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COMPUTER ORGANIZATION AND DESIGN 5th Edition The Hardware/Software Interface Chapter 4 The Processor §4.1 Introduction Introduction n CPU p...
COMPUTER ORGANIZATION AND DESIGN 5th Edition The Hardware/Software Interface Chapter 4 The Processor §4.1 Introduction Introduction n CPU performance factors n Instruction count n Determined by ISA and compiler n CPI and Cycle time n Determined by CPU hardware n We will examine two MIPS implementations n A simplified version n A more realistic pipelined version n Simple subset, shows most aspects n Memory reference: lw, sw n Arithmetic/logical: add, sub, and, or, slt n Control transfer: beq, j Chapter 4 — The Processor — 2 Instruction Execution n PC → instruction memory, fetch instruction n Register numbers → register file, read registers n Depending on instruction class n Use ALU to calculate n Arithmetic result n Memory address for load/store n Branch target address n Access data memory for load/store n PC ← target address (for jumps) or PC + 4 (for next instruction) Chapter 4 — The Processor — 3 CPU Overview Chapter 4 — The Processor — 4 Multiplexers n Can’t just join wires together n Use multiplexers Chapter 4 — The Processor — 5 A Multiplexer A AND A 0 M U C inverter OR C X B 1 B AND S S Chapter 4 — The Processor — 6 A Multiplexer A AND A 1 A 0 M U C OR A X B 1 B AND S 0 0 S Chapter 4 — The Processor — 7 A Multiplexer A AND 0 0 A 0 M U C OR B X B 1 B AND S B 1 S Chapter 4 — The Processor — 8 Control Chapter 4 — The Processor — 9 §4.2 Logic Design Conventions Logic Design Basics n Information encoded in binary n Low voltage = 0, High voltage = 1 n One wire per bit n Multi-bit data encoded on multi-wire buses n Combinational element n Operate on data n Output is a function of input (no internal storage) n State (sequential) elements n Store information Chapter 4 — The Processor — 10 Combinational Elements n AND-gate n Adder A Y + n Y=A&B n Y=A+B B A Y B n Arithmetic/Logic Unit n Multiplexer n Y = F(A, B) n Y = S ? I1 : I0 A I0 M u Y ALU Y I1 x B S F Chapter 4 — The Processor — 11 Sequential Elements n Register: stores data in a circuit n Uses a clock signal to determine when to update the stored value n Edge-triggered: update when Clk changes from 0 to 1 Clk D Q D Clk Q Chapter 4 — The Processor — 12 Sequential Elements n Register with write control n Only updates on clock edge when write control input is 1 n Used when stored value is required later Clk D Q Write Write D Clk Q Chapter 4 — The Processor — 13 Clocking Methodology n Combinational logic transforms data during clock cycles n Between clock edges n Input from state elements, output to state element n Longest delay determines clock period Chapter 4 — The Processor — 14 §4.3 Building a Datapath Building a Datapath n Datapath n Elements that process data and addresses in the CPU n Registers, ALUs, mux’s, memories, … n We will build a MIPS datapath incrementally n Refining the overview design Chapter 4 — The Processor — 15 Instruction Fetch Increment by 4 for next 32-bit instruction register Chapter 4 — The Processor — 16 R-Format Instructions add $t1, $t2, $t3 n Read two register operands (e.g., $t2, $t3) n Perform arithmetic/logical operation n Write register result (e.g., $t1) Chapter 4 — The Processor — 17 Load/Store Instructions lw $t1, off($t2) n Read register operands ($t2) n Calculate address using 16-bit offset n Use ALU, but sign-extend offset n Load: Read memory and update register (e.g., $t1) n Store: Write register value to memory Chapter 4 — The Processor — 18 Branch Instructions beq $t1, $t2, offset n Read register operands ($t1, $t2) n Compare operands n Use ALU, subtract and check Zero output n Calculate target address n Sign-extend displacement n Shift left 2 places (word displacement) n Add to PC + 4 n Already calculated by instruction fetch Chapter 4 — The Processor — 19 Branch Instructions beq $t1, $t2, offset Just re-routes wires Sign-bit wire replicated Chapter 4 — The Processor — 20 Composing the Elements n First-cut data path does an instruction in one clock cycle n Each datapath element can only do one function at a time n Hence, we need separate instruction and data memories n Use multiplexers where alternate data sources are used for different instructions Chapter 4 — The Processor — 21 R-Type/Load/Store Datapath 0 0 add $t1, $t2, $t3 lw $t1, off($t2) sw $t1, off($t2) Chapter 4 — The Processor — 22 R-Type/Load/Store Datapath $t2 0 $t3 $t1 0 add $t1, $t2, $t3 Chapter 4 — The Processor — 23 R-Type/Load/Store Datapath $t2 $t2 0 $t3 0 $t1 $t3 0 add $t1, $t2, $t3 Chapter 4 — The Processor — 24 R-Type/Load/Store Datapath add $t2+$t3 $t2 $t2 0 $t3 0 0 $t1 $t3 0 add $t1, $t2, $t3 Chapter 4 — The Processor — 25 R-Type/Load/Store Datapath add $t2+$t3 $t2 $t2 0 $t3 0 0 $t1 $t3 1 0 $t2+$t3 add $t1, $t2, $t3 Chapter 4 — The Processor — 26 R-Type/Load/Store Datapath $t2 0 $t1 off 1 lw $t1, off($t2) Chapter 4 — The Processor — 27 R-Type/Load/Store Datapath $t2 $t2 0 1 $t1 off 1 32-bit off lw $t1, off($t2) Chapter 4 — The Processor — 28 R-Type/Load/Store Datapath add $t2+off $t2 $t2 0 1 $t1 off 1 32-bit off lw $t1, off($t2) Chapter 4 — The Processor — 29 R-Type/Load/Store Datapath add $t2+off $t2 $t2 0 1 1 $t1 off 1 32-bit off lw $t1, off($t2) Chapter 4 — The Processor — 30 R-Type/Load/Store Datapath add $t2+off $t2 $t2 0 1 1 $t1 off 1 32-bit off Mem [$t2+off] lw $t1, off($t2) Chapter 4 — The Processor — 31 R-Type/Load/Store Datapath add $t2+off $t2 $t2 0 1 1 $t1 1 off 1 32-bit off Mem [$t2+off] lw $t1, off($t2) Chapter 4 — The Processor — 32 R-Type/Load/Store Datapath $t2 1 $t1 off 0 sw $t1, off($t2) Chapter 4 — The Processor — 33 R-Type/Load/Store Datapath $t2 $t2 1 $t1 1 $t1 off 0 32-bit off sw $t1, off($t2) Chapter 4 — The Processor — 34 R-Type/Load/Store Datapath add $t2+off $t2 $t2 1 $t1 1 $t1 off 0 32-bit off sw $t1, off($t2) Chapter 4 — The Processor — 35 Full Datapath Chapter 4 — The Processor — 36 §4.4 A Simple Implementation Scheme ALU Control n ALU used for n Load/Store: F = add n Branch: F = subtract n R-type: F depends on funct field ALU control Function 0000 AND 0001 OR 0010 add 0110 subtract 0111 set-on-less-than 1100 NOR Chapter 4 — The Processor — 37 ALU Control n Assume 2-bit ALUOp derived from opcode n Combinational logic derives ALU control opcode ALUOp Operation funct ALU function ALU control lw 00 load word XXXXXX add 0010 sw 00 store word XXXXXX add 0010 beq 01 branch equal XXXXXX subtract 0110 R-type 10 add 100000 add 0010 subtract 100010 subtract 0110 AND 100100 AND 0000 OR 100101 OR 0001 set-on-less-than 101010 set-on-less-than 0111 Chapter 4 — The Processor — 38 The Main Control Unit n Control signals derived from instruction R-type 0 rs rt rd shamt funct 31:26 25:21 20:16 15:11 10:6 5:0 Load/ 35 or 43 rs rt address Store 31:26 25:21 20:16 15:0 Branch 4 rs rt address 31:26 25:21 20:16 15:0 opcode always read, write for sign-extend read except R-type and add for load and load Chapter 4 — The Processor — 39 Datapath With Control 4 2 Chapter 4 — The Processor — 40 R-Type 0 31:26 rs 25:21 rt 20:16 rd 15:11 shamt 10:6 funct 5:0 4 2 Chapter 4 — The Processor — 41 Load 35 or 43 31:26 rs 25:21 rt 20:16 address 15:0 4 2 Chapter 4 — The Processor — 42 BEQ 4 31:26 rs 25:21 rt 20:16 address 15:0 4 2 Chapter 4 — The Processor — 43 Implementing Jumps Jump 2 address 31:26 25:0 n Jump uses word address n Update PC with concatenation of n Top 4 bits of old PC n 26-bit jump address n 00 n Need an extra control signal decoded from opcode Chapter 4 — The Processor — 44 Jump 2 31:26 address 25:0 4 2 Chapter 4 — The Processor — 45 ALU Control ALU Control Lines Function 0000 AND 0001 OR 0010 add 0110 subtract 0111 set on less than 1100 NOR ALUOp Operation 00 Add (lw/sw) 01 Subtract (beq) 10 Operation determined by function field (0-5) of R-type instructions Chapter 4 — The Processor — 46 ALU Control Instruction ALUOp Instruction Func Field Desired ALU op code operation ALU control operation input LW 00 Load word XXXXXX Add 0010 SW 00 Store word XXXXXX Add 0010 Branch eq 01 Branch eq XXXXXX Subtract 0110 R-type 10 Add 100000 Add 0010 R-type 10 Subtract 100010 Subtract 0110 R-type 10 AND 100100 AND 0000 R-type 10 OR 100101 OR 0001 R-type 10 Set on < 101010 Set on < 0111 Chapter 4 — The Processor — 47 Control Unit Design Instr. RegDst ALU- Mem-to Reg- Mem Mem- Branch ALU ALU Src Reg Write Read Write Op1 Op0 R- 1 0 0 1 0 0 0 1 0 type lw 0 1 1 1 1 0 0 0 0 sw X 1 X 0 0 1 0 0 0 beq X 0 X 0 0 0 1 0 1 Chapter 4 — The Processor — 48 Performance Issues n Longest delay determines clock period n Critical path: load instruction n Instruction memory → register file → ALU → data memory → register file n Not feasible to vary period for different instructions n Violates design principle n Making the common case fast n We will improve performance by pipelining Chapter 4 — The Processor — 49 §4.5 An Overview of Pipelining Pipelining Analogy n Pipelined laundry: overlapping execution n Parallelism improves performance n Four loads: n Speedup = 8/3.5 = 2.3 n Non-stop: n Speedup = 2n/0.5n + 1.5 ≈ 4 = number of stages Chapter 4 — The Processor — 50 §4.5 An Overview of Pipelining Pipelining Analogy n With pipelining, how long does it take for a full load to be ready (from washing to being put away) assuming that each of the four steps takes 30 minutes? n In steady state, what is the rate (i.e., throughput) at which full loads are complete? Chapter 4 — The Processor — 51 §4.5 An Overview of Pipelining Pipelining Analogy n With pipelining, how long does it take for a full load to be ready (from washing to being put away) assuming that each of the four steps takes 30 minutes? n A: 4 x 30 minutes = 2 hours. Same as without pipelining! n In steady state, what is the rate (i.e., throughput) at which full loads are complete? n A: 1/30 minutes. Four times the throughput without pipelining. Chapter 4 — The Processor — 52 MIPS Pipeline n Five stages, one step per stage 1. IF: Instruction fetch from memory 2. ID: Instruction decode & register read 3. EX: Execute operation or calculate address 4. MEM: Access memory operand 5. WB: Write result back to register Chapter 4 — The Processor — 53 Pipeline Performance n Assume time for stages is n 100ps for register read or write n 200ps for other stages n Compare pipelined datapath with single-cycle datapath Instr Instr fetch Register ALU op Memory Register Total time read access write lw 200ps 100 ps 200ps 200ps 100 ps 800ps sw 200ps 100 ps 200ps 200ps 700ps R-format 200ps 100 ps 200ps 100 ps 600ps beq 200ps 100 ps 200ps 500ps Chapter 4 — The Processor — 54 Pipeline Performance Single-cycle (Tc= 800ps) Pipelined (Tc= 200ps) Chapter 4 — The Processor — 55 Pipeline Performance Single-cycle (Tc= 800ps) Pipelined (Tc= 200ps) Chapter 4 — The Processor — 56 Pipeline Performance Single-cycle (Tc= 800ps) Pipelined (Tc= 200ps) Chapter 4 — The Processor — 57 Pipeline Performance Assume as before that a single cycle implementation takes 800 ps per instruction and that a 5-stage pipeline completes one instruction each 200 ps. How long does it