Instruction Level Parallelism 2. Superscalar and VLIW processors

Superscalar and VLIW Processors Scalar processors fetch and issue max 1 operation in each clock cycle. Multiple-issue processors: Superscalar (issue a varying number of instructions at each clock cycle). VLIW (issue a fixed number of instructions at each clock cycle). Vittorio Zaccaria – ST 2001

Superscalar Processors Issues from 1 to 8 instructions at each clock cycle. If instructions are dependent, only the instructions preceding that one are issued (in-order issue). This decision is made at run-time by the processor. => Variability in the issue rate.

Superscalar Processors Can be: Statically scheduled: Do not allow (issue) instructions behind stalls to proceed or Dynamically scheduled and speculative (allow instructions behind RAW hazards to proceed).

How to optimize code for Superscalar Processors (1) Loop:LDF0,0(R1) ADDDF4,F0,F2 SD0(R1),F4 SUBIR1,R1,#8 BNEZR1,LOOP The loop is unrolled 4 times (load/addd/store) in which RAW hazards have been reduced, but there are resource conflicts on the pipelines... Loop:LDF0,0(R1) LDF6,-8(R1) LDF10,-16(R1) LDF14,-24(R1) ADDDF4,F0,F2 ADDDF8,F6,F2 ADDDF12,F10,F2 ADDDF16,F14,F2 SD0(R1),F4 SD-8(R1),F8 SD-16(R1),F12 SUBIR1,R1,#32 BNEZR1,LOOP SD (R1),F16 ; 8-32 = -24

How to optimize code for Superscalar processors (2) Integer instructionFP instruction Loop:LD F0,0(R1)// LD F6,-8(R1)// LD F10,-16(R1)ADDD F4,F0,F2 LD F14,-24(R1)ADDD F8,F6,F2 LD F18,-32(R1)ADDD F12,F10,F2 SD 0(R1),F4ADDD F16,F14,F2 SD -8(R1),F8ADDD F20,F18,F2 SD -16(R1),F12// SD -24(R1),F16// SUBI R1,R1,#40// BNEZ R1,LOOP// SD -32(R1),F20// 5 times unrolled loop.

The PowerPC 620 [’94] Superscalar Architecture Similar to: MIPS R10000 HP PA 8000 Fetch, issue and completion of up to 4 instructions per clock cycle. Six separate execution units buffered with reservation stations.

PowerPC functional units 2 integer units (XSU0, XSU1), 0 cycles latency [+,-,shift..] 1 complex integer function unit MCFXU for integer (pipelined *, unpipelined /). Latency from 3 to 20 cycles). 1 Load store unit. Latency=1 for integer loads, 2 for FP loads.

PowerPC functional units 1 FPU with latencies of: 2 cycles for multiply,add, multiply-add 31 for DP FP divide. (fully pipelined except for divide). 1 BRU, completes branches and informs the fetch unit of mispredictions. Includes the condition register used for conditional branches.

PowerPC Architecture Speculative Tomasulo with register renaming. Extendend register file holds speculative result of an instruction until the instruction commits. The ROB enforces only in-order commit. Advantages: operands are available from a single location (no need for additional complex logic to access ROB result values)

PowerPC 620 architecture

PowerPC Pipeline Fetch: The Fetch unit loads the decode queue with instructions from the cache. Next address is predicted through a 256-entry, two-way set associative BTB. A BPB is used if there is a miss in the BTB.

PowerPC Pipeline Instruction decode: Instructions are decoded and inserted into an 8-entry instruction queue. Instruction Issue: 4 Instructions are taken from the 8-entry instruction queue and are issued to the RS. Allocate a rename register and a reorder buffer entry for the instruction issued. If we can’t, stall.

PowerPC Pipeline Execution: Proceeds with execution when all operands are available. At the end, the result is written on the result bus. The completion unit is notified that the instruction has completed.

PowerPC Pipeline If the instruction is a (mispredicted) branch, IFU and IC(ompletion)U are notified. Instruction fetch restarts, and ICU discards all the speculated instructions after the branch and free the rename buffers. Commit: When all previous instructions have been committed, commit the result into the RF and free the rename buffer. Stores also commit from store buffer to memory.

Performance results IPC from under 1 to 1.8. We do not reach IPC=4 due to: Fus are not replicated for each instruction (structural hazards) Limited instruction level parallelism or limited buffering (insufficient buffers).

P6 Processor Family: Intel Pentium II/III 3-way superscalar. Basic Idea, three engines:

P6 Pipeline Fetch/Decode Unit: decodes instructions and puts them in the instruction pool in-order. converts the instructions in micro-ops that represent instruction code. Dispatch/Execute Unit: out-of-order issue from the instruction pool in a reservation station and out-of-order execution of micro-ops. Retire Unit Reorders the instructions and commits speculative results to the architectural state.

P6 Instruction Decode The decoder fetches 16 bytes at each clock cycle from the cache 3 parallel decoders convert most of the instructions into one or more triadic micro-ops. Some instruction need microcode (several micro-ops) to be executed. Register Alias Table unit converts logical reg. ref. into physical reg. ref. In the ROB (register renaming)

P6 Instruction Dispatch/Execute The dispatch unit dispatches out-of-order the microops in the instruction pool through the reservation station unit This happens when: All the operands are ready The resource needed is ready. Maximum throughput: 5 micro-ops/cycle. If micro-ops are branches, their execution is compared with the predicted address (in the Fetch phase). If mispredicted the JEU changes the status of all the micro-ops behind the branch and removes them from the instruction pool.

