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September 24 th , 2003 Prof. John Kubiatowicz

CS252 Graduate Computer Architecture Lecture 8 ILP 2: Precise Interrupts and Getting the CPI < 1. September 24 th , 2003 Prof. John Kubiatowicz http://www.cs.berkeley.edu/~kubitron/courses/cs252-F03. Review: Dynamic hardware techniques for out-of-order execution. HW exploitation of ILP

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September 24 th , 2003 Prof. John Kubiatowicz

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  1. CS252Graduate Computer ArchitectureLecture 8 ILP 2:Precise Interrupts and Getting the CPI < 1 September 24th, 2003 Prof. John Kubiatowicz http://www.cs.berkeley.edu/~kubitron/courses/cs252-F03

  2. Review: Dynamic hardware techniques for out-of-order execution • HW exploitation of ILP • Works when can’t know dependence at compile time. • Code for one machine runs well on another • Scoreboard (ala CDC 6600 in 1963) • Centralized control structure • No register renaming, no forwarding • Pipeline stalls for WAR and WAW hazards. • Are these fundamental limitations??? (No) • Reservation stations (ala IBM 360/91 in 1966) • Distributed control structures • Implicit renaming of registers (dispatched pointers) • WAR and WAW hazards eliminated by register renaming • Results broadcast to all reservation stations for RAW

  3. Review: Tomasulo Organization FP Registers From Mem FP Op Queue Load Buffers Load1 Load2 Load3 Load4 Load5 Load6 Store Buffers Add1 Add2 Add3 Mult1 Mult2 Reservation Stations To Mem FP adders FP multipliers Common Data Bus (CDB)

  4. Review: Three Stages of Tomasulo Algorithm 1. Issue—get instruction from FP Op Queue If reservation station free (no structural hazard), control issues instr & sends operands (renames registers). 2. Execution—operate on operands (EX) When both operands ready then execute; if not ready, watch Common Data Bus for result 3. Write result—finish execution (WB) Write on Common Data Bus to all awaiting units; mark reservation station available • Common data bus: data + source (“come from” bus) • 64 bits of data + 4 bits of Functional Unit source address • Write if matches expected Functional Unit (produces result) • Does the broadcast

  5. Review: Loop Example Cycle 9 • Dataflow graph constructed completely in hardware • Renaming detaches early iterations from registers

  6. Stream of Instructions To Execute Instruction Fetch with Branch Prediction Out-Of-Order Execution Unit Correctness Feedback On Branch Results Problem: “Fetch” unit • Instruction fetch decoupled from execution • Often issue logic (+ rename) included with Fetch

  7. Branches must be resolved quickly for loop overlap! • In our loop-unrolling example, we relied on the fact that branches were under control of “fast” integer unit in order to get overlap! Loop: LD F0 0 R1 MULTD F4 F0 F2 SD F4 0 R1 SUBI R1 R1 #8 BNEZ R1 Loop • What happens if branch depends on result of multd?? • We completely lose all of our advantages! • Need to be able to “predict” branch outcome. • If we were to predict that branch was taken, this would be right most of the time. • Problem much worse for superscalar machines!

  8. Prediction: Branches, Dependencies, Data • Prediction has become essential to getting good performance from scalar instruction streams. • We will discuss predicting branches. However, architects are now predicting everything: data dependencies, actual data, and results of groups of instructions: • At what point does computation become a probabilistic operation + verification? • We are pretty close with control hazards already… • Why does prediction work? • Underlying algorithm has regularities. • Data that is being operated on has regularities. • Instruction sequence has redundancies that are artifacts of way that humans/compilers think about problems. • Prediction  Compressible information streams?

  9. What about Precise Exceptions/Interrupts? • Both Scoreboard and Tomasulo have: • In-order issue, out-of-order execution, out-of-order completion • Recall: An interrupt or exception is precise if there is a single instruction for which: • All instructions before that have committed their state • No following instructions (including the interrupting instruction) have modified any state. • Need way to resynchronize execution with instruction stream (I.e. with issue-order) • Easiest way is with in-order completion (i.e. reorder buffer) • Other Techniques (Smith paper): Future File, History Buffer

  10. Reorder Buffer FP Op Queue FP Regs Res Stations Res Stations FP Adder FP Adder HW support for precise interrupts • Concept of Reorder Buffer (ROB): • Holds instructions in FIFO order, exactly as they were issued • Each ROB entry contains PC, dest reg, result, exception status • When instructions complete, results placed into ROB • Supplies operands to other instruction between execution complete & commit  more registers like RS • Tag results with ROB buffer number instead of reservation station • Instructions commit values at head of ROB placed in registers • As a result, easy to undo speculated instructions on mispredicted branches or on exceptions Commit path

  11. Recall: Four Steps of Speculative Tomasulo Algorithm 1. Issue—get instruction from FP Op Queue If reservation station and reorder buffer slot free, issue instr & send operands &reorder buffer no. for destination (this stage sometimes called “dispatch”) 2. Execution—operate on operands (EX) When both operands ready then execute; if not ready, watch CDB for result; when both in reservation station, execute; checks RAW (sometimes called “issue”) 3. Write result—finish execution (WB) Write on Common Data Bus to all awaiting FUs & reorder buffer; mark reservation station available. 4. Commit—update register with reorder result When instr. at head of reorder buffer & result present, update register with result (or store to memory) and remove instr from reorder buffer. Mispredicted branch flushes reorder buffer (sometimes called “graduation”)

  12. Reorder Buffer FP Op Queue Compar network Program Counter Exceptions? Dest Reg FP Regs Result Valid Reorder Table Res Stations Res Stations FP Adder FP Adder What are the hardware complexities with reorder buffer (ROB)? • How do you find the latest version of a register? • As specified by Smith paper, need associative comparison network • Could use future file or just use the register result status buffer to track which specific reorder buffer has received the value • Need as many ports on ROB as register file

  13. 1 10+R2 Tomasulo With Reorder buffer: Done? FP Op Queue ROB7 ROB6 ROB5 ROB4 ROB3 ROB2 ROB1 Newest Reorder Buffer Oldest F0 LD F0,10(R2) N Registers To Memory Dest from Memory Dest Dest Reservation Stations FP adders FP multipliers

  14. F10 ADDD F10,F4,F0 N F0 LD F0,10(R2) N 1 10+R2 Tomasulo With Reorder buffer: Done? FP Op Queue ROB7 ROB6 ROB5 ROB4 ROB3 ROB2 ROB1 Newest Reorder Buffer Oldest Registers To Memory Dest from Memory Dest 2 ADDD R(F4),ROB1 Dest Reservation Stations FP adders FP multipliers

  15. F2 DIVD F2,F10,F6 N F10 ADDD F10,F4,F0 N F0 LD F0,10(R2) N 3 DIVD ROB2,R(F6) 1 10+R2 Tomasulo With Reorder buffer: Done? FP Op Queue ROB7 ROB6 ROB5 ROB4 ROB3 ROB2 ROB1 Newest Reorder Buffer Oldest Registers To Memory Dest from Memory Dest 2 ADDD R(F4),ROB1 Dest Reservation Stations FP adders FP multipliers

  16. F0 ADDD F0,F4,F6 N F4 LD F4,0(R3) N -- BNE F2,<…> N F2 DIVD F2,F10,F6 N F10 ADDD F10,F4,F0 N F0 LD F0,10(R2) N 3 DIVD ROB2,R(F6) Tomasulo With Reorder buffer: Done? FP Op Queue ROB7 ROB6 ROB5 ROB4 ROB3 ROB2 ROB1 Newest Reorder Buffer Oldest Registers To Memory Dest from Memory Dest 2 ADDD R(F4),ROB1 6 ADDD ROB5, R(F6) Dest Reservation Stations 1 10+R2 6 0+R3 FP adders FP multipliers

  17. -- ROB5 ST 0(R3),F4 N F0 ADDD F0,F4,F6 N F4 LD F4,0(R3) N -- BNE F2,<…> N F2 DIVD F2,F10,F6 N F10 ADDD F10,F4,F0 N F0 LD F0,10(R2) N 3 DIVD ROB2,R(F6) Tomasulo With Reorder buffer: Done? FP Op Queue ROB7 ROB6 ROB5 ROB4 ROB3 ROB2 ROB1 Newest Reorder Buffer Oldest Registers To Memory Dest from Memory Dest 2 ADDD R(F4),ROB1 6 ADDD ROB5, R(F6) Dest Reservation Stations 1 10+R2 6 0+R3 FP adders FP multipliers

