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CMU CS 15213 - Lecture

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Code Optimization IISeptember 27, 2006Code Optimization IISeptember 27, 2006TopicsTopics Machine Dependent Optimizationsz Understanding Processor Operationsz Branches and Branch Predictionclass09.ppt15-213“The course that gives CMU its Zip!”–2–15-213, F’06Getting High PerformanceGetting High PerformanceDonDon’’t Do Anything Stupidt Do Anything Stupid Watch out for hidden algorithmic inefficiencies Write compiler-friendly codez Help compiler past optimization blockers: function calls & memory refs.Tune Code For MachineTune Code For Machine Exploit instruction-level parallelism Avoid unpredictable branches Make code cache friendlyz Covered later in course–3–15-213, F’06Modern CPU DesignModern CPU DesignExecutionExecutionFunctionalUnitsInstruction ControlInstruction ControlInteger/BranchFPAddFPMult/DivLoad StoreInstructionCacheDataCacheFetchControlInstructionDecodeAddressInstrs.OperationsPrediction OK?DataDataAddr.Addr.GeneralIntegerOperation ResultsRetirementUnitRegisterFileRegister Updates–4–15-213, F’06CPU Capabilities of Pentium IVCPU Capabilities of Pentium IVMultiple Instructions Can Execute in ParallelMultiple Instructions Can Execute in Parallel 1 load, with address computation 1 store, with address computation 2 simple integer (one may be branch) 1 complex integer (multiply/divide) 1 FP/SSE3 unit 1 FP move (does all conversions)Some Instructions Take > 1 Cycle, but Can be PipelinedSome Instructions Take > 1 Cycle, but Can be Pipelined Instruction Latency Cycles/Issue Load / Store 5 1 Integer Multiply 10 1 Integer/Long Divide 36/106 36/106 Single/Double FP Multiply 7 2 Single/Double FP Add 5 2 Single/Double FP Divide 32/46 32/46–5–15-213, F’06Instruction ControlInstruction ControlGrabs Instruction Bytes From MemoryGrabs Instruction Bytes From Memory Based on current PC + predicted targets for predicted branches Hardware dynamically guesses whether branches taken/not taken and (possibly) branch targetTranslates Instructions Into Translates Instructions Into Operations Operations (for CISC style CPUs)(for CISC style CPUs) Primitive steps required to perform instruction Typical instruction requires 1–3 operationsConverts Register References Into Converts Register References Into TagsTags Abstract identifier linking destination of one operation with sources of later operationsInstruction ControlInstruction ControlInstructionCacheFetchControlInstructionDecodeAddressInstrs.OperationsRetirementUnitRegisterFile–6–15-213, F’06Translating into OperationsTranslating into OperationsGoal: Each Operation Utilizes Single Functional UnitGoal: Each Operation Utilizes Single Functional Unit Requires: Load, Integer arithmetic, Store Exact form and format of operations is trade secret Operations: split up instruction into simpler pieces Devise temporary names to describe how result of one operation gets used by other operationsaddq %rax, 8(%rbx,%rdx,4)load 8(%rbx,%rdx,4) Î temp1imull %rax, temp1 Î temp2store temp2, 8(%rbx,%rdx,4)–7–15-213, F’06Traditional View of Instruction ExecutionTraditional View of Instruction ExecutionImperative ViewImperative View Registers are fixed storage locationsz Individual instructions read & write them Instructions must be executed in specified sequence to guarantee proper program behavioraddq %rax, %rbx # I1andq %rbx, %rdx # I2mulq %rcx, %rbx # I3xorq %rbx, %rdi # I4rax+rbx rdx rcx rdiI1I2I3I4&*^–8–15-213, F’06Dataflow View of Instruction ExecutionDataflow View of Instruction ExecutionFunctional ViewFunctional View View each write as creating new instance of value Operations can be performed as soon as operands available No need to execute in original sequenceaddq %rax, %rbx # I1andq %rbx, %rdx # I2mulq %rcx, %rbx # I3xorq %rbx, %rdi # I4rax.0+rbx.0 rdx.0 rcx.0 rdi.0I1I2/I3I4&*^rbx.1rdx.1rbx.2rdi.0–9–15-213, F’06Example ComputationExample ComputationData TypesData Types Use different declarations for data_t int float doublevoid combine4(vec_ptr v, data_t *dest){int i;int length = vec_length(v);data_t *d = get_vec_start(v);data_t t = IDENT;for (i = 0; i < length; i++)t = t OP d[i];*dest = t;}OperationsOperations Use different definitions of OP and IDENT + / 0 * / 1–10–15-213, F’06Cycles Per ElementCycles Per Element Convenient way to express performance of program that operators on vectors or lists Length = n T = CPE*n + Overhead010020030040050060070080090010000 50 100 150 200Ele mentsCyclesvsum1Slope = 4.0vsum2Slope = 3.5–11–15-213, F’06x86-64 Compilation of Combine4x86-64 Compilation of Combine4Inner Loop (Integer Multiply)Inner Loop (Integer Multiply)PerformancePerformance 5 instructions in 2 clock cyclesL33: # Loop:movl (%eax,%edx,4), %ebx # temp = d[i]incl %edx # i++imull %ebx, %ecx # x *= tempcmpl %esi, %edx # i:lengthjl L33 # if < goto Loop7.005.0010.002.20Combine4Floating PointIntegerMethod–12–15-213, F’06Serial ComputationSerial ComputationComputation (length=12)Computation (length=12)((((((((((((1 * d[0]) * d[1]) * d[2]) * d[3]) * d[4]) * d[5]) * d[6]) * d[7]) * d[8]) * d[9]) * d[10]) * d[11])PerformancePerformance N elements, D cycles/operation N*D cycles **11dd00dd11*dd22*dd33*dd44*dd55*dd66*dd77*dd88*dd99*dd1010*dd11117.005.0010.002.20Combine4Floating PointIntegerMethod–13–15-213, F’06Loop UnrollingLoop Unrolling Perform 2x more useful work per iterationvoid unroll2a_combine(vec_ptr v, data_t *dest){int length = vec_length(v);int limit = length-1;data_t *d = get_vec_start(v);data_t x = IDENT;int i;/* Combine 2 elements at a time */for (i = 0; i < limit; i+=2) {x = (x OPER d[i]) OPER d[i+1];}/* Finish any remaining elements */for (; i < length; i++) {x = x OPER d[i];}*dest = x;}–14–15-213, F’067.005.0010.001.50Unroll 27.005.0010.002.20Combine4Floating PointIntegerMethodEffect of Loop UnrollingEffect of Loop UnrollingHelps Integer SumHelps Integer Sum Before: 5 operations per element After: 6 operations per 2 elements z = 3 operations per elementOthers DonOthers Don’’t Improvet Improve Sequential dependencyz Each operation must wait until previous one completesx = (x OPER d[i]) OPER d[i+1];–15–15-213, F’06Loop Unrolling with ReassociationLoop Unrolling with Reassociation Could change numerical results for FPvoid unroll2aa_combine(vec_ptr v, data_t *dest){int


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