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CMU CS 15213 - Code Optimization II

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Code Optimization IIDec. 2, 2008Code Optimization IIDec. 2, 2008TopicsTopics Machine Dependent Optimizationsz Understanding Processor Operationsz Branches and Branch Predictionclass26.ppt15-213“The course that gives CMU its Zip!”–2 –15-213, F’08Motivate: Compute FactorialsMotivate: Compute FactorialsMachinesMachines Intel Pentium 4 Nocona, 3.2 GHzz Fish Machines Intel Core 2, 2.7 GHzCompiler VersionsCompiler Versions GCC 3.4.2 (current on Fish machines) GCC 4.1.2 (most recent available)int rfact(int n){if (n <= 1)return 1;return n * rfact(n-1);}int fact(int n){int i;int result = 1;for (i = n; i > 0; i--)result = result * i;return result;}Machine Nocona Core 2Compiler 3.4 3.4 4.1rfact15.5 6.0 3.0fact10.0 3.0 3.0Cycles Per ElementCycles Per Element–3 –15-213, F’08Faster Versions (1)Faster Versions (1)Loop UnrollingLoop Unrolling Compute more values per iteration Does not help hereint fact_u3a(int n){int i;int result = 1;for (i = n; i >= 3; i-=3) {result =result * i * (i-1) * (i-2);}for (; i > 0; i--)result *= i;return result;}Machine Nocona Core 2Compiler 3.4 3.4 4.1rfact15.5 6.0 3.0fact10.0 3.0 3.0fact_u3a10.0 3.0 3.0Cycles Per ElementCycles Per Element–4 –15-213, F’08Faster Versions (2)Faster Versions (2)Loop Unrolling + ReassociationLoop Unrolling + Reassociation ~3X drop for GCC 3.4 No improvement for GCC 4.1z Very strange!int fact_u3b(int n){int i;int result = 1;for (i = n; i >= 3; i-=3) {result =result * (i * (i-1) * (i-2));}for (; i > 0; i--)result *= i;return result;}Machine Nocona Core 2Compiler 3.4 3.4 4.1rfact15.5 6.0 3.0fact10.0 3.0 3.0fact_u3a10.0 3.0 3.0fact_u3b3.3 1.03.0Cycles Per ElementCycles Per Element–5 –15-213, F’08Faster Versions (3)Faster Versions (3)Loop Unrolling + Multiple AccumulatorsLoop Unrolling + Multiple Accumulators ~3X drop for all machinesint fact_u3c(int n){int i;int result0 = 1;int result1 = 1;int result2 = 1;for (i = n; i >= 3; i-=3) {result0 *= i;result1 *= (i-1);result2 *= (i-2);}for (; i > 0; i--)result0 *= i;return result0 * result1 * result2;}Machine Nocona Core 2Compiler 3.4 3.4 4.1rfact15.5 6.0 3.0fact10.0 3.0 3.0fact_u3a10.0 3.0 3.0fact_u3b3.3 1.0 3.0fact_u3c3.3 1.0 1.0Cycles Per ElementCycles Per Element–6 –15-213, F’08Getting 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–7 –15-213, F’08Modern CPU DesignModern CPU DesignExecutionExecutionFunctionalUnitsInstruction ControlInstruction ControlInteger/BranchFPAddFPMult/DivLoad StoreInstructionCacheDataCacheFetchControlInstructionDecodeAddressInstrs.OperationsPrediction OK?DataDataAddr.Addr.GeneralIntegerOperation ResultsRetirementUnitRegisterFileRegister Updates–8 –15-213, F’08Pentium IV Nocona CPUPentium IV Nocona CPUMultiple 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 PipelinedInstruction 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 32/46–9 –15-213, F’08CPUs: Nocona vs. Core 2CPUs: Nocona vs. Core 2Nocona (3.2 GHz) (Saltwater fish machines)Nocona (3.2 GHz) (Saltwater fish machines)Instruction Latency Cycles/Issue Load / Store 10 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 32/46Core 2 (2.7 GHz) (Recent Intel microprocessors)Core 2 (2.7 GHz) (Recent Intel microprocessors) Load / Store 5 1 Integer Multiply 3 1 Integer/Long Divide 18/50 18/50 Single/Double FP Multiply 4 1 Single/Double FP Add 3 1 Single/Double FP Divide 18/32 18/32–10 –15-213, F’08Instruction 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–11 –15-213, F’08Translating 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)–12 –15-213, F’08Traditional 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&*^–13 –15-213, F’08Dataflow 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


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CMU CS 15213 - Code Optimization II

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