  18. -- M[10] ST 0(R3),F4 Y F0 ADDD F0,F4,F6 N F4 M[10] LD F4,0(R3) Y -- BNE F2,<…> N F2 DIVD F2,F10,F6 N F10 ADDD F10,F4,F0 N F0 LD F0,10(R2) N 3 DIVD ROB2,R(F6) 1 10+R2 Tomasulo With Reorder buffer: Done? FP Op Queue ROB7 ROB6 ROB5 ROB4 ROB3 ROB2 ROB1 Newest Reorder Buffer Oldest Registers To Memory Dest from Memory Dest 2 ADDD R(F4),ROB1 6 ADDD M[10],R(F6) Dest Reservation Stations FP adders FP multipliers

  19. -- M[10] ST 0(R3),F4 Y F0 <val2> ADDD F0,F4,F6 Ex F4 M[10] LD F4,0(R3) Y -- BNE F2,<…> N F2 DIVD F2,F10,F6 N F10 ADDD F10,F4,F0 N F0 LD F0,10(R2) N 3 DIVD ROB2,R(F6) 1 10+R2 Tomasulo With Reorder buffer: Done? FP Op Queue ROB7 ROB6 ROB5 ROB4 ROB3 ROB2 ROB1 Newest Reorder Buffer Oldest Registers To Memory Dest from Memory Dest 2 ADDD R(F4),ROB1 Dest Reservation Stations FP adders FP multipliers

  20. -- M[10] ST 0(R3),F4 Y F0 <val2> ADDD F0,F4,F6 Ex F4 M[10] LD F4,0(R3) Y -- BNE F2,<…> N What about memory hazards??? 3 DIVD ROB2,R(F6) 1 10+R2 Tomasulo With Reorder buffer: Done? FP Op Queue ROB7 ROB6 ROB5 ROB4 ROB3 ROB2 ROB1 Newest Reorder Buffer F2 DIVD F2,F10,F6 N F10 ADDD F10,F4,F0 N Oldest F0 LD F0,10(R2) N Registers To Memory Dest from Memory Dest 2 ADDD R(F4),ROB1 Dest Reservation Stations FP adders FP multipliers

  21. Memory Disambiguation:Sorting out RAW Hazards in memory • Question: Given a load that follows a store in program order, are the two related? • (Alternatively: is there a RAW hazard between the store and the load)? Eg: st 0(R2),R5 ld R6,0(R3) • Can we go ahead and start the load early? • Store address could be delayed for a long time by some calculation that leads to R2 (divide?). • We might want to issue/begin execution of both operations in same cycle. • Today: Answer is that we are not allowed to start load until we know that address 0(R2)  0(R3) • Next Week: We might guess at whether or not they are dependent (called “dependence speculation”) and use reorder buffer to fixup if we are wrong.

  22. Hardware Support for Memory Disambiguation • Need buffer to keep track of all outstanding stores to memory, in program order. • Keep track of address (when becomes available) and value (when becomes available) • FIFO ordering: will retire stores from this buffer in program order • When issuing a load, record current head of store queue (know which stores are ahead of you). • When have address for load, check store queue: • If any store prior to load is waiting for its address, stall load. • If load address matches earlier store address (associative lookup), then we have a memory-induced RAW hazard: • store value available  return value • store value not available  return ROB number of source • Otherwise, send out request to memory • Actual stores commit in order, so no worry about WAR/WAW hazards through memory.

  23. Memory Disambiguation: Done? FP Op Queue ROB7 ROB6 ROB5 ROB4 ROB3 ROB2 ROB1 Newest Reorder Buffer -- LD F4, 10(R3) N F2 R[F5] ST 10(R3), F5 N F0 LD F0,32(R2) N Oldest -- <val 1> ST 0(R3), F4 Y Registers To Memory Dest from Memory Dest Dest Reservation Stations 2 32+R2 4ROB3 FP adders FP multipliers

  24. Relationship between precise interrupts and speculation: • Speculation is a form of guessing • Branch prediction, data prediction • If we speculate and are wrong, need to back up and restart execution to point at which we predicted incorrectly • This is exactly same as precise exceptions! • Branch prediction is a very important! • Need to “take our best shot” at predicting branch direction. • If we issue multiple instructions per cycle, lose lots of potential instructions otherwise: • Consider 4 instructions per cycle • If take single cycle to decide on branch, waste from 4 - 7 instruction slots! • Technique for both precise interrupts/exceptions and speculation: in-order completion or commit • This is why reorder buffers in all new processors

  25. Administrative • Assignment #1 up later this week • Usually have 1.5 to 2 weeks to do assignment • Can work together for brainstorming, but must do own work • Midterm I: Monday 10/13 Location: 306 Soda Hall TIME: 5:30 - 8:30 • Can have 1 sheet of 8½x11 handwritten notes – both sides • No microfiche of the book! • This info is on the Lecture page (has been) • Meet at LaVal’s afterwards for Pizza and Beverages • Great way for me to get to know you better • I’ll Buy!

