Chapter 8 Pipelining

A strategy for employing parallelism to achieve better performance Taking the “assembly line” approach to fetching and executing instructions

The Cycle The control unit: Fetch Execute Fetch Execute Etc.

The Cycle How about separate components for fetching the instruction and executing it? Then fetch unit: fetch instruction execute unit: execute instruction So, how about fetch while execute?

clock cycle

Overlapping fetch with execute Two stage pipeline

Both components busy during each clock cycle F4F4

The Cycle The cycle can be divided into four parts fetch instruction decode instruction execute instruction write result back to memory So, how about four components?

The four components operating in parallel

buffer for instruction buffer for operands buffer for result

Instruction I3 Operands for I2 Operation info for I2 Write info for I2 Result of instruction I1

One clock cycle for each pipeline stage Therefore cycle time must be long enough for the longest stage A unit is idle if it requires less time than another Best if all stages are about the same length Cache memory helps

Fetching (instructions or data) from main memory may take 10 times as long as an operation such as ADD Cache memory (especially if on the same chip) allows fetching as quickly as other operations

One clock cycle per component, four cycles total to complete an instruction

Completes an instruction each clock cycle Therefore, four times as fast as without pipeline

Completes an instruction each clock cycle Therefore, four times as fast as without pipeline as long as nothing takes more than one cycle But sometimes things take longer -- for example, most executes such as ADD take one clock, but suppose DIVIDE takes three

and other stages idle Write has nothing to write Decode can’t use its “out” buffer Fetch can’t use its “out” buffer

A data “hazard” has caused the pipeline to “stall” no data for Write

An instruction “hazard” (or “control hazard”) has caused the pipeline to “stall” Instruction I2 not in the cache, required a main memory access

Structural Hazards Conflict over use of a hardware resource Memory –Can’t fetch an instruction while another instruction is fetching an operand, for example –Cache: same Unless cache has multiple ports Or separate caches for instructions, data Register file One access at a time, again unless multiple ports

Structural Hazards Conflict over use of a hardware resource- -such as the register file Example: LOAD X(R1), R2 ( LOAD R2, X(R1) in MIPS ) address of memory location i.e., the address in R1 + X Load that word from memory (cache) into R2 X + [R1]

I2 writing to register file I3 must wait for register file I2 takes extra cycle for cache access as part of execution calculate the address I5 fetch delayed

Data Hazards Situations that cause the pipeline to stall because data to be operated on is delayed –execute takes extra cycle, for example

Data Hazards Or, because of data dependencies –Pipeline stalls because an instruction depends on data from another instruction

Concurrency A 3 + A B 4 x A A 5 x C B 20 + C Can’t be performed concurrently--result incorrect if new value of A is not used Can be performed concurrently (or in either order) without affecting result

Concurrency A 3 + A B 4 x A A 5 x C B 20 + C Second operation depends on completion of first operation The two operations are independent

MUL R2, R3, R4 ADD R5, R4, R6 ( dependent on result in R4 from previous instruction) will write result in R4 can’t finish decoding until result is in R4

Data Forwarding Pipeline stalls in previous example waiting for I1’s result to be stored in R4 Delay can be reduced if result is forwarded directly to I2

pipeline stall data forwarding

MUL R2, R3, R4 from R2 R2 x R3 to R4 to I2 from R3

ADD R5, R4, R6 MUL R2, R3, R4 R2, R3 R2 x R3 R5R4 + R5 R4 R6

If solved by software: MUL R2, R3, R4 NOOP ADD R5, R4, R6 2 cycle stall introduced by hardware (if no data forwarding)

Side Effects ADD (R1)+, R2, R3 –Not only changes destination register, but also changes R1 ADD R1, R3 ADDWCR2, R4 –Add with carry dependent on condition code flag set by previous ADD—an implicit dependency

Side Effects Data dependency on something other than the result destination Multiple dependencies Pipelining clearly works better if side effects are avoided in the instruction set –Simple instructions

Instruction Hazards Pipeline depends on steady stream of instructions from the instruction fetch unit pipeline stall from a cache miss

Decode, execute, and write units are all idle for the “extra” clock cycles

Branch Instructions Their purpose is to change the content of the PC and fetch another instruction Consequently, the fetch unit may be fetching an “unwanted” instruction

