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Pipelinig hazardous
- 2. Pipeline Hazards • Hazards are situations in pipelining where one instruction cannot immediately follow another. • Hazards reduce the ideal speedup gained from pipelining and are classified into three classes: – Structural hazards: Arise from hardware resource conflicts when the available hardware cannot support all possible combinations of instructions. – Data hazards: Arise when an instruction depends on the results of a previous instruction in a way that is exposed by the overlapping of instructions in the pipeline – Control hazards: Arise from the pipelining of conditional branches and other instructions that change the PC • Can always resolve hazards by waiting 2
- 3. Structural Hazards • In pipelined processors, overlapped instruction execution requires pipelining of functional units and duplication of resources to allow all possible combinations of instructions in the pipeline. • If a resource conflict arises due to a hardware resource being required by more than one instruction in a single cycle, and one or more such instructions cannot be accommodated, then a structural hazard has occurred, for example: – when a machine has only one register file write port – or when a pipelined machine has a shared single-memory pipeline for data and instructions. 3
- 4. Register File/Structural Hazards Operation on register set by 2 different instructions in the same clock cycle Time (clock cycles) Instr 4 Ifetch DMem Reg Ifetch ALU Reg Reg Reg Ifetch Reg DMem Reg DMem Reg ALU Instr 3 Ifetch DMem Cycle 6 Cycle 7 ALU O r d e r Instr 2 Reg ALU I Load Ifetch n s t Instr 1 r. ALU Cycle 1 Cycle 2 Cycle 3 Cycle 4 Cycle 5 Reg Reg DMem 4
- 5. Register File/Structural Hazards We need 3 stall cycles In order to solve this hazard Time (clock cycles) Instr 3 Instr 4 Reg Ifetch Reg DMem Reg ALU Ifetch DMem Cycle 6 Cycle 7 Reg DMem 3 stalls cycles Reg Ifetch Reg ALU O r d e r Instr 2 Reg ALU I Load Ifetch n s t Instr 1 r. ALU Cycle 1 Cycle 2 Cycle 3 Cycle 4 Cycle 5 Ifetch 5
- 6. Register File/Structural Hazards Allow writing registers in first ½ of cycle and reading in 2nd ½ of cycle Instr 3 Instr 4 Ifetch DMem Reg Ifetch ALU Reg No stalls are required Reg Ifetch Reg DMem Reg DMem Reg ALU O r d e r Instr 2 Ifetch DMem Cycle 6 Cycle 7 Reg ALU I Load Ifetch n s t Instr 1 r. Reg ALU Cycle 1 Cycle 2 Cycle 3 Cycle 4 Cycle 5 ALU Time (clock cycles) Reg Reg DMem 6
- 7. 1 Memory Port/Structural Hazards Instruction4 Mem Reg Mem Reg Mem Reg Mem Reg Mem Reg Mem Reg ALU Instruction3 Reg Operation on Memory by 2 different instructions in the same clock cycle ALU Instruction2 Mem Mem ALU Instruction1 Reg ALU Load Mem ALU Instruction Order Time (in Cycles) Reg Mem Reg 7
- 8. Inserting Bubbles (Stalls) Stall Stall Stall Instruction3 Reg Mem Reg Reg Mem 3 stall cycles with 1-port memory Reg Mem Reg Bubble Bubble Bubble Bubble Bubble Bubble Bubble Bubble Bubble Bubble Bubble Bubble Bubble Bubble Bubb Mem Reg ALU Instruction2 Mem Mem ALU Instruction1 Reg ALU Load Mem ALU Time (in Cycles) 8 Mem
- 9. 2 Memory Port/Structural Hazards (Read & Write at the same time) Instruction4 Mem Reg Mem Mem Reg Mem Reg Mem Reg Mem Reg ALU Instruction3 Reg No stall with 2-memory ports Reg ALU Instruction2 Mem Mem ALU Instruction1 Reg ALU Load Mem ALU Instruction Order Time (in Cycles) Reg Mem Reg 9
- 10. Pipeline Hazards • Hazards reduce the ideal speedup gained from pipelining and are classified into three classes: – Structural hazards: Arise from hardware resource conflicts when the available hardware cannot support all possible combinations of instructions. – Data hazards: Arise when an instruction depends on the results of a previous instruction in a way that is exposed by the overlapping of instructions in the pipeline – Control hazards: Arise from the pipelining of conditional branches and other instructions that change the PC • Can always resolve hazards by waiting 10
- 11. Data Hazard on R1 Time (clock cycles) and r6,r1,r7 or r8,r1,r9 xor r10,r1,r11 Ifetch DMem Reg DMem Ifetch Reg DMem Reg DMem Reg ALU sub r4,r1,r3 Reg ALU Ifetch ALU O r d e r add r1,r2,r3 WB ALU I n s t r. MEM ALU IF ID/RF EX Ifetch Reg Ifetch Reg Reg Reg DMem 11 Reg
- 12. Data Hazard Classification • • • • Given two instructions I, J, with I occurring before J in an instruction stream: RAW (read after write): A true data dependence J tried to read a source before I writes to it, so J incorrectly gets the old value. WAW (write after write): A name dependence J tries to write an operand before it is written by I The writes end up being performed in the wrong order. WAR (write after read): A name dependence J tries to write to a destination before it is read by I, so I incorrectly gets the new value. RAR (read after read): Not a hazard. 12
- 13. I (Write) Data Hazard Classification I (Read) Shared Operand J (Read) Shared Operand J (Write) Read after Write (RAW) Write after Read (WAR) I (Write) I (Read) Shared Operand Shared Operand J (Write) Write after Write (WAW) J (Read) Read after Read (RAR) not a hazard 13
- 14. Data Hazards Present in Current MIPS Pipeline • Read after Write (RAW) Hazards: Possible? – Caused by a “Dependence” (in compiler nomenclature). This hazard results from an actual need for communication. – Yes possible, when an instruction requires an operand generated by a preceding instruction with distance less than four. I: add r1,r2,r3 J: sub r4,r1,r3 Inst i Inst j 1 2 1 3 4 2 3 Read read the old data. Write 5 4 5 – Resolved by: • Forwarding or Stalling. 14
- 15. Data Hazards Present in Current MIPS Pipeline – Write After Read (WAR) – not possible Error if InstrJ tries to write operand before InstrI reads it I: sub r4,r1,r3 J: add r1,r2,r3 K: mul r6,r1,r7 Inst i Inst j 1 Read 2 1 Always read the correct data. 3 4 5 2 3 4 Write 5 – Called an “anti-dependence” by compiler writers. This results from reuse of the name “r1”. 15
- 16. Data Hazards Present in Current MIPS Pipeline – Write After Write (WAW) - not possible – Error if InstrJ tries to write operand before InstrI writes it. I: sub r1,r4,r3 J: add r1,r2,r3 K: mul r6,r1,r7 Inst i Inst j 1 2 3 4 Write 5 1 2 3 4 5 Write – Called an “output dependence” by compiler writers This also results from the reuse of name “r1”. 16
- 17. Data Hazards • Solutions for Data Hazards – Stalling – Forwarding: • connect new value directly to next stage – Reordering 17
- 18. Minimizing Data Hazard Stalls by Forwarding • Forwarding is a hardware-based technique (also called register bypassing or short-circuiting) used to eliminate or minimize data hazard stalls. • Using forwarding hardware, the result of an instruction is copied directly from where it is produced (ALU, memory read port etc.), to where subsequent instructions need it (ALU, input register, memory write port etc.) Zero? Data Memory M/W Buffer MUX A/M Buffer MUX D/A Buffer ALU 18
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