Computer Architecture Vector Computer

2 Supercomputers Definition of a supercomputer: • Fastest machine in world at given task • A device to turn a compute-bound problem into an I/O bound problem • Any machine costing $30M+ • Any machine designed by Seymour Cray CDC6600 (Cray, 1964) regarded as first supercomputer

3/10/2009 3 CDC 6600 Seymour Cray, 1963 • A fast pipelined machine with 60-bit words – 128 Kword main memory capacity, 32 banks • Ten functional units (parallel, unpipelined) – Floating Point: adder, 2 multipliers, divider – Integer: adder, 2 incrementers, ... • Hardwired control (no microcoding) • Scoreboard for dynamic scheduling of instructions • Ten Peripheral Processors for Input/Output – a fast multi-threaded 12-bit integer ALU • Very fast clock, 10 MHz (FP add in 4 clocks) • >400,000 transistors, 750 sq. ft., 5 tons, 150 kW, novel freon-based technology for cooling • Fastest machine in world for 5 years (until 7600) – over 100 sold ($7-10M each)

4 IBM Memo on CDC6600 Thomas Watson Jr., IBM CEO, August 1963: “Last week, Control Data ... announced the 6600 system. I understand that in the laboratory developing the system there are only 34 people including the janitor. Of these, 14 are engineers and 4 are programmers... Contrasting this modest effort with our vast development activities, I fail to understand why we have lost our industry leadership position by letting someone else offer the world's most powerful computer.” To which Cray replied: “It seems like Mr. Watson has answered his own question.”

5 • Separate instructions to manipulate three types of reg. 8 60-bit data registers (X) 8 18-bit address registers (A) 8 18-bit index registers (B) • All arithmetic and logic instructions are reg-to-reg • Only Load and Store instructions refer to memory! Touching address registers 1 to 5 initiates a load 6 to 7 initiates a store - very useful for vector operations opcode i j k Ri  (Rj) op (Rk) CDC 6600: A Load/Store Architecture opcode i j disp Ri M[(Rj) + disp] 6 3 3 3 6 3 3 18

6 CDC 6600: Datapath Address Regs Index Regs 8 x 18-bit 8 x 18-bit Operand Regs 8 x 60-bit Inst. Stack 8 x 60-bit IR 10 Functional Units Central Memory 128K words, 32 banks, 1ms cycle result addr result operand operand addr

7 CDC6600 ISA designed to simplify high-performance implementation • Use of three-address, register-register ALU instructions simplifies pipelined implementation – No implicit dependencies between inputs and outputs • Decoupling setting of address register (Ar) from retrieving value from data register (Xr) simplifies providing multiple outstanding memory accesses – Software can schedule load of address register before use of value – Can interleave independent instructions inbetween • CDC6600 has multiple parallel but unpipelined functional units – E.g., 2 separate multipliers • Follow-on machine CDC7600 used pipelined functional units – Foreshadows later RISC designs

8 CDC6600: Vector Addition B0  - n loop: JZE B0, exit A0  B0 + a0 load X0 A1  B0 + b0 load X1 X6  X0 + X1 A6  B0 + c0 store X6 B0  B0 + 1 jump loop Ai = address register Bi = index register Xi = data register

9 Supercomputer Applications Typical application areas • Military research (nuclear weapons, cryptography) • Scientific research • Weather forecasting • Oil exploration • Industrial design (car crash simulation) • Bioinformatics • Cryptography All involve huge computations on large data sets In 70s-80s, Supercomputer  Vector Machine

Vector Programming Model 10 + + + + + + [0] [1] [VLR-1] Vector Arithmetic Instructions ADDV v3, v1, v2 v3 v2 v1 Scalar Registers r0 r15 Vector Registers v0 v15 [0] [1] [2] [VLRMAX-1] VLRVector Length Register v1 Vector Load and Store Instructions LV v1, r1, r2 Base, r1 Stride, r2 Memory Vector Register