P6 Instruction Retire The retire unit looks for micro-ops that have been executed and can be removed from the pool. The original architectural target of the micro-ops is written. This is done in-order by committing an instruction only if: Previous instructions have been committed The instruction has been executed. Up to 3 micro-ops can be retired at each clock cycle.

Pentium 4

New NetBurst micro-architecture 20 pipeline stages (hyper-pipeline) 1.4 GHz to 2GHz 3 prefetching mechanisms Harware instruction prefetcher (based on BTB). Software controlled data cache prefetching. L3->L2 data and instruction hardware prefetcher

Pentium 4 Execution Trace Cache TC stores decoded IA-32 instructions or micro-ops. Removes decoding costs 12K micro-ops, 3 micro-ops per cycle fetch bandwidth It stores traces built across predicted branches. However some instructions need micro-code from ROM.

Pentium 4 Branch penalty delay can be much more than 10 cycles Uses BTB In case of a miss in the BTB, static prediction is used (back=T, forw=NT) Use of software branch hints during the trace construction that override static prediction.

Pentium 4 Execution Units and Issue Ports

Pentium 4 1 load and 1 store issue for each cycle. Loads can be reordered w.r.t. other loads and stores Loads can be executed speculatively Up to 4 outstanding load misses. Load/store forwarding

AMD Athlon K7 Nine-issue (micro-ops), super-pipelined, superscalar x86 processor Multiple x86 instruction decoders (into triadic micro- ops) Three out-of-order, superscalar, fully pipelined floating point execution units. Three out-of-order, superscalar, pipelined integer units. Three out-of-order, superscalar, pipelined address calculation units. 72-entry instruction control unit (ROB)

AMD Athlon K7

The Instruction Control Unit contains a reorder buffer and distributed reservation stations to hold operands while OP’s wait to be scheduled. The Integer Instruction Scheduler is an instruction scheduling logic that picks OP’s for execution based on their operand availability and issues them to functional units or address generation units. The function units perform transformations on data and return their results to the reorder buffer, while the address-generation units send calculated memory addresses to the Load/Store Unit for further processing.

Clustered VLIW

Multi-Ported Register File Limits Area of the register file grows approximately with the square of the number of ports

Multiported Register File Read Access time of a register file grows approximately linearly with the number of ports Internal Bit Cell loading becomes larger Larger area of register file causes longer wire delays What is reasonable today in terms of number of ports? Changes with technology, ports is currently about the maximum (read ports + write ports)

Clustered VLIW To solve the bottleneck, create partitioned register files connected to small numbers of Executions Units

Register File Communication Architecturally Invisible Partitioned RFs appear as one large register file to the compiler Copying between RFs is done by control Detection of when copying is needed can be complicated; goes against VLIW philosophy of minimal control overhead

Register File Communication Architecturally Visible Remote and Local versions of instructions Explicit copy primitives Remote Instructions: have one or more operands in non-local RF Copying of remote operands to local RFs takes clock cycles. Because copying is ‘atomic’ part of remote instruction, execution unit is idle while copying is done => performance

Register File Communication Copy instructions: Separation of copy and execution allows more flexible scheduling by compiler move r1, r60 //(r60 in another RF) independent instr a //do not waste useful independent instr b //clock cycles add r2, r1, r3

Instruction Comression Embedded Processors often put a limit on code size How to reduce size? NOPs are common, use only a few bits (2-3) to represent a NOP. Mark explicitly start and stop of the long instruction and do not insert nop.

Instructions Decompression On Instruction Cache fill ICache has to hold uncompressed instructions - limits cache size On instruction fetch Decompression in critical path of fetch stage, may have to add one or more pipeline stages just for decompression

VLIW Architectures: Some real world example

TMS320C6X CPU 8 independent execution units Execution unit types: L : Integer adder, Logical, Bit Counting, FP adder, FP conversion S : Integer adder, Logical, Bit Manipulation, Shifting, Constant, Branch/Control, FP compare D : Integer adder, Load-Store M : Integer Multiplier, FP multiplier Split into two identical datapaths, each contains the same four units (L, S, D, M)

TMS320C6X CPU (cont). Max clock speed of 200 Mhz Each datapath has a 16 x 32bit Register file

Instruction Encoding Internal Execution path is 256 bits-wide Each operation is 32 bits wide => 8 operations per clock A fetch packet is a group of instructions fetched simultaneously. Fetch packet has 8 instructions. A execute packet is a group of instructions beginning execution in parallel. Execute packet has 8 instructions

Instruction Encoding Instructions in ICache have an associated P-bit (Parallel-bit). Fetch packet expanded to 1 to 8 Execute packets during fetch stage depending on P-bits

Fetch Packet to Execute Packet Expansion

Philips TM 1000/Multimedia Processor

Philips Trimedia Five Execution Units => Five operations per clock issued 15 Read and 5 Write Ports on register File Need 15 read ports for 5 Execution Units because each operation requires two operands and a Guard operand. Guard operand makes each operation conditional based upon value of LSB of the guard operand => Predicated Execution. 128 Registers (r0, r1 always 0)

Philips Trimedia Instructions Multiple operation sizes 2 bits for NOP, 26 bits, 34 bits, and 44 bits.