  26. Quick Recap: Explicit Register Renaming • Make use of a physical register file that is larger than number of registers specified by ISA • Keep a translation table: • ISA register => physical register mapping • When register is written, replace table entry with new register from freelist. • Physical register becomes free when not being used by any instructions in progress. Fetch Decode/ Rename Execute Rename Table

  27. P0 P2 P4 F6 F8 P10 P12 P14 P16 P18 P20 P22 P24 p26 P28 P30 Newest Done? Oldest  P32 P34 P36 P38 P60 P62 Explicit register renaming:R10000 Freelist Management • Physical register file larger than ISA register file • On issue, each instruction that modifies a register is allocated new physical register from freelist • Used on: R10000, Alpha 21264, HP PA8000 Current Map Table Freelist

  28. P32 P2 P4 F6 F8 P10 P12 P14 P16 P18 P20 P22 P24 p26 P28 P30 Newest Oldest  P34 P36 P38 P40 P60 P62 Explicit register renaming:R10000 Freelist Management • Note that physical register P0 is “dead” (or not “live”) past the point of this load. • When we go to commit the load, we free up Done? Current Map Table Freelist F0 P0 LD P32,10(R2) N

  29. P32 P2 P4 F6 F8 P34 P12 P14 P16 P18 P20 P22 P24 p26 P28 P30 Newest Oldest  P36 P38 P40 P42 P60 P62 Explicit register renaming:R10000 Freelist Management Done? Current Map Table F10 P10 ADDD P34,P4,P32 N Freelist F0 P0 LD P32,10(R2) N

  30. P32 P32 P36 P36 P4 P4 F6 F6 F8 F8 P34 P34 P12 P12 P14 P14 P16 P16 P18 P18 P20 P20 P22 P22 P24 P24 p26 p26 P28 P28 P30 P30 Newest Done? -- Oldest -- BNE P36,<…> N F2 P2 DIVD P36,P34,P6 N F10 P10 ADDD P34,P4,P32 N  P38 P38 P40 P40 P44 P44 P48 P48 P60 P62 F0 P0 LD P32,10(R2) N Explicit register renaming:R10000 Freelist Management Current Map Table Freelist  Checkpoint at BNE instruction P60 P62

  31. P40 P32 P36 P36 P38 P4 F6 F6 F8 F8 P34 P34 P12 P12 P14 P14 P16 P16 P18 P18 P20 P20 P22 P22 P24 P24 p26 p26 P28 P28 P30 P30 Newest Oldest  P42 P38 P40 P44 P44 P48 P50 P48 P0 P10 Explicit register renaming:R10000 Freelist Management Done? Current Map Table -- ST 0(R3),P40 Y F0 P32 ADDD P40,P38,P6 Y F4 P4 LD P38,0(R3) Y -- BNE P36,<…> N F2 P2 DIVD P36,P34,P6 N F10 P10 ADDD P34,P4,P32 y Freelist F0 P0 LD P32,10(R2) y  Checkpoint at BNE instruction P60 P62

  32. P32 P32 P36 P36 P4 P4 F6 F6 F8 F8 P34 P34 P12 P12 P14 P14 P16 P16 P18 P18 P20 P20 P22 P22 P24 P24 p26 p26 P28 P28 P30 P30 Newest Oldest  P38 P38 P40 P40 P44 P44 P48 P48 P0 P10 Explicit register renaming:R10000 Freelist Management Done? Current Map Table F2 P2 DIVD P36,P34,P6 N F10 P10 ADDD P34,P4,P32 y Freelist F0 P0 LD P32,10(R2) y Error fixed by restoring map table and merging freelist  Checkpoint at BNE instruction P60 P62

  33. Advantages of Explicit Renaming • Decouples renamingfrom scheduling: • Pipeline can be exactly like “standard” DLX pipeline (perhaps with multiple operations issued per cycle) • Or, pipeline could be tomasulo-like or a scoreboard, etc. • Standard forwarding or bypassing could be used • Allows data to be fetched from single register file • No need to bypass values from reorder buffer • This can be important for balancing pipeline • Many processors use a variant of this technique: • R10000, Alpha 21264, HP PA8000 • Another way to get precise interrupt points: • All that needs to be “undone” for precise break pointis to undo the table mappings • Provides an interesting mix between reorder buffer and future file • Results are written immediately back to register file • Registers names are “freed” in program order (by ROB)

