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Lecture 18: Core Design Today: basics of implementing a correct ooo core: register renaming, commit, LSQ, issue queue
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The Alpha 21264 Out-of-Order Implementation
Reorder Buffer (ROB) Branch prediction and instr fetch Instr 1 Instr 2 Instr 3 Instr 4 Instr 5 Instr 6 Committed Reg Map R1P1 R2P2 Register File P1-P64 R1 R1+R2 R2 R1+R3 BEQZ R2 R3 R1+R2 R1 R3+R2 Decode & Rename P33 P1+P2 P34 P33+P3 BEQZ P34 P35 P33+P34 P36 P35+P34 ALU ALU ALU Speculative Reg Map R1P36 R2P34 Instr Fetch Queue Results written to regfile and tags broadcast to IQ Issue Queue (IQ)
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Rename A lr1 lr2 + lr3 pr7 pr2 + pr3 B lr2 lr4 + lr5
C lr6 lr1 + lr3 D lr6 lr1 + lr2 RAR lr3 RAW lr1 WAR lr2 WAW lr6 A ; BC ; D pr7 pr2 + pr3 pr8 pr4 + pr5 pr9 pr7 + pr3 pr10 pr7 + pr8 RAR pr3 RAW pr7 WAR x WAW x AB ; CD
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Commit Example Map Old / New lr1 pr1 pr7 lr2 pr2 pr8 lr6 pr6 pr9
Assume a processor with 6 logical regs and 10 physical regs Map Old / New lr1 pr1 pr7 lr2 pr2 pr8 lr6 pr6 pr9 lr6 pr9 pr10 lr3 pr3 pr1 lr4 pr4 pr2 A lr1 lr2 + lr3 B lr2 lr4 + lr5 C lr6 lr1 + lr3 D lr6 lr1 + lr2 E lr3 lr6 + lr2 F lr4 lr3 + lr4 pr7 pr2 + pr3 pr8 pr4 + pr5 pr9 pr7 + pr3 pr10 pr7 + pr8 pr1 pr10 + pr8 pr2 pr1 + pr4
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Out-of-Order Loads/Stores
Ld R1 [R2] Ld R3 [R4] St R5 [R6] Ld R7 [R8] Ld R9[R10]
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Memory Dependence Checking
Ld 0x abcdef The issue queue checks for register dependences and executes instructions as soon as registers are ready Loads/stores access memory as well – must check for RAW, WAW, and WAR hazards for memory as well Hence, first check for register dependences to compute effective addresses; then check for memory dependences Ld St Ld Ld 0x abcdef St 0x abcd00 Ld 0x abc000 Ld 0x abcd00
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Memory Dependence Checking
Load and store addresses are maintained in program order in the Load/Store Queue (LSQ) Loads can issue if they are guaranteed to not have true dependences with earlier stores Stores can issue only if we are ready to modify memory (can not recover if an earlier instr raises an exception) Ld 0x abcdef Ld St Ld Ld 0x abcdef St 0x abcd00 Ld 0x abc000 Ld 0x abcd00
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The Alpha 21264 Out-of-Order Implementation
Reorder Buffer (ROB) Branch prediction and instr fetch Instr 1 Instr 2 Instr 3 Instr 4 Instr 5 Instr 6 Instr 7 Committed Reg Map R1P1 R2P2 Register File P1-P64 R1 R1+R2 R2 R1+R3 BEQZ R2 R3 R1+R2 R1 R3+R2 LD R4 8[R3] ST R4 8[R1] Decode & Rename P33 P1+P2 P34 P33+P3 BEQZ P34 P35 P33+P34 P36 P35+P34 P37 8[P35] P37 8[P36] ALU ALU ALU Speculative Reg Map R1P36 R2P34 Results written to regfile and tags broadcast to IQ Instr Fetch Queue Issue Queue (IQ) ALU P37 [P35 + 8] P37 [P36 + 8] D-Cache LSQ
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Speculative Issue Instr I1 leaves the issue queue at start of cycle 6; the instr then reads operands from the regfile, wires are traversed, instruction executes, result is available at end of cycle 8 If operand availability is broadcast to issue queue in cycle 9, dependent instruction leaves in cycle 10 This causes a 4-cycle gap between successive instrs Hence, if we know that the instruction takes a cycle to execute, the operand is broadcast to the issue queue in cycle 6 and the dependent instr leaves issue queue in cycle 7; the input operand is correctly bypassed at the FU
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Load Hit Speculation The previous optimization assumes that we know the exact latency for every operation This is true for all ops except loads (cache hit or miss?) Assume hit and schedule accordingly; on a cache miss, must squash all speculatively issued instructions; an instruction therefore sits in the queue until load hits are determined
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Register Rename Logic Map Table Physical Source Regs Physical Dest
Logical Source Regs Mux Free Pool Logical Dest Regs Dependence Check Logic Logical Source Reg
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Shadow copies (shift register)
Map Table – RAM 7-bits 7-bits 7-bits 7-bits 7-bits Phys reg id Num entries = Num logical regs Shadow copies (shift register)
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Map Table – CAM 5-bits 1-bit 1-bit Logical reg id v a l i d
Num entries = Num phys regs Shadow copies
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Wakeup Logic . . … = = tag1 tagIW or or rdyL tagL tagR rdyR rdyL tagL
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Selection Logic For multiple FUs, will need sequential selectors
Issue window req grant enable anyreq Arbiter cell enable For multiple FUs, will need sequential selectors
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Structure Complexities
Critical structures: register map tables, issue queue, LSQ, register file, register bypass Cycle time is heavily influenced by: window size (physical register size), issue width (#FUs) Conflict between the desire to increase IPC and clock speed
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Title Bullet
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