take to execute a program with 1,000,000 instructions in each case? What is the speedup of the pipelining implementation? Chapter 4 — The Processor — 58 Pipeline Performance Assume as before that a single cycle implementation takes 800 ps per instruction and that a 5-stage pipeline completes one instruction each 200 ps. How long does it take to execute a program with 1,000,000 instructions in each case? A: no pipelined implementation: 1,000,000 * 800 ps = 8 * 108 ps = 8 * 108 * 10-12 sec = 0.0008 sec pipelined implementation 1,000,000 * 200 ps = 2 * 108 ps = 2 * 108 * 10-12 sec = 0.0002 sec What is the speedup of the pipelining implementation? A: 0.0008 / 0.0002 = 4 Chapter 4 — The Processor — 59 Pipeline Speedup n If all stages are balanced n i.e., all take the same time n Time between instructionspipelined = Time between instructionsnonpipelined Number of stages n If not balanced, speedup is less n Speedup due to increased throughput n Latency (time for each instruction) does not decrease Chapter 4 — The Processor — 60 Pipelining and ISA Design n MIPS ISA designed for pipelining n All instructions are 32-bits n Easier to fetch and decode in one cycle n c.f. x86: 1- to 17-byte instructions n Few and regular instruction formats n Can decode and read registers in one step n Load/store addressing n Can calculate address in 3rd stage, access memory in 4th stage n Alignment of memory operands n Memory access takes only one cycle Chapter 4 — The Processor — 61 Hazards n Situations that prevent starting the next instruction in the next cycle n Structural hazards n A required resource is busy n Data hazard n Need to wait for previous instruction to complete its data read/write n Control hazard n Deciding on control action depends on previous instruction Chapter 4 — The Processor — 62 Hazards n Situations that prevent starting the next instruction in the next cycle n Structural hazards n A required resource is busy n Data hazard n Need to wait for previous instruction to complete its data read/write n Control hazard n Deciding on control action depends on previous instruction Chapter 4 — The Processor — 63 Structural Hazards n Conflict for use of a resource n In MIPS pipeline with a single memory n Load/store requires data access n Instruction fetch would have to stall for that cycle n Would cause a pipeline “bubble” n Hence, pipelined datapaths require separate instruction/data memories n Or separate instruction/data caches Chapter 4 — The Processor — 64 Structural Hazard with Single Memory Resource conflict if instruction and data memory are the same! Chapter 4 — The Processor — 65 Hazards n Situations that prevent starting the next instruction in the next cycle n Structural hazards n A required resource is busy n Data hazard n Need to wait for previous instruction to complete its data read/write n Control hazard n Deciding on control action depends on previous instruction Chapter 4 — The Processor — 66 Data Hazards n An instruction depends on completion of data access by a previous instruction n add $s0, $t0, $t1 sub $t2, $s0, $t3 Chapter 4 — The Processor — 67 Forwarding (aka Bypassing) n Use result when it is computed n Don’t wait for it to be stored in a register n Requires extra connections in the datapath Chapter 4 — The Processor — 68 Load-Use Data Hazard n Can’t always avoid stalls by forwarding n If value not computed when needed n Can’t forward backward in time! Chapter 4 — The Processor — 69 Code Scheduling to Avoid Stalls n Reorder code to avoid use of load result in the next instruction n C code for A = B + E; C = B + F; lw $t1, 0($t0) lw $t1, 0($t0) lw $t2, 4($t0) lw $t2, 4($t0) stall add $t3, $t1, $t2 lw $t4, 8($t0) sw $t3, 12($t0) add $t3, $t1, $t2 lw $t4, 8($t0) sw $t3, 12($t0) stall add $t5, $t1, $t4 add $t5, $t1, $t4 sw $t5, 16($t0) sw $t5, 16($t0) 13 cycles 11 cycles Chapter 4 — The Processor — 70 Hazards n Situations that prevent starting the next instruction in the next cycle n Structural hazards n A required resource is busy n Data hazard n Need to wait for previous instruction to complete its data read/write n Control hazard n Deciding on control action depends on previous instruction Chapter 4 — The Processor — 71 Control Hazards n Branch determines flow of control n Fetching next instruction depends on branch outcome n Pipeline can’t always fetch correct instruction n Still working on ID stage of branch n In MIPS pipeline n Need to compare registers and compute target early in the pipeline n Add hardware to do it in ID stage Chapter 4 — The Processor — 72 Stall on Branch n Wait until branch outcome determined before fetching next instruction Assumes extra hardware to compare registers in the ID stage. Chapter 4 — The Processor — 73 Stall on Branch Increases CPI n Branch instructions represent 17% of SPECint2006 benchmark. If they took two clock cycles and the other instructions one clock cycle, what is the CPI? Chapter 4 — The Processor — 74 Stall on Branch Increases CPI n Branch instructions represent 17% of SPECint2006 benchmark. If they took two clock cycles and the other instructions one clock cycle, what is the CPI? n A: CPI = 0.17 * 2 + 0.83 * 1 = 1.17 Chapter 4 — The Processor — 75 Branch Prediction n Longer pipelines can’t readily determine branch outcome early n Stall penalty becomes unacceptable n Predict outcome of branch n Only stall if prediction is wrong n In MIPS pipeline n Can predict branches not taken n Fetch instruction after branch, with no delay Chapter 4 — The Processor — 76 MIPS with Predict Not Taken Prediction