SW R1, A BUN K LW R5, B two stage pipeline fetch instruction 3 discard instruction 3 and fetch instruction K instead computes new PC value

the lost cycle is the “branch penalty”

four stage pipeline instruction 3 fetched and decoded instruction 4 fetched instructions 3 and 4 discarded, instruction K fetched

In a four stage pipeline, the penalty is two clock cycles

Unconditional Branch Instructions Reducing the branch penalty requires computing the branch address earlier Hardware in the fetch and decode units –Identify branch instructions –Compute branch target address (instead of doing it in the execute stage)

fetched and decoded discarded penalty reduced to one cycle

Instruction Queue and Prefetching Fetching instructions into a “queue” Dispatch unit (added to decode) to take instructions from queue Enlarging the “buffer” zone between fetch and decode

buffer for instruction buffer for operands buffer for result

buffer for multiple instructions buffer for operands buffer for result

“oldest instruction in the queue--next to be dispatched “newest instruction in the queue--most recently fetched

previous out, F1 in F1 out, F2 in F2 out, F3 in F4 inF5 in

instructions 3 and 4 instructions 3,4, and 5 keeps fetching despite stall

calculates branch target concurrently “branch folding” discards F6 and fetches K

completes an instruction each clock cycle no branch penalty

Instruction Queue Avoiding branch penalty requires that the queue contain other instructions for processing while branch target is calculated Queue can mitigate cache misses--if instruction not in cache, execution unit can continue as long as queue has instructions

Instruction Queue So, it is important to keep queue full Helped by increasing rate at which instructions move from cache to queue Multiple word moves (in parallel) cacheinstruction queue one clock cycle n words

Conditional Branching Added hazard of dependency on previous instruction Can’t calculate target until a previous instruction completes execution SUBW R1, A FEDW FD 1 2 fetch K or fetch 3? BGTZ K

Conditional Branching Occur frequently, perhaps 20% of all instruction executions Would present serious problems for pipelining if not handled Several possibilities

Delayed Branching Location(s) following a branch instruction have been fetched and must be discarded These positions called “branch delay slots”

the penalty is two clock cycles in a four stage pipeline two branch delay slots if branch address calculated here

fetched and decoded discarded penalty reduced to one cycle one branch delay slot

Delayed Branching Instructions in delay slots always fetched and partially executed So, place useful instructions in those positions and execute whether branch is taken or not If no such instructions, use NOOPs

shift R1 N times R2 contains N fetched and discarded on every iteration branch if not zero

shift R1 N times R2 contains N do the shifting while the counter is being decremented and tested branch delayed for one instruction

appears to “branch” here actually branches here Branch instruction waits for an instruction cycle before actually branching—hence “delayed branch” “delay slot”

If there is no useful instruction to put here, put NOOP “delay slot” If branches are “delayed branches then the next instruction will always be fetched and executed

next to last pass through the loop last pass through the loop

will branch, so fetch the decrement instruction won’t branch, so fetch the add instruction

Delayed Branching Compiler must recognize and rearrange instructions One branch delay slot can usually be filled--filling two is much more difficult If adding pipeline stages increases number of branch delay slots, benefit may be lost

Branch Prediction Hardware attempts to predict which path will be taken For example, assumes branch will not take place Does “speculative execution” of the instructions on that path Must not change any registers or memory until branch decision is known

fetch unit predicts branch will not be taken results of compare known in cycle 4 if branch taken these instructions discarded

Static Branch Prediction Hardware attempts to predict which path will be taken (shouldn’t always assume the same result) Based on target address of branch—is it higher or lower than current address Software (compiler) can make better prediction and for example set a bit in the branch instruction

Dynamic Branch Prediction Processor keeps track of branch decisions Determines likelihood of future branches One bit for each branch instruction –LT branch likely to be taken –LNT branch likely not to be taken

Four states ST: strongly likely SNT: strongly not likely

Dynamic Branch Prediction If prediction wrong (LNT) at end of loop, it is changed (to ST), therefore is correct until the last iteration Stays correct for subsequent executions of the loop (only changes if wrong twice in a row) Initial prediction can be guessed by the hardware, better if set by the compiler

Pipelining’s Effect on Instruction Sets Multiple addressing modes –Facilitate use of data structures Indexing, offsets –Provide flexibility –One instruction instead of many Can cause problems for the pipeline

Structural Hazards Conflict over use of a hardware resource- -such as the register file Example: (indirect addressing with offset) LOAD X(R1), R2 ( LOAD R2, X(R1) in MIPS ) address of memory location i.e., the address in R1 + X Load that word from memory (cache) into R2 X + [R1]