11 Vector Code Example # Scalar Code LI R4, 64 loop: L.D F0, 0(R1) L.D F2, 0(R2) ADD.D F4, F2, F0 S.D F4, 0(R3) DADDIU R1, 8 DADDIU R2, 8 DADDIU R3, 8 DSUBIU R4, 1 BNEZ R4, loop # Vector Code LI VLR, 64 LV V1, R1 LV V2, R2 ADDV.D V3, V1, V2 SV V3, R3 # C code for (i=0; i<64; i++) C[i] = A[i] + B[i];

12 Vector Supercomputers Epitomized by Cray-1, 1976: • Scalar Unit – Load/Store Architecture • Vector Extension – Vector Registers – Vector Instructions • Implementation – Hardwired Control – Highly Pipelined Functional Units – Interleaved Memory System – No Data Caches – No Virtual Memory

13 Cray-1 (1976) Single Port Memory 16 banks of 64-bit words + 8-bit SECDED 80MW/sec data load/store 320MW/sec instruction buffer refill 4 Instruction Buffers 64-bitx16 NIP LIP CIP (A0) ( (Ah) + j k m ) 64 T Regs (A0) ( (Ah) + j k m ) 64 B Regs S0 S1 S2 S3 S4 S5 S6 S7 A0 A1 A2 A3 A4 A5 A6 A7 Si Tjk Ai Bjk FP Add FP Mul FP Recip Int Add Int Logic Int Shift Pop Cnt Sj Si Sk Addr Add Addr Mul Aj Ai Ak memory bank cycle 50 ns processor cycle 12.5 ns (80MHz) V0 V1 V2 V3 V4 V5 V6 V7 Vk Vj Vi V. Mask V. Length64 Element Vector Registers

14 Vector Instruction Set Advantages • Compact – one short instruction encodes N operations • Expressive, tells hardware that these N operations: – are independent – use the same functional unit – access disjoint registers – access registers in same pattern as previous instructions – access a contiguous block of memory (unit-stride load/store) – access memory in a known pattern (strided load/store) • Scalable – can run same code on more parallel pipelines (lanes)

Vector Arithmetic Execution 15 • Use deep pipeline (=> fast clock) to execute element operations • Simplifies control of deep pipeline because elements in vector are independent (=> no hazards!) V 1 V 2 V 3 V3 <- v1 * v2 Six stage multiply pipeline

16 Vector Instruction Execution ADDV C,A,B C[1] C[2] C[0] A[3] B[3] A[4] B[4] A[5] B[5] A[6] B[6] Execution using one pipelined functional unit C[4] C[8] C[0] A[12] B[12] A[16] B[16] A[20] B[20] A[24] B[24] C[5] C[9] C[1] A[13] B[13] A[17] B[17] A[21] B[21] A[25] B[25] C[6] C[10] C[2] A[14] B[14] A[18] B[18] A[22] B[22] A[26] B[26] C[7] C[11] C[3] A[15] B[15] A[19] B[19] A[23] B[23] A[27] B[27] Execution using four pipelined functional units

Interleaved Vector Memory System 17 0 1 2 3 4 5 6 7 8 9 A B C D E F + Base Stride Vector Registers Memory Banks Address Generator Cray-1, 16 banks, 4 cycle bank busy time, 12 cycle latency • Bank busy time: Time before bank ready to accept next request

18 Vector Unit Structure Lane Functional Unit Vector Registers Memory Subsystem Elements 0, 4, 8, … Elements 1, 5, 9, … Elements 2, 6, 10, … Elements 3, 7, 11, …

19 T0 Vector Microprocessor (UCB/ICSI, 1995) LaneVector register elements striped over lanes [0] [8] [16] [24] [1] [9] [17] [25] [2] [10] [18] [26] [3] [11] [19] [27] [4] [12] [20] [28] [5] [13] [21] [29] [6] [14] [22] [30] [7] [15] [23] [31]

20 load Vector Instruction Parallelism Can overlap execution of multiple vector instructions – example machine has 32 elements per vector register and 8 lanes load mul mul add add Load Unit Multiply Unit Add Unit time Instruction issue Complete 24 operations/cycle while issuing 1 short instruction/cycle

21 Vector Chaining • Vector version of register bypassing – introduced with Cray-1 Memory V 1 Load Unit Mult. V 2 V 3 Chain Add V 4 V 5 Chain LV v1 MULV v3,v1,v2 ADDV v5, v3, v4

