, R6, R7 OR R9, R1, R7 No action required Detecting Control Signals18 Multicycle Instructions Functional unit Latency Initiation interval Integer ALU 1 1 Data memory 2 1 FP add 4 1 FP multiply 7 1 FP divide 25 2519 Effects of Multicycle Instructions • Structural hazards if the unit is not fully pipelined (divider) • Frequent RAW hazard stalls • Potentially multiple writes to the register file in a cycle • WAW hazards because of out-of-order instr completion • Imprecise exceptions because of o-o-o instr completion Note: Can also increase the “width” of the processor: handle multiple instructions at the same time: for example, fetch two instructions, read registers for both, execute both, etc.20 Precise Exceptions • On an exception: must save PC of instruction where program must resume all instructions after that PC that might be in the pipeline must be converted to NOPs (other instructions continue to execute and may raise exceptions of their own) temporary program state not in memory (in other words, registers) has to be stored in memory potential problems if a later instruction has already modified memory or registers • A processor that fulfils all the above conditions is said to provide precise exceptions (useful for debugging and of course, correctness)21 Dealing with these Effects • Multiple writes to the register file: increase the number of ports, stall one of the writers during ID, stall one of the writers during WB (the stall will propagate) • WAW hazards: detect the hazard during ID and stall the later instruction • Imprecise exceptions: buffer the results if they complete early or save more pipeline state so that you can return to exactly the same state that you left at22 ILP • Instruction-level parallelism: overlap among instructions: pipelining or multiple instruction execution • What determines the degree of ILP? dependences: property of the
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