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Published byHenry Booth Modified over 8 years ago
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Optimization
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How to Optimize Code Conventional Wisdom: 1.Don't do it 2.(For experts only) Don't do it yet
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Why Not??? You can't optimize something that doesn't work Biggest improvement is picking the right algorithm 90 / 10 rule – 90% of the time is spent in 10% of the code Need to be able to verify speedup Optimized code is usually: – Less readable/maintainable – More complex/brittle
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Code Tricks Prefer ints to floats Minimize float->int conversions Use multiplication instead of division Put common branches first in if/else sequences
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Fancy Tricks Loop unrolling : – Reducing iterations by increasing work done per step Less Overhead Gives compiler/processor more to rearrange for (i = 1; i <= 30; i+=3) { a[i] = a[i] + b[i] * c; a[i+1] = a[i+1] + b[i+1] * c; a[i+2] = a[i+2] + b[i+2] * c; } for (i = 1; i <= 30; i++) a[i] = a[i] + b[i] * c;
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Fancy Tricks Loop fusion: – Combine loops doing unrelated jobs to avoid overhead Will they fight over cache??? for (i = 0; i < N; i++) C[i] = A[i] + B[i]; for (i = 0; i < N; i++) D[i] = E[i] + C[i]; for (i = 0; i < N; i++) C[i] = A[i] + B[i]; D[i] = E[i] + C[i]; }
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Fancy Tricks Loop peeling: – Break up loops to avoid branches within them A[1] = 0; for (i = 2; i < N; i++) A[i] = A[i] + 8; A[N] = N; for (i = 1; i <= N; i++) { if (i==1) A[i] = 0; else if (i == N) A[i] = N; else A[i] = A[i] + 8; }
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Strength Reductions Avoid costlier operations (divide/multiply) by using cheaper ones (add/subtract/shift) int total = 0; for (i = 0; i < 1000; i++) total += i * 3; int total = 0; for (i = 0; i < 3000; i+=3) total += i;
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Compiler Optimization Compilers can – Eliminate unneeded code – Do strength reductions – Unroll loops http://goo.gl/TK2k06 http://goo.gl/rvJ7rq http://goo.gl/TK2k06 http://goo.gl/rvJ7rq Compiler likes specific information – Use constants – Make variables as local as possible
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Compiler Optimization gcc has multiple optimizations levels – 0 = no optimization – 1 = optimize, but don't spend too much time – 2 = spend lots of time optimizing – 3 = aggressively optimize for speed and allow code size to increase if need be Activate with command line flag like -O2 Oh-2
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Samples C++ Code:
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Samples O0 : Faithful (but efficient) line by line translation O1 : – Try to do work in registers instead of stack frame – Remove obviously worthless code O2 : – Aggressively do work ahead of time
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How To Optimize - Branching Pipelined processors HATE branch mispredications – 15+ cycle penalty
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How To Optimize - Branching May be able to rearrange data for consistent branching Some compilers can use runtime profiles to reorder code Come compilers allow "expect" – hints about how to order instructions
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How To Optimize - Branching Maybe better off without it:
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How To Optimize - Branching Maybe better off without it:
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How To Optimize - Cache Cache: – Modern CPU's live/die by cache Like contiguous data Avoid repeated swapping between multiple items https://www.youtube.com/watch?v=rX0ItVEVjHc&start=1555
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Bad Situations for Cache Data with poor locality – Complex object oriented programming Large 2D arrays traversed in column major order… 012 0 768276 1 838494 2 889383 012345678 768276838494889383 Row Major Access Col Major Access
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How to Optimize Right Rules 1.Start with clean, working code and good tests 2.Test any optimizations 3.If it doesn't help, take it out
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