22 Vector Chaining Advantage • With chaining, can start dependent instruction as soon as first result appears Load Mul Add Load Mul AddTime • Without chaining, must wait for last element of result to be written before starting dependent instruction

23 Vector Startup Two components of vector startup penalty – functional unit latency (time through pipeline) – dead time or recovery time (time before another vector instruction can start down pipeline) R X X X W R X X X W R X X X W R X X X W R X X X W R X X X W R X X X W R X X X W R X X X W R X X X W Functional Unit Latency Dead Time First Vector Instruction Second Vector Instruction Dead Time

24 Dead Time and Short Vectors Cray C90, Two lanes 4 cycle dead time Maximum efficiency 94% with 128 element vectors 4 cycles dead time T0, Eight lanes No dead time 100% efficiency with 8 element vectors No dead time 64 cycles active

25 Vector Memory-Memory versus Vector Register Machines • Vector memory-memory instructions hold all vector operands in main memory • The first vector machines, CDC Star-100 (‘73) and TI ASC (‘71), were memory-memory machines • Cray-1 (’76) was first vector register machine for (i=0; i<N; i++) { C[i] = A[i] + B[i]; D[i] = A[i] - B[i]; } Example Source Code ADDV C, A, B SUBV D, A, B Vector Memory-Memory Code LV V1, A LV V2, B ADDV V3, V1, V2 SV V3, C SUBV V4, V1, V2 SV V4, D Vector Register Code

26 Vector Memory-Memory vs. Vector Register Machines • Vector memory-memory architectures (VMMA) require greater main memory bandwidth, why? – All operands must be read in and out of memory • VMMAs make if difficult to overlap execution of multiple vector operations, why? – Must check dependencies on memory addresses • VMMAs incur greater startup latency – Scalar code was faster on CDC Star-100 for vectors < 100 elements – For Cray-1, vector/scalar breakeven point was around 2 elements Apart from CDC follow-ons (Cyber-205, ETA-10) all major vector machines since Cray-1 have had vector register architectures (we ignore vector memory-memory from now on)

27 Automatic Code Vectorization for (i=0; i < N; i++) C[i] = A[i] + B[i]; load load add store load load add store Iter. 1 Iter. 2 Scalar Sequential Code Vectorization is a massive compile-time reordering of operation sequencing  requires extensive loop dependence analysis Vector Instruction load load add store load load add store Iter. 1 Iter. 2 Vectorized Code Time

28 Vector Stripmining Problem: Vector registers have finite length Solution: Break loops into pieces that fit in registers, “Stripmining” ANDI R1, N, 63 # N mod 64 MTC1 VLR, R1 # Do remainder loop: LV V1, RA DSLL R2, R1, 3 # Multiply by 8 DADDU RA, RA, R2 # Bump pointer LV V2, RB DADDU RB, RB, R2 ADDV.D V3, V1, V2 SV V3, RC DADDU RC, RC, R2 DSUBU N, N, R1 # Subtract elements LI R1, 64 MTC1 VLR, R1 # Reset full length BGTZ N, loop # Any more to do? for (i=0; i<N; i++) C[i] = A[i]+B[i]; + + + A B C 64 elements Remainder

29 Vector Conditional Execution Problem: Want to vectorize loops with conditional code: for (i=0; i<N; i++) if (A[i]>0) then A[i] = B[i]; Solution: Add vector mask (or flag) registers – vector version of predicate registers, 1 bit per element …and maskable vector instructions – vector operation becomes NOP at elements where mask bit is clear Code example: CVM # Turn on all elements LV vA, rA # Load entire A vector SGTVS.D vA, F0 # Set bits in mask register where A>0 LV vA, rB # Load B vector into A under mask SV vA, rA # Store A back to memory under mask

30 Masked Vector Instructions C[4] C[5] C[1] Write data port A[7] B[7] M[3]=0 M[4]=1 M[5]=1 M[6]=0 M[2]=0 M[1]=1 M[0]=0 M[7]=1 Density-Time Implementation – scan mask vector and only execute elements with non-zero masks C[1] C[2] C[0] A[3] B[3] A[4] B[4] A[5] B[5] A[6] B[6] M[3]=0 M[4]=1 M[5]=1 M[6]=0 M[2]=0 M[1]=1 M[0]=0 Write data portWrite Enable A[7] B[7]M[7]=1 Simple Implementation – execute all N operations, turn off result writeback according to mask