I2 writing to register file I3 must wait for register file I2 takes extra cycle for cache access as part of execution calculate the address (register access) I5 fetch delayed

LOAD ( X(R1) ), R2 double indirect with offset two clock pipeline stall while address is calculated and data fetched

ADD #X, R1, R2 LOAD (R2), R2 same seven clock cycles three simpler instructions to do the same thing

Condition Codes Conditional branch instructions dependent on condition codes set by a previous instruction For example, COMPARE R3, R4 sets a bit in the PSW to be tested by BRANCH if ZERO

Branch decision must wait for completion of Compare Can’t take place in decode, must wait for execution stage

result of compare is in PSW by the time Branch instruction is decoded depends on Add instruction not affecting condition codes

Condition Codes and Pipelining Compiler must be able to reorder instructions Condition codes set by few instructions Compiler should be able to control which instructions set condition codes

Datapath: registers, ALU, interconnecting bus single internal bus general registers

with a single internal bus, one thing at a time over the bus single internal bus

1) PC out, MAR in, Read, Select 4, Add, Z in 2) Z out, PC in, Y in, wait for memory 3) MDR out, IR in 4) Offset field of IR out, Add, Z in 5) Z out, PC in, end unconditional branch

three internal buses three port register file transfers three at a time PC incrementer for address incrementing (multiple word transfers)

1) PC out, R=B, MAR in, Read, Inc PC 2) Wait for memory 3) MDR out B, R=B, IR in 4) R4 out A, R5 out B, select A, Add, R6 in, end ADD R4, R5, R6

three internal bus organization modified for pipelining two caches--one for instructions, one for data separate MARs one for each cache

PC connected directly to IMAR, can transfer concurrently with ALU operation data address can come from register file or from ALU

separate MDRs for read and write buffer registers for ALU inputs and output

buffering for control signals following Decode and Execute instruction queue loaded directly from cache

Can perform simultaneously in any combination: Reading an instruction from instruction cache Incrementing the PC Decoding an instruction Reading from or writing into data cache Reading contents of up to two registers from the register file Writing into one register of the register file Performing an ALU operation

Superscalar Operation Pipelining fetches one instruction per cycle, completes one per cycle (if no hazards) Adding multiple processing units for each stage would allow more than one instruction to be fetched, and moved through the pipeline, during each cycle

Superscalar Operation Starting more than one instruction in each clock cycle is called multiple issue Such processors are called superscalar

a processor with two execution units

instruction queue and multiple word moves enables fetching n instructions

dispatch unit capable of decoding two instructions from queue

so, if top two instructions are: ADDF R1, R2, R3 ADD R4, R5, R6

Floating point execution unit is pipelined also

instructions complete out of order OK if no dependencies problem if error (imprecise interrupt/exception)

imprecise interrupt error occurs here later instructions have already completed!

results written in program order

error occurs here later instructions discarded results written in program order precise interrupts

temporary registers allow greater flexibility

register renaming ADD R4, R5, R6 would write to R6 writes to another register “renamed” as R6, used by subsequent instructions using that result

R0 R1 R2. Rn n012...n the “architectural” registers the physical registers changeable mapping register renaming

Superscalar Statically scheduled –Variable number of instructions issued each clock cycle –Issued in-order (as ordered by compiler) –Much effort required by compiler Dynamically scheduled –Issued out-of-order (determined by hardware)

Superscalar Two issue, dual issue--capable of issuing two instructions at a time (as in previous example) Four issue--four at a time Etc. Overhead grows with issue width

Closing the Performance Gap At the “micro architecture” level Interpreting the CISC x86 instruction set with hardware that translates to “RISC- like” operators Pipelining and superscalar execution of those micro-ops

microcode compiler x86 machine language C++ program hard wired logic early x86 processors

micro op translator compiler x86 machine language C++ program micro-ops hard wired logic current x86 processors

micro op translator compiler x86 machine language C++ program micro-ops hard wired logic microcode current x86 processors some instructions

micro op translator compiler x86 machine language C++ program micro-ops hard wired logic compiler MIPS machine language C++ program hard wired logic

micro op translator compiler x86 machine language C++ program micro-ops hard wired logic micro op translator compiler x86 machine language Java program micro-ops hard wired logic JVM Java byte code

micro op translator compiler x86 machine language C++ program micro-ops hard wired logic