correct Prediction incorrect Chapter 4 — The Processor — 77 More-Realistic Branch Prediction n Static branch prediction n Based on typical branch behavior n Example: loop and if-statement branches n Predict backward branches taken n Predict forward branches not taken n Dynamic branch prediction n Hardware measures actual branch behavior n e.g., record recent history of each branch n Assume future behavior will continue the trend n When wrong, stall while re-fetching, and update history Chapter 4 — The Processor — 78 Pipeline Summary The BIG Picture n Pipelining improves performance by increasing instruction throughput n Executes multiple instructions in parallel n Each instruction has the same latency n Subject to hazards n Structure, data, control n Instruction set design affects complexity of pipeline implementation Chapter 4 — The Processor — 79 Checking Pipeline Understanding Does the following code sequence must stall, can avoid stalls using forwarding only, or can execute without stalling or forwarding? n lw $t0, 0($t0) n add $t1, $t0, $t0 Chapter 4 — The Processor — 80 Checking Pipeline Understanding Does the following code sequence must stall, can avoid stalls using forwarding only, or can execute without stalling or forwarding? n lw $t0, 0($t0) $t0 available n add $t1, $t0, $t0 must stall $t0 needed Chapter 4 — The Processor — 81 Checking Pipeline Understanding Does the following code sequence must stall, can avoid stalls using forwarding only, or can execute without stalling or forwarding? n add $t1, $t0, $t0 n addi $t2, $t0, #5 n addi $t4, $t1, #5 Chapter 4 — The Processor — 82 Checking Pipeline Understanding Does the following code sequence must stall, can avoid stalls using forwarding only, or can execute without stalling or forwarding? n add $t1, $t0, $t0 n addi $t2, $t0, #5 $t1 available n addi $t4, $t1, #5 $t2 available $t1 needed Chapter 4 — The Processor — 83 Checking Pipeline Understanding Does the following code sequence must stall, can avoid stalls using forwarding only, or can execute without stalling or forwarding? n add $t1, $t0, $t0 n addi $t2, $t0, #5 $t1 computed > forward n addi $t4, $t1, #5 $t2 available $t1 needed Chapter 4 — The Processor — 84 Checking Pipeline Understanding Does the following code sequence must stall, can avoid stalls using forwarding only, or can execute without stalling or forwarding? n addi $t1, $t0, #1 n addi $t2, $t0, #2 n addi $t3, $t0, #2 n addi $t3, $t0, #4 n add $t5, $t2, $t1 Chapter 4 — The Processor — 85 Checking Pipeline Understanding Does the following code sequence must stall, can avoid stalls using forwarding only, or can execute without stalling or forwarding? n addi $t1, $t0, #1 n addi $t2, $t0, #2 No stalling or forwarding needed. n addi $t3, $t0, #2 $t1 available n addi $t3, $t0, #4 n add $t5, $t2, $t1 $t2 available $t3 available $t3 available $t5 av. Chapter 4 — The Processor — 86 §4.6 Pipelined Datapath and Control MIPS Pipelined Datapath MEM Right-to-left WB flow leads to hazards Chapter 4 — The Processor — 87 Pipeline registers n Need registers between stages n To hold information produced in previous cycle Chapter 4 — The Processor — 88 Pipeline Operation n Cycle-by-cycle flow of instructions through the pipelined datapath n “Single-clock-cycle” pipeline diagram n Shows pipeline usage in a single cycle n Highlight resources used n c.f. “multi-clock-cycle” diagram n Graph of operation over time n We’ll look at “single-clock-cycle” diagrams for load & store Chapter 4 — The Processor — 89 IF for Load, Store, … Chapter 4 — The Processor — 90 ID for Load, Store, … Chapter 4 — The Processor — 91 EX for Load Chapter 4 — The Processor — 92 MEM for Load Chapter 4 — The Processor — 93 WB for Load Wrong register number Chapter 4 — The Processor — 94 Corrected Datapath for Load Chapter 4 — The Processor — 95 EX for Store Chapter 4 — The Processor — 96 MEM for Store Chapter 4 — The Processor — 97 WB for Store Chapter 4 — The Processor — 98 Multi-Cycle Pipeline Diagram n Form showing resource usage Chapter 4 — The Processor — 99 Multi-Cycle Pipeline Diagram n Traditional form Chapter 4 — The Processor — 100 Single-Cycle Pipeline Diagram n State of pipeline in a given cycle Chapter 4 — The Processor — 101 Pipelined Control (Simplified) Chapter 4 — The Processor — 102 Pipelined Control n Control signals derived from instruction n As in single-cycle implementation Chapter 4 — The Processor — 103 Pipelined Control Chapter 4 — The Processor — 104 §4.7 Data Hazards: Forwarding vs. Stalling Data Hazards in ALU Instructions n Consider this sequence: sub $2, $1,$3 and $12,$2,$5 or $13,$6,$2 add $14,$2,$2 sw $15,100($2) n We can resolve hazards with forwarding n How do we detect when to forward? Chapter 4 — The Processor — 105 Dependencies & Forwarding Chapter 4 — The Processor — 106 Remembering n Control signals derived from instruction R-type 0 rs rt rd shamt funct 31:26 25:21 20:16 15:11 10:6 5:0 Load/ 35 or 43 rs rt address Store 31:26 25:21 20:16 15:0 Branch 4 rs rt address 31:26 25:21 20:16 15:0 opcode always read, write for sign-extend read except R-type and add for load and load Chapter 4 — The Processor — 107 Detecting the Need to Forward n Pass register numbers along pipeline n e.g., ID/EX.RegisterRs = register number for Rs sitting in ID/EX pipeline register n ALU operand register numbers in EX stage are given by n ID/EX.RegisterRs, ID/EX.RegisterRt n Data hazards when Fwd from 1a. EX/MEM.RegisterRd = ID/EX.RegisterRs EX/MEM pipeline reg 1b. EX/MEM.RegisterRd = ID/EX.RegisterRt 2a. MEM/WB.RegisterRd = ID/EX.RegisterRs Fwd from MEM/WB 2b. MEM/WB.RegisterRd = ID/EX.RegisterRt pipeline reg Chapter 4 — The Processor — 108 add $t1, $t2, $t5 Pipelined Control sub $t3, $t1, $t4 add rd=$t1 rs=$t2 rt = $t5 Chapter 4 — The Processor — 109 add $t1, $t2, $t5 Pipelined Control sub $t3, $t1, $t4 sub add rd=$t3 rd=$t1 rs=$t1 rt = $t4 Chapter 4 — The Processor — 110 add $t1, $t2, $t5 Pipelined Control sub $t3, $t1, $t4 sub add rd=$t3 rd=$t1 rs=$t1 rt = $t4 Forward from the EX/MEM pipeline register Chapter 4 — The Processor — 111 Detecting the Need to Forward n But only if forwarding instruction will write to a register! n EX/MEM.RegWrite, MEM/WB.RegWrite n And only if Rd for that instruction is not $zero n EX/MEM.RegisterRd ≠ 0, MEM/WB.RegisterRd ≠ 0 Chapter 4 — The Processor — 112 Forwarding Paths 00 01 10 00 01 10 Chapter 4 — The Processor — 113 Forwarding Conditions Mux Control Source Explanation ForwardA = 00 ID/EX The first ALU operand comes from the register file. ForwardA = 10 EX/MEM The first ALU operand is forwarded from the prior ALU result. ForwardA = 01 MEM/WB The first ALU operand is forwarded from data memory or an earlier ALU result. ForwardB = 00 ID/EX The second ALU operand comes from the register file. ForwardB = 10 EX/MEM The second ALU operand is forwarded from the prior ALU result. ForwardB = 01 MEM/WB The second ALU operand is forwarded from data memory or an earlier ALU result. Chapter 4 — The Processor — 114 Forwarding Conditions n EX hazard n if (EX/MEM.RegWrite and (EX/MEM.RegisterRd ≠ 0) and (EX/MEM.RegisterRd = ID/EX.RegisterRs)) ForwardA = 10 n if (EX/MEM.RegWrite and (EX/MEM.RegisterRd ≠ 0) and (EX/MEM.RegisterRd = ID/EX.RegisterRt)) ForwardB = 10 n MEM hazard n if (MEM/WB.RegWrite and (MEM/WB.RegisterRd ≠ 0) and (MEM/WB.RegisterRd = ID/EX.RegisterRs)) ForwardA = 01 n if (MEM/WB.RegWrite and (MEM/WB.RegisterRd ≠ 0) and (MEM/WB.RegisterRd = ID/EX.RegisterRt)) ForwardB = 01 Chapter 4 — The Processor — 115 Double Data Hazard n Consider the sequence: add $1,$1,$2 add $1,$1,$3 add $1,$1,$4 n Both hazards occur n Want to use the most recent n Revise MEM hazard condition n Only fwd if EX hazard condition isn’t true Chapter 4 — The Processor — 116 Revised Forwarding Condition n MEM hazard n if (MEM/WB.RegWrite and (MEM/WB.RegisterRd ≠ 0) and not (EX/MEM.RegWrite and (EX/MEM.RegisterRd ≠ 0) and (EX/MEM.RegisterRd = ID/EX.RegisterRs)) and (MEM/WB.RegisterRd = ID/EX.RegisterRs)) ForwardA = 01 n if (MEM/WB.RegWrite and (MEM/WB.RegisterRd ≠ 0) and not (EX/MEM.RegWrite and (EX/MEM.RegisterRd ≠ 0) and (EX/MEM.RegisterRd = ID/EX.RegisterRt)) and (MEM/WB.RegisterRd = ID/EX.RegisterRt)) ForwardB = 01 Chapter 4 — The Processor — 117 Datapath with Forwarding Chapter 4 — The Processor — 118 Load-Use Data Hazard Need to stall for one cycle Chapter 4 — The Processor — 119 Load-Use Hazard Detection n Check when using instruction is decoded in ID stage n ALU operand register numbers in ID stage are given by n IF/ID.RegisterRs, IF/ID.RegisterRt n Load-use hazard when n ID/EX.MemRead and ((ID/EX.RegisterRt = IF/ID.RegisterRs) or (ID/EX.RegisterRt = IF/ID.RegisterRt)) n If detected, stall and insert bubble Chapter 4 — The Processor — 120 How to Stall the Pipeline n Force control values in ID/EX register to 0 n EX, MEM and WB do nop (no-operation) n Prevent update of PC and IF/ID register n Using instruction is decoded again n Following instruction is fetched again n 1-cycle stall allows MEM to read data for lw n Can subsequently forward to EX stage Chapter 4 — The Processor — 121 Stall/Bubble in the Pipeline Stall inserted here Chapter 4 — The Processor — 122 Stall/Bubble in the Pipeline Or, more accurately… Chapter 4 — The Processor — 123 Datapath with Hazard Detection Chapter 4 — The Processor — 124 Stalls and Performance The BIG Picture n Stalls reduce performance n But are required to get correct results n Compiler can arrange code to avoid hazards and stalls n Requires knowledge of the pipeline structure Chapter 4 — The Processor — 125 §4.8 Control Hazards Branch Hazards n If branch outcome determined in MEM Flush these instructions (Set control values to 0) PC Chapter 4 — The Processor — 126 Reducing Branch Delay n Move hardware to determine outcome to ID stage n Target address adder n Register comparator n Example: branch taken 36: sub $10, $4, $8 40: beq $1, $3, 7 44: and $12, $2, $5 48: or $13, $2, $6 52: add $14, $4, $2 56: slt $15, $6, $7... 72: lw $4, 50($7) Chapter 4 — The Processor — 127 Example: Branch Taken Chapter 4 — The Processor — 128 Example: Branch Taken Chapter 4 — The Processor — 129 Data Hazards for Branches n If a comparison register is a destination of 2nd or 3rd preceding ALU instruction add $1, $2, $3 IF ID EX MEM WB add $4, $5, $6 IF ID EX MEM WB … IF ID EX MEM WB beq $1, $4, target IF ID EX MEM WB n Can resolve using forwarding Chapter 4 — The Processor — 130 Data Hazards for Branches n If a comparison register is a destination of preceding ALU instruction or 2nd preceding load instruction n Need 1 stall cycle lw $1, addr IF ID EX MEM WB add $4, $5, $6 IF ID EX MEM WB beq stalled IF ID beq $1, $4, target ID EX MEM WB Chapter 4 — The Processor — 131 Data Hazards for Branches n If a comparison register is a destination of immediately preceding load instruction n Need 2 stall cycles lw $1, addr IF ID EX MEM WB beq stalled IF ID beq stalled ID beq $1, $0, target ID EX MEM WB Chapter 4 — The Processor — 132 Dynamic Branch Prediction n In deeper and superscalar pipelines, branch penalty is more significant n Use dynamic prediction n Branch