  34. Getting CPI < 1: IssuingMultiple Instructions/Cycle • Two variations • Superscalar: varying no. instructions/cycle (1 to 8), scheduled by compiler or by HW (Tomasulo) • IBM PowerPC, Sun UltraSparc, DEC Alpha, HP 8000 • (Very) Long Instruction Words (V)LIW: fixed number of instructions (4-16) scheduled by the compiler; put ops into wide templates • Joint HP/Intel agreement in 1999/2000? • Intel Architecture-64 (IA-64) 64-bit address • Style: “Explicitly Parallel Instruction Computer (EPIC)” • Anticipated success lead to use of Instructions Per Clock cycle (IPC) vs. CPI

  35. Getting CPI < 1: IssuingMultiple Instructions/Cycle • Superscalar DLX: 2 instructions, 1 FP & 1 anything else – Fetch 64-bits/clock cycle; Int on left, FP on right – Can only issue 2nd instruction if 1st instruction issues – More ports for FP registers to do FP load & FP op in a pair Type Pipe Stages Int. instruction IF ID EX MEM WB FP instruction IF ID EX MEM WB Int. instruction IF ID EX MEM WB FP instruction IF ID EX MEM WB Int. instruction IF ID EX MEM WB FP instruction IF ID EX MEM WB • 1 cycle load delay expands to 3 instructions in SS • instruction in right half can’t use it, nor instructions in next slot

  36. Review: Unrolled Loop that Minimizes Stalls for Scalar 1 Loop: LD F0,0(R1) 2 LD F6,-8(R1) 3 LD F10,-16(R1) 4 LD F14,-24(R1) 5 ADDD F4,F0,F2 6 ADDD F8,F6,F2 7 ADDD F12,F10,F2 8 ADDD F16,F14,F2 9 SD 0(R1),F4 10 SD -8(R1),F8 11 SD -16(R1),F12 12 SUBI R1,R1,#32 13 BNEZ R1,LOOP 14 SD 8(R1),F16 ; 8-32 = -24 14 clock cycles, or 3.5 per iteration LD to ADDD: 1 Cycle ADDD to SD: 2 Cycles

  37. Loop Unrolling in Superscalar Integer instruction FP instruction Clock cycle Loop: LD F0,0(R1) 1 LD F6,-8(R1) 2 LD F10,-16(R1) ADDD F4,F0,F2 3 LD F14,-24(R1) ADDD F8,F6,F2 4 LD F18,-32(R1) ADDD F12,F10,F2 5 SD 0(R1),F4 ADDD F16,F14,F2 6 SD -8(R1),F8 ADDD F20,F18,F2 7 SD -16(R1),F12 8 SD -24(R1),F16 9 SUBI R1,R1,#40 10 BNEZ R1,LOOP 11 SD -32(R1),F20 12 • Unrolled 5 times to avoid delays (+1 due to SS) • 12 clocks, or 2.4 clocks per iteration (1.5X)

  38. Dynamic Scheduling in Superscalar • How to issue two instructions and keep in-order instruction issue for Tomasulo? • Assume 1 integer + 1 floating point • 1 Tomasulo control for integer, 1 for floating point • Issue 2X Clock Rate, so that issue remains in order • Only FP loads might cause dependency between integer and FP issue: • Replace load reservation station with a load queue; operands must be read in the order they are fetched • Load checks addresses in Store Queue to avoid RAW violation • Store checks addresses in Load Queue to avoid WAR,WAW • Called “decoupled architecture”

  39. Multiple Issue Challenges • While Integer/FP split is simple for the HW, get CPI of 0.5 only for programs with: • Exactly 50% FP operations • No hazards • If more instructions issue at same time, greater difficulty of decode and issue: • Even 2-scalar => examine 2 opcodes, 6 register specifiers, & decide if 1 or 2 instructions can issue • Multiported rename logic: must be able to rename same register multiple times in one cycle! • Rename logic one of key complexities in the way of multiple issue! • VLIW: tradeoff instruction space for simple decoding • The long instruction word has room for many operations • By definition, all the operations the compiler puts in the long instruction word are independent => execute in parallel • E.g., 2 integer operations, 2 FP ops, 2 Memory refs, 1 branch • 16 to 24 bits per field => 7*16 or 112 bits to 7*24 or 168 bits wide • Need compiling technique that schedules across several branches

