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Module 12: View
Lecture 24: Code Optimization
The Lecture Contains:
Code Optimization
Most Important Optimizations
Low Level Model of Optimization
Mixed Model of Optimization
Placement of Optimizations
Placement of Optimizations: Continued
Common Subexpression Elimination
Copy Propagation
Dead Code Elimination
Algebraic Transformation
Loop Optimizations
Loop-unrolling
Induction Variable Simplification
Loop Jamming
Module 12: View
Lecture 24: Code Optimization
Code Optimization
int a,b,c,d c = a+b d = c+
ldw a,r ldw b,r add r1,r2,r stw r3,c ldw c,r add r3,1,r stw r4,d
add r1,r2,r add r3,1,r
source code naive sparc code 10 cycles
optimized code 2 cycles
Most Important Optimizations
Loop Optimizations Moving loop invariant computations Simplifying or eliminating computations on induction variables Global register allocation Instruction scheduling Some optimizations may be relevant to a particular program and may vary according to the structure and details of the programs A highly recursive program may benefit from tail call optimization A program with few loops and large basic blocks may benefit from loop distribution In-lining may decrease call over heads; in-lining may increase the code size and may have negative effect on performance by increasing cache misses
Module 12: View
Lecture 24: Code Optimization
Mixed Model of Optimization
More easily to be adapted to new architectures May be more efficient at compile time Used in Sun Sparc, Digital Alpha, Intel, SGI MIPS More appropriate for same language and many architectures
Placement of Optimizations
Module 12: View
Lecture 24: Code Optimization
Placement of Optimizations: Cont’d
Code Optimization
Criteria for Code Improving Transformation
Preserve the meaning Must speed up the program Must be worth the effort The analysis must be fast
Local transformation : Within basic blocks Global transformation : Across basic blocks
Module 12: View
Lecture 24: Code Optimization
Copy Propagation
Use g for f after assignment f = g
Dead Code Elimination
Dead Operation : Unreachable by any path produces a value not used
If whose true and false arcs are same If whose B expr known at compile time loop not to be executed procedure not to be called
debug := false ... if (debug) { ... }
while( i <= limit - 2 ) { * statement not changing limit *
}
t := limit - 2 while( i <= t ) { statement }
Module 12: View
Lecture 24: Code Optimization
Renaming Temporary Variable Rename temporary variable t to u and replace all the occurrences of t by u. This transformation increases parallelism. Interchange Statements Two statements may be interchanged if value of the block is not affected.
t1 = b + c t2 = X + Y
t2 = X + Y t1 = b + c
Constant folding Evaluate constant expressions at compile time.
X = 3 + 5 Y = X * 2
X = 8
Y = 16
Algebraic Transformation
Eliminate addition/subtraction with 0 X = X ± 0 should be eliminated. Eliminate multiplication/division by 1 X = X * 1 or X = X/1 should be eliminated. Eliminate multiplication by 0 X = X * 0 should be replaced with X = 0
Strength reduction Costly operators should be replaced by cheaper operators
Replace T = X * * *2 by T = X * X Replace T = X * 4 by T = ls(X, 2) Replace 2 * X by X + X Replace X * 0.5 by X/ Replace X/2 by rs(X, 1)
Loop Optimizations
Code Motion: Expression not evaluated in the code must be moved out of loop
file:///D|/...ry,%20Dr.%20Sanjeev%20K%20Aggrwal%20&%20Dr.%20Rajat%20Moona/Multi-core_Architecture/lecture%2024/24_10.htm[6/14/2012 12:07:23 PM]
Module 12: View
Lecture 24: Code Optimization
Induction Variable Simplification
Loop Jamming
Two adjacent loops may me merged into a single loop
can be replaced by