prediction buffer (aka branch history table) n Indexed by recent branch instruction addresses n Stores outcome (taken/not taken) n To execute a branch n Check table, expect the same outcome n Start fetching from fall-through or target n If wrong, flush pipeline and flip prediction Chapter 4 — The Processor — 133 1-Bit Predictor: Shortcoming n Inner loop branches mispredicted twice! outer: … … inner: … … beq …, …, inner … beq …, …, outer n Mispredict as taken on last iteration of inner loop n Then mispredict as not taken on first iteration of inner loop next time around Chapter 4 — The Processor — 134 1-Bit Predictor: Shortcoming n Assume inner loop branch is executed 10 times. outer: … … inner: … … beq …, …, inner … beq …, …, outer n Correctly predict branch is taken 80% of the time. n But, branch is taken 90% of the time! Chapter 4 — The Processor — 135 2-Bit Predictor (improvement) n Only change prediction on two successive mispredictions 4 states: 2 bits Chapter 4 — The Processor — 136 Calculating the Branch Target n Even with predictor, still need to calculate the target address n 1-cycle penalty for a taken branch n Branch target buffer n Cache of target addresses n Indexed by PC when instruction fetched n If hit and instruction is branch predicted taken, can fetch target immediately Chapter 4 — The Processor — 137 §4.9 Exceptions Exceptions and Interrupts n “Unexpected” events requiring change in flow of control n Different ISAs use the terms differently n Exception n Arises within the CPU n e.g., undefined opcode, overflow, syscall, … n Interrupt n From an external I/O controller n Dealing with them without sacrificing performance is hard Chapter 4 — The Processor — 138 Handling Exceptions n In MIPS, exceptions managed by a System Control Coprocessor (CP0) n Save PC of offending (or interrupted) instruction n In MIPS: Exception Program Counter (EPC) n Save indication of the problem n In MIPS: Cause register n We’ll assume 1-bit n 0 for undefined opcode, 1 for overflow n Jump to handler at 8000 00180 Chapter 4 — The Processor — 139 An Alternate Mechanism n Vectored Interrupts n Handler address determined by the cause n Example: n Undefined opcode: C000 0000 n Overflow: C000 0020 n …: C000 0040 n Instructions either n Deal with the interrupt, or n Jump to real handler Chapter 4 — The Processor — 140 Handler Actions n Read cause, and transfer to relevant handler n Determine action required n If restartable n Take corrective action n use EPC to return to program n Otherwise n Terminate program n Report error using EPC, cause, … Chapter 4 — The Processor — 141 Exceptions in a Pipeline n Another form of control hazard n Consider overflow on add in EX stage add $1, $2, $1 n Prevent $1 from being clobbered n Complete previous instructions n Flush add and subsequent instructions n Set Cause and EPC register values n Transfer control to handler n Similar to mispredicted branch n Use much of the same hardware Chapter 4 — The Processor — 142 Pipeline with Exceptions Chapter 4 — The Processor — 143 Pipeline with Exceptions Initial address of exception handler Chapter 4 — The Processor — 144 Exception Properties n Restartable exceptions n Pipeline can flush the instruction n Handler executes, then returns to the instruction n Refetched and executed from scratch n PC saved in EPC register n Identifies causing instruction n Actually PC + 4 is saved n Handler must adjust by subtracting 4 from EPC Chapter 4 — The Processor — 145 Exception Example n Exception on add in 40 sub $11, $2, $4 44 and $12, $2, $5 48 or $13, $2, $6 4C add $1, $2, $1 50 slt $15, $6, $7 54 lw $16, 50($7) … n Handler 80000180 sw $25, 1000($0) 80000184 sw $26, 1004($0) … Chapter 4 — The Processor — 146 Exception Example Chapter 4 — The Processor — 147 Exception Example Chapter 4 — The Processor — 148 Multiple Exceptions n Pipelining overlaps multiple instructions n Could have multiple exceptions at once n Simple approach: deal with exception from earliest instruction n Flush subsequent instructions n “Precise” exceptions n In complex pipelines n Multiple instructions issued per cycle n Out-of-order completion n Maintaining precise exceptions is difficult! Chapter 4 — The Processor — 149 Imprecise Exceptions n Just stop pipeline and save state n Including exception cause(s) n Let the handler work out n Which instruction(s) had exceptions n Which to complete or flush n May require “manual” completion n Simplifies hardware, but more complex handler software n Not feasible for complex multiple-issue out-of-order pipelines Chapter 4 — The Processor — 150 §4.10 Parallelism via Instructions Instruction-Level Parallelism (ILP) n Pipelining: executing multiple instructions in parallel n To increase ILP n Deeper pipeline n Less work per stage ⇒ shorter clock cycle n Multiple issue n Replicate pipeline stages ⇒ multiple pipelines n Start multiple instructions per clock cycle n CPI < 1, so use Instructions Per Cycle (IPC) n E.g., 4GHz 4-way multiple-issue n 16 BIPS, peak CPI = 0.25, peak IPC = 4 n But dependencies reduce this in practice Chapter 4 — The Processor — 151 Multiple Issue n Static multiple issue n Compiler groups instructions to be issued together n Packages them into “issue slots” n Compiler detects and avoids hazards n Dynamic multiple issue n CPU examines instruction stream and chooses instructions to issue each cycle n Compiler can help by reordering instructions n CPU resolves hazards using advanced techniques at runtime Chapter 4 — The Processor — 152 Speculation n “Guess” what to do with an