31 Vector Reductions Problem: Loop-carried dependence on reduction variables sum = 0; for (i=0; i<N; i++) sum += A[i]; # Loop-carried dependence on sum Solution: Re-associate operations if possible, use binary tree to perform reduction # Rearrange as: sum[0:VL-1] = 0 # Vector of VL partial sums for(i=0; i<N; i+=VL) # Stripmine VL-sized chunks sum[0:VL-1] += A[i:i+VL-1]; # Vector sum # Now have VL partial sums in one vector register do { VL = VL/2; # Halve vector length sum[0:VL-1] += sum[VL:2*VL-1] # Halve no. of partials } while (VL>1)

32 Vector Scatter/Gather Want to vectorize loops with indirect accesses: for (i=0; i<N; i++) A[i] = B[i] + C[D[i]] Indexed load instruction (Gather) LV vD, rD # Load indices in D vector LVI vC, rC, vD # Load indirect from rC base LV vB, rB # Load B vector ADDV.D vA,vB,vC # Do add SV vA, rA # Store result

33 Vector Scatter/Gather Histogram example: for (i=0; i<N; i++) A[B[i]]++; Is following a correct translation? LV vB, rB # Load indices in B vector LVI vA, rA, vB # Gather initial A values ADDV vA, vA, 1 # Increment SVI vA, rA, vB # Scatter incremented values

34 A Modern Vector Super: NEC SX-9 (2008) • 65nm CMOS technology • Vector unit (3.2 GHz) – 8 foreground VRegs + 64 background VRegs (256x64-bit elements/VReg) – 64-bit functional units: 2 multiply, 2 add, 1 divide/sqrt, 1 logical, 1 mask unit – 8 lanes (32+ FLOPS/cycle, 100+ GFLOPS peak per CPU) – 1 load or store unit (8 x 8-byte accesses/cycle) • Scalar unit (1.6 GHz) – 4-way superscalar with out-of-order and speculative execution – 64KB I-cache and 64KB data cache • Memory system provides 256GB/s DRAM bandwidth per CPU • Up to 16 CPUs and up to 1TB DRAM form shared-memory node – total of 4TB/s bandwidth to shared DRAM memory • Up to 512 nodes connected via 128GB/s network links (message passing between nodes)

35 Multimedia Extensions (aka SIMD extensions) • Very short vectors added to existing ISAs for microprocessors • Use existing 64-bit registers split into 2x32b or 4x16b or 8x8b – Lincoln Labs TX-2 from 1957 had 36b datapath split into 2x18b or 4x9b – Newer designs have wider registers » 128b for PowerPC Altivec, Intel SSE2/3/4 » 256b for Intel AVX • Single instruction operates on all elements within register 16b 16b 16b 16b 32b 32b 64b 8b 8b 8b 8b 8b 8b 8b 8b 16b 16b 16b 16b 16b 16b 16b 16b 16b 16b 16b 16b + + + +4x16b adds

36 Multimedia Extensions versus Vectors • Limited instruction set: – no vector length control – no strided load/store or scatter/gather – unit-stride loads must be aligned to 64/128-bit boundary • Limited vector register length: – requires superscalar dispatch to keep multiply/add/load units busy – loop unrolling to hide latencies increases register pressure • Trend towards fuller vector support in microprocessors – Better support for misaligned memory accesses – Support of double-precision (64-bit floating-point) – New Intel AVX spec (announced April 2008), 256b vector registers (expandable up to 1024b)

37 Acknowledgements • These slides contain material developed and copyright by: – Arvind (MIT) – Krste Asanovic (MIT/UCB) – Joel Emer (Intel/MIT) – James Hoe (CMU) – John Kubiatowicz (UCB) – David Patterson (UCB) • MIT material derived from course 6.823 • UCB material derived from course CS252

Computer Architecture Vector Computer

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