  40. Loop Unrolling in VLIW Memory Memory FP FP Int. op/ Clockreference 1 reference 2 operation 1 op. 2 branch LD F0,0(R1) LD F6,-8(R1) 1 LD F10,-16(R1) LD F14,-24(R1) 2 LD F18,-32(R1) LD F22,-40(R1) ADDD F4,F0,F2 ADDD F8,F6,F2 3 LD F26,-48(R1) ADDD F12,F10,F2 ADDD F16,F14,F2 4 ADDD F20,F18,F2 ADDD F24,F22,F2 5 SD 0(R1),F4 SD -8(R1),F8 ADDD F28,F26,F2 6 SD -16(R1),F12 SD -24(R1),F16 7 SD -32(R1),F20 SD -40(R1),F24 SUBI R1,R1,#48 8 SD -0(R1),F28 BNEZ R1,LOOP 9 Unrolled 7 times to avoid delays 7 results in 9 clocks, or 1.3 clocks per iteration (1.8X) Average: 2.5 ops per clock, 50% efficiency Note: Need more registers in VLIW (15 vs. 6 in SS)

  41. Recall: Software Pipelining withLoop Unrolling in VLIW Memory Memory FP FP Int. op/ Clock reference 1 reference 2 operation 1 op. 2 branch LD F0,-48(R1) ST 0(R1),F4 ADDD F4,F0,F2 1 LD F6,-56(R1) ST -8(R1),F8 ADDD F8,F6,F2 SUBI R1,R1,#24 2 LD F10,-40(R1) ST 8(R1),F12 ADDD F12,F10,F2 BNEZ R1,LOOP 3 • Software pipelined across 9 iterations of original loop • In each iteration of above loop, we: • Store to m,m-8,m-16 (iterations I-3,I-2,I-1) • Compute for m-24,m-32,m-40 (iterations I,I+1,I+2) • Load from m-48,m-56,m-64 (iterations I+3,I+4,I+5) • 9 results in 9 cycles, or 1 clock per iteration • Average: 3.3 ops per clock, 66% efficiency Note: Need less registers for software pipelining (only using 7 registers here, was using 15)

  42. Advantages of HW (Tomasulo) vs. SW (VLIW) Speculation • HW determines address conflicts • HW better branch prediction • HW maintains precise exception model • HW does not execute bookkeeping instructions • Works across multiple implementations • SW speculation is much easier for HW design

  43. Smaller code size Binary compatability across generations of hardware Simplified Hardware for decoding, issuing instructions No Interlock Hardware (compiler checks?) More registers, but simplified Hardware for Register Ports (multiple independent register files?) Superscalar v. VLIW

  44. Intel/HP “Explicitly Parallel Instruction Computer (EPIC)” • 3 Instructions in 128 bit “groups”; field determines if instructions dependent or independent • Smaller code size than old VLIW, larger than x86/RISC • Groups can be linked to show independence > 3 instr • 64 integer registers + 64 floating point registers • Not separate filesper funcitonal unit as in old VLIW • Hardware checks dependencies (interlocks => binary compatibility over time) • Predicated execution (select 1 out of 64 1-bit flags) => 40% fewer mispredictions? • IA-64 : instruction set architecture; EPIC is type • Merced is name of first implementation (1999/2000?) • LIW = EPIC?

  45. Summary #1 • Dynamic hardware schemes can unroll loops dynamically in hardware • Form of limited dataflow • Reorder Buffer • In-order issue, Out-of-order execution, In-order commit • Holds results until they can be commited in order • Serves as source of info until instructions committed • Provides support for precise exceptions/Speculation: simply throw out instructions later than excepted instruction. • Memory Disambiguation: • Tracking of RAW hazards through memory • Keep program-order queue of storesWhen have address for load, check store queue: • If any store prior to load is waiting for its address, stall load. • If load address matches earlier store address (associative lookup), then we have a memory-induced RAW hazard: • Otherwise, send out request to memory

  46. Summary #2 • Explicit Renaming: more physical registers than needed by ISA. • Separates renaming from scheduling • Opens up lots of options for resolving RAW hazards • Rename table: tracks current association between architectural registers and physical registers • Potentially complicated rename table management • Superscalar and VLIW: CPI < 1 (IPC > 1) • Dynamic issue vs. Static issue • More instructions issue at same time => larger hazard penalty • Limitation is often number of instructions that you can successfully fetch and decode per cycle  “Flynn barrier” • Other models of parallelism: Vector processing

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