1. Optimization

2. How to Optimize Code Conventional Wisdom: Don't do it
(For experts only) Don't do it yet

3. 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

4. Code Tricks Prefer ints to floats Minimize float->int conversions
Use multiplication instead of division

5. 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;

6. Fancy Tricks Make if the common case:
If path usually gets unconditional branch
Static branch prediction usually skips forward branches
Else relies on conditional branch

7. 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++)
a[i] = a[i] + b[i] * c;
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; }

8. 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];
D[i] = E[i] + C[i];
for (i = 0; i < N; i++) C[i] = A[i] + B[i];
D[i] = E[i] + C[i]; }

9. Fancy Tricks Loop peeling:
Break up loops to avoid branches within them
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; }
A[1] = 0;
for (i = 2; i < N; i++)
A[i] = A[i] + 8;
A[N] = N;

10. How To Optimize - Branching
Pipelined processors HATE branch miss- predictions
15+ cycle penalty

11. 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

12. How To Optimize - Branching
Maybe better off without it:

13. How To Optimize - Branching
Maybe better off without it:

14. How To Optimize - Cache Cache: Modern CPU's live/die by cache
Like contiguous data
Avoid repeated swapping between multiple items
25:00-35:00 and beyond

15. How to Optimize Right Rules
Start with clean, working code and good tests
Test any optimizations
If it doesn't help, take it out

16. Compiler Optimization
Compilers can
Eliminate unneeded code
Do strength reductions
Unroll loops
Compiler likes specific information
Use constants
Make variables as local as possible

17. 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

18. Samples
C++ Code: