Lecture 11: Out-of-order Processors. Topics: more ooo design details, timing, load-store queue
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1 Lecture 11: Out-of-order Processors Topics: more ooo design details, timing, load-store queue 1
2 Problem 0 Show the renamed version of the following code: Assume that you have 36 physical registers and 32 architected registers. When does each instr leave the IQ? R1 R2+R3 R1 R1+R5 BEQZ R1 R1 R4 + R5 R4 R1 + R7 R1 R6 + R8 R4 R3 + R1 R1 R5 + R9 2
3 Problem 0 Show the renamed version of the following code: Assume that you have 36 physical registers and 32 architected registers. When does each instr leave the IQ? R1 R2+R3 P33 P2+P3 cycle i R1 R1+R5 P34 P33+P5 i+1 BEQZ R1 BEQZ P34 i+2 R1 R4 + R5 P35 P4+P5 i R4 R1 + R7 P36 P35+P7 i+1 R1 R6 + R8 P1 P6+P8 j R4 R3 + R1 P33 P3+P1 j+1 R1 R5 + R9 P34 P5+P9 j+2 Width is assumed to be 4. j depends on the #stages between issue and commit. 3
4 OOO Example IQ Assume there are 36 physical registers and 32 logical registers, and width is 4 Estimate the issue time, completion time, and commit time for the sample code 4
5 Assumptions IQ Perfect branch prediction, instruction fetch, caches ADD dep has no stall; LD dep has one stall An instr is placed in the IQ at the end of its 5 th stage, an instr takes 5 more stages after leaving the IQ (ld/st instrs take 6 more stages after leaving the IQ) 5
6 OOO Example IQ Original code ADD R1, R2, R3 LD R2, 8(R1) ADD R2, R2, 8 ST R1, (R3) SUB R1, R1, R5 LD R1, 8(R2) ADD R1, R1, R2 Renamed code 6
7 OOO Example IQ Original code Renamed code ADD R1, R2, R3 ADD P33, P2, P3 LD R2, 8(R1) LD P34, 8(P33) ADD R2, R2, 8 ADD P35, P34, 8 ST R1, (R3) ST P33, (P3) SUB R1, R1, R5 SUB P36, P33, P5 LD R1, 8(R2) Must wait ADD R1, R1, R2 7
8 OOO Example IQ Original code Renamed code InQ Iss Comp Comm ADD R1, R2, R3 ADD P33, P2, P3 LD R2, 8(R1) LD P34, 8(P33) ADD R2, R2, 8 ADD P35, P34, 8 ST R1, (R3) ST P33, (P3) SUB R1, R1, R5 SUB P36, P33, P5 LD R1, 8(R2) ADD R1, R1, R2 8
9 OOO Example IQ Original code Renamed code InQ Iss Comp Comm ADD R1, R2, R3 ADD P33, P2, P3 i i+1 i+6 i+6 LD R2, 8(R1) LD P34, 8(P33) i i+2 i+8 i+8 ADD R2, R2, 8 ADD P35, P34, 8 i i+4 i+9 i+9 ST R1, (R3) ST P33, (P3) i i+2 i+8 i+9 SUB R1, R1, R5 SUB P36, P33, P5 i+1 i+2 i+7 i+9 LD R1, 8(R2) ADD R1, R1, R2 9
10 OOO Example IQ Original code Renamed code InQ Iss Comp Comm ADD R1, R2, R3 ADD P33, P2, P3 i i+1 i+6 i+6 LD R2, 8(R1) LD P34, 8(P33) i i+2 i+8 i+8 ADD R2, R2, 8 ADD P35, P34, 8 i i+4 i+9 i+9 ST R1, (R3) ST P33, (P3) i i+2 i+8 i+9 SUB R1, R1, R5 SUB P36, P33, P5 i+1 i+2 i+7 i+9 LD R1, 8(R2) LD P1, 8(P35) i+7 i+8 i+14 i+14 ADD R1, R1, R2 ADD P2, P1, P35 i+9 i+10 i+15 i+15 10
11 The Alpha Out-of-Order Implementation Reorder Buffer (ROB) Branch prediction and instr fetch R1 R1+R2 R2 R1+R3 BEQZ R2 R3 R1+R2 R1 R3+R2 Instr Fetch Queue Decode & Rename Speculative Reg Map R1 P36 R2 P34 Instr 1 Instr 2 Instr 3 Instr 4 Instr 5 Instr 6 Committed Reg Map R1 P1 R2 P2 P33 P1+P2 P34 P33+P3 BEQZ P34 P35 P33+P34 P36 P35+P34 Register File P1-P64 ALU ALU ALU Results written to regfile and tags broadcast to IQ Issue Queue (IQ) 11
12 Additional Details When does the decode stage stall? When we either run out of registers, or ROB entries, or issue queue entries Issue width: the number of instructions handled by each stage in a cycle. High issue width high peak ILP Window size: the number of in-flight instructions in the pipeline. Large window size high ILP No more WAR and WAW hazards because of rename registers must only worry about RAW hazards 12
13 Branch Mispredict Recovery On a branch mispredict, must roll back the processor state: throw away IFQ contents, ROB/IQ contents after branch Committed map table is correct and need not be fixed The speculative map table needs to go back to an earlier state To facilitate this spec-map-table rollback, it is checkpointed at every branch 13