instruction n Start operation as soon as possible n Check whether guess was right n If so, complete the operation n If not, roll-back and do the right thing n Common to static and dynamic multiple issue n Examples n Speculate on branch outcome n Roll back if path taken is different n Speculate on load n Roll back if location is updated Chapter 4 — The Processor — 153 Compiler/Hardware Speculation n Compiler can reorder instructions n e.g., move load before branch n Can include “fix-up” instructions to recover from incorrect guess n Hardware can look ahead for instructions to execute n Buffer results until it determines they are actually needed n Flush buffers on incorrect speculation Chapter 4 — The Processor — 154 Speculation and Exceptions n What if exception occurs on a speculatively executed instruction? n e.g., speculative load before null-pointer check n Static speculation n Can add ISA support for deferring exceptions n Dynamic speculation n Can buffer exceptions until instruction completion (which may not occur) Chapter 4 — The Processor — 155 Static Multiple Issue n Compiler groups instructions into “issue packets” n Group of instructions that can be issued on a single cycle n Determined by pipeline resources required n Think of an issue packet as a very long instruction n Specifies multiple concurrent operations n ⇒ Very Long Instruction Word (VLIW) Chapter 4 — The Processor — 156 Scheduling Static Multiple Issue n Compiler must remove some/all hazards n Reorder instructions into issue packets n No dependencies with a packet n Possibly some dependencies between packets n Varies between ISAs; compiler must know! n Pad with nop if necessary Chapter 4 — The Processor — 157 MIPS with Static Dual Issue n Two-issue packets n One ALU/branch instruction n One load/store instruction n 64-bit aligned n ALU/branch, then load/store n Pad an unused instruction with nop Address Instruction type Pipeline Stages n ALU/branch IF ID EX MEM WB n+4 Load/store IF ID EX MEM WB n+8 ALU/branch IF ID EX MEM WB n + 12 Load/store IF ID EX MEM WB n + 16 ALU/branch IF ID EX MEM WB n + 20 Load/store IF ID EX MEM WB Chapter 4 — The Processor — 158 MIPS with Static Dual Issue Chapter 4 — The Processor — 159 Hazards in the Dual-Issue MIPS n More instructions executing in parallel n EX data hazard n Forwarding avoided stalls with single-issue n Now can’t use ALU result in load/store in same packet n add $t0, $s0, $s1 load $s2, 0($t0) n Split into two packets, effectively a stall n Load-use hazard n Still one cycle use latency, but now two instructions n More aggressive scheduling required Chapter 4 — The Processor — 160 Scheduling Example n Schedule this for dual-issue MIPS Loop: lw $t0, 0($s1) # $t0=array element addu $t0, $t0, $s2 # add scalar in $s2 sw $t0, 0($s1) # store result addi $s1, $s1,–4 # decrement pointer bne $s1, $zero, Loop # branch $s1!=0 Best schedule ALU/branch Load/store cycle Loop: nop lw $t0, 0($s1) 1 addi $s1, $s1,–4 nop 2 addu $t0, $t0, $s2 nop 3 bne $s1, $zero, Loop sw $t0, 4($s1) 4 n IPC = 5/4 = 1.25 (c.f. peak IPC = 2) Chapter 4 — The Processor — 161 Loop Unrolling n Replicate loop body to expose more parallelism n Reduces loop-control overhead n Use different registers per replication n Called “register renaming” n Avoid loop-carried “anti-dependencies” n Store followed by a load of the same register n Aka “name dependence” n Reuse of a register name Chapter 4 — The Processor — 162 Loop Unrolling Example ALU/branch Load/store cycle Loop: addi $s1, $s1,–16 lw $t0, 0($s1) 1 nop lw $t1, 12($s1) 2 addu $t0, $t0, $s2 lw $t2, 8($s1) 3 addu $t1, $t1, $s2 lw $t3, 4($s1) 4 addu $t2, $t2, $s2 sw $t0, 16($s1) 5 addu $t3, $t4, $s2 sw $t1, 12($s1) 6 nop sw $t2, 8($s1) 7 bne $s1, $zero, Loop sw $t3, 4($s1) 8 n IPC = 14/8 = 1.75 n Closer to 2, but at cost of registers and code size Chapter 4 — The Processor — 163 Dynamic Multiple Issue n “Superscalar” processors n CPU decides whether to issue 0, 1, 2, … each cycle n Avoiding structural and data hazards n Avoids the need for compiler scheduling n Though it may still help n Code semantics ensured by the CPU Chapter 4 — The Processor — 164 Dynamic Pipeline Scheduling n Allow the CPU to execute instructions out of order to avoid stalls n But commit result to registers in order n Example lw $t0, 20($s2) addu $t1, $t0, $t2 sub $s4, $s4, $t3 slti $t5, $s4, 20 n Can start sub while addu is waiting for lw Chapter 4 — The Processor — 165 Dynamically Scheduled CPU Preserves dependencies Hold pending operands Results also sent to any waiting reservation stations Reorders buffer for register writes Can supply operands for issued instructions In-order commit Chapter 4 — The Processor — 166 Register Renaming n Reservation stations and reorder buffer effectively provide register renaming n On instruction issue to reservation station n If operand is available in register file or reorder buffer n Copied to reservation station n No longer required in the register; can be overwritten n If operand is not yet available n It will be provided to the reservation station by a function unit n Register update may not be required Chapter 4 — The Processor — 167 Speculation n Predict branch and continue issuing n Don’t commit until branch outcome determined n Load speculation n Avoid load and cache miss delay n Predict the effective address n Predict loaded value n Load before completing outstanding stores n Bypass stored values to load unit n Don’t commit load until speculation cleared Chapter 4 — The Processor — 168 Why Do Dynamic Scheduling? n Why not just let the compiler schedule code? n Not all stalls are predicable n e.g., cache misses n Can’t always schedule around branches n Branch outcome is dynamically determined n Different implementations of an ISA have different latencies and hazards Chapter 4 — The Processor — 169 Does Multiple Issue Work? The BIG Picture n Yes, but not as much as we’d like n Programs have real dependencies that limit ILP n Some dependencies are hard to eliminate n e.g., pointer aliasing n Some parallelism is hard to expose n Limited window size during instruction issue n Memory delays and limited bandwidth n Hard to keep pipelines full n Speculation can help if done well Chapter 4 — The Processor — 170 Power Efficiency n Complexity of dynamic scheduling and speculations requires power n Multiple simpler cores may be better Microprocessor Year Clock Rate Pipeline Issue Out-of-order/ Cores Power Stages width Speculation i486 1989 25MHz 5 1 No 1 5W Pentium 1993 66MHz 5 2 No 1 10W Pentium Pro 1997 200MHz 10 3 Yes 1 29W P4 Willamette 2001 2000MHz 22 3 Yes 1 75W P4 Prescott 2004 3600MHz 31 3 Yes 1 103W Core 2006 2930MHz 14 4 Yes 2 75W UltraSparc III 2003 1950MHz 14 4 No 1 90W UltraSparc T1 2005 1200MHz 6 1 No 8 70W Chapter 4 — The Processor — 171 §4.11 Real Stuff: The ARM Cortex-A8 and Intel Core i7 Pipelines Cortex A8 and Intel i7 Processor ARM A8 Intel Core i7 920 Market Personal Mobile Device Server, cloud Thermal design power 2 Watts 130 Watts Clock rate 1 GHz 2.66 GHz Cores/Chip 1 4 Floating point? No Yes Multiple issue? Dynamic Dynamic Peak instructions/clock cycle 2 4 Pipeline stages 14 14 Pipeline schedule Static in-order Dynamic out-of-order with speculation Branch prediction 2-level 2-level 1st level caches/core 32 KiB I, 32 KiB D 32 KiB I, 32 KiB D 2nd level caches/core 128-1024 KiB 256 KiB 3rd level caches (shared) - 2- 8 MB Chapter 4 — The Processor — 172 ARM Cortex-A8 Pipeline Chapter 4 — The Processor — 173 ARM Cortex-A8 Performance Chapter 4 — The Processor — 174 Core i7 Pipeline Chapter 4 — The Processor — 175 Core i7 Performance SPEC2006 integer benchmark programs Chapter 4 — The Processor — 176 §4.12 Instruction-Level Parallelism and Matrix Multiply Matrix Multiply n Unrolled C code 1 #include 2 #define UNROLL (4) 3 4 void dgemm (int n, double* A, double* B, double* C) 5 { 6 for ( int i = 0; i < n; i+=UNROLL*4 ) 7 for ( int j = 0; j < n; j++ ) { 8 __m256d c; 9 for ( int x = 0; x < UNROLL; x++ ) 10 c[x] = _mm256_load_pd(C+i+x*4+j*n); 11 12 for( int k = 0; k < n; k++ ) 13 { 14 __m256d b = _mm256_broadcast_sd(B+k+j*n); 15 for (int x = 0; x < UNROLL; x++) 16 c[x] = _mm256_add_pd(c[x], 17 _mm256_mul_pd(_mm256_load_pd(A+n*k+x*4+i), b)); 18 } 19 20 for ( int x = 0; x < UNROLL; x++ ) 21 _mm256_store_pd(C+i+x*4+j*n, c[x]); 22 } 23 } Chapter 4 — The Processor — 177 §4.12 Instruction-Level Parallelism and Matrix Multiply Matrix Multiply n Assembly code: 1 vmovapd (%r11),%ymm4 # Load 4 elements of C into %ymm4 2 mov %rbx,%rax # register %rax = %rbx 3 xor %ecx,%ecx # register %ecx = 0 4 vmovapd 0x20(%r11),%ymm3 # Load 4 elements of C into %ymm3 5 vmovapd 0x40(%r11),%ymm2 # Load 4 elements of C into %ymm2 6 vmovapd 0x60(%r11),%ymm1 # Load 4 elements of C into %ymm1 7 vbroadcastsd (%rcx,%r9,1),%ymm0 # Make 4 copies of B element 8 add $0x8,%rcx # register %rcx = %rcx + 8 9 vmulpd (%rax),%ymm0,%ymm5 # Parallel mul %ymm1,4 A elements 10 vaddpd %ymm5,%ymm4,%ymm4 # Parallel add %ymm5, %ymm4 11 vmulpd 0x20(%rax),%ymm0,%ymm5 # Parallel mul %ymm1,4 A elements 12 vaddpd %ymm5,%ymm3,%ymm3 # Parallel add %ymm5, %ymm3 13 vmulpd 0x40(%rax),%ymm0,%ymm5 # Parallel mul %ymm1,4 A elements 14 vmulpd 0x60(%rax),%ymm0,%ymm0 # Parallel mul %ymm1,4 A elements 15 add %r8,%rax # register %rax = %rax + %r8 16 cmp %r10,%rcx # compare %r8 to %rax 17 vaddpd %ymm5,%ymm2,%ymm2 # Parallel add %ymm5, %ymm2 18 vaddpd %ymm0,%ymm1,%ymm1 # Parallel add %ymm0, %ymm1 19 jne 68 # jump if not %r8 != %rax 20 add $0x1,%esi # register % esi = % esi + 1 21 vmovapd %ymm4,(%r11) # Store %ymm4 into 4 C elements 22 vmovapd %ymm3,0x20(%r11) # Store %ymm3 into 4 C elements 23 vmovapd %ymm2,0x40(%r11) # Store %ymm2 into 4 C elements 24 vmovapd %ymm1,0x60(%r11) # Store %ymm1 into 4 C elements Chapter 4 — The Processor — 178 Performance Impact Chapter 4 — The Processor — 179 §4.14 Fallacies and Pitfalls Fallacies n Pipelining is easy (!) n The basic idea is easy n The devil is in the details n e.g., detecting data hazards n Pipelining is independent of technology n So why haven’t we always done pipelining? n More transistors make more advanced techniques feasible n Pipeline-related ISA design needs to take account of technology trends n e.g., predicted instructions Chapter 4 — The Processor — 180 Pitfalls n Poor ISA design can make pipelining harder n e.g., complex instruction sets (VAX, IA-32) n Significant overhead to make pipelining work n IA-32 micro-op approach n e.g., complex addressing modes n Register update side effects, memory indirection n e.g., delayed branches n Advanced pipelines have long delay slots Chapter 4 — The Processor — 181 §4.14 Concluding Remarks Concluding Remarks n ISA influences design of datapath and control n Datapath and control influence design of ISA n Pipelining improves instruction throughput using parallelism n More instructions completed per second n Latency for each instruction not reduced n Hazards: structural, data, control n Multiple issue and dynamic scheduling (ILP) n Dependencies limit achievable parallelism n Complexity leads to the power wall Chapter 4 — The Processor — 182