14 Waking Up a Dependent In an in-order pipeline, an instruction leaves the decode stage when it is known that the inputs can be correctly received, not when the inputs are computed Similarly, an instruction leaves the issue queue before its inputs are known, i.e., wakeup is speculative based on the expected latency of the producer instruction 14
15 Out-of-Order Loads/Stores St R1 [R2] R3 [R4] R5 [R6] R7 [R8] R9 [R10] What if the issue queue also had load/store instructions? Can we continue executing instructions out-of-order? 15
16 Memory Dependence Checking St St 0x abcdef 0x abcdef 0x abcd00 0x abc000 0x abcd00 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 16
17 Memory Dependence Checking St St 0x abcdef 0x abcdef 0x abcd00 0x abc000 0x abcd00 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) happens at commit 17
18 The Alpha Out-of-Order Implementation Branch prediction and instr fetch R1 R1+R2 R2 R1+R3 BEQZ R2 R3 R1+R2 R1 R3+R2 LD R4 8[R3] ST R4 8[R1] Instr Fetch Queue Decode & Rename Speculative Reg Map R1 P36 R2 P34 Reorder Buffer (ROB) Instr 1 Committed Instr 2 Reg Map Instr 3 R1 P1 Instr 4 R2 P2 Instr 5 Instr 6 Instr 7 P33 P1+P2 P34 P33+P3 BEQZ P34 P35 P33+P34 P36 P35+P34 P37 8[P35] P37 8[P36] Issue Queue (IQ) P37 [P35 + 8] P37 [P36 + 8] LSQ Register File P1-P64 ALU ALU ALU Results written to regfile and tags broadcast to IQ ALU D-Cache 18
19 Problem 1 Consider the following LSQ and when operands are available. Estimate when the address calculation and memory accesses happen for each ld/st. Assume no memory dependence prediction. Ad. Op St. Op Ad.Val Ad.Cal LD R1 [R2] 3 abcd LD R3 [R4] 6 adde ST R5 [R6] 4 7 abba LD R7 [R8] 2 abce ST R9 [R10] 8 3 abba LD R11 [R12] 1 abba Mem.Acc 19
20 Problem 1 Consider the following LSQ and when operands are available. Estimate when the address calculation and memory accesses happen for each ld/st. Assume no memory dependence prediction. Ad. Op St. Op Ad.Val Ad.Cal Mem.Acc LD R1 [R2] 3 abcd 4 5 LD R3 [R4] 6 adde 7 8 ST R5 [R6] 4 7 abba 5 commit LD R7 [R8] 2 abce 3 6 ST R9 [R10] 8 3 abba 9 commit LD R11 [R12] 1 abba
21 Problem 2 Consider the following LSQ and when operands are available. Estimate when the address calculation and memory accesses happen for each ld/st. Assume no memory dependence prediction. Ad. Op St. Op Ad.Val Ad.Cal LD R1 [R2] 3 abcd LD R3 [R4] 6 adde ST R5 [R6] 5 7 abba LD R7 [R8] 2 abce ST R9 [R10] 1 4 abba LD R11 [R12] 2 abba Mem.Acc 21
22 Problem 2 Consider the following LSQ and when operands are available. Estimate when the address calculation and memory accesses happen for each ld/st. Assume no memory dependence prediction. Ad. Op St. Op Ad.Val Ad.Cal Mem.Acc LD R1 [R2] 3 abcd 4 5 LD R3 [R4] 6 adde 7 8 ST R5 [R6] 5 7 abba 6 commit LD R7 [R8] 2 abce 3 7 ST R9 [R10] 1 4 abba 2 commit LD R11 [R12] 2 abba
23 Problem 3 Consider the following LSQ and when operands are available. Estimate when the address calculation and memory accesses happen for each ld/st. Assume memory dependence prediction. Ad. Op St. Op Ad.Val Ad.Cal LD R1 [R2] 3 abcd LD R3 [R4] 6 adde ST R5 [R6] 4 7 abba LD R7 [R8] 2 abce ST R9 [R10] 8 3 abba LD R11 [R12] 1 abba Mem.Acc 23
24 Problem 3 Consider the following LSQ and when operands are available. Estimate when the address calculation and memory accesses happen for each ld/st. Assume memory dependence prediction. Ad. Op St. Op Ad.Val Ad.Cal Mem.Acc LD R1 [R2] 3 abcd 4 5 LD R3 [R4] 6 adde 7 8 ST R5 [R6] 4 7 abba 5 commit LD R7 [R8] 2 abce 3 4 ST R9 [R10] 8 3 abba 9 commit LD R11 [R12] 1 abba 2 3/10 24
25 Title Bullet 25
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