Compiler optimization transforms programs to equivalent programs that use fewer resources by applying techniques like: 1) Combining multiple simple operations like increments into a single optimized operation 2) Evaluating constant expressions at compile-time rather than run-time 3) Eliminating redundant computations and storing values in registers rather than memory when possible 4) Optimizing loops, conditionals, and expressions to minimize computations Compiler optimization aims to minimize program execution time, memory usage, and power consumption by transforming programs in various ways before producing executable code. Some key techniques include instruction combining, constant folding, common subexpression elimination, strength reduction, dead code elimination, and loop optimizations. This improves program efficiency and performance.

1. Compiler Optimization
Name : Khaled Hasan Rony
ID : 011132009
Compiler Lab - Section SB
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2. What is compiler optimization ?
compiler optimization = code optimization
transformations, algorithms which take a program and transform it to
an equivalent output program that uses fewer resources.
• minimizing program executing time
• minimizing memory use
• minimizing the power consumed by a program
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3. Instruction Combining
int i;
void f()
{
i++;
i++;
}
int i;
void f()
{
i += 2;
}
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4. Constant Folding
int f()
{
// do other work
return (3 + 5);
}
int f()
{
// do other work
return 8;
}
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5. Constant Propagation
void f()
{
x = 3;
y = x + 4;
}
void f()
{
x = 3;
y = 7;
}
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6. Common SubExpression (CSE) Elimination
void f()
{
i = x + y + 1;
j = x + y;
}
void f()
{
t1 = x + y;
i = t1 + 1;
j = t1;
}
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7. Integer Multiply Optimization
int f(int i)
{
return i * 4;
}
int f(int i)
{
return i << 2;
}
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8. Integer Divide Optimization
int f(int i)
{
return i / 2;
}
int f(int i)
{
return i >> 1;
}
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9. Loop Fusion
void f()
{
int i;
for( i = 0; i < 100; i++ )
a[i] += 10;
for( i = 0; i < 100; i++ )
b[i] += 20;
}
void f()
{
int i;
for( i = 0; i < 100; i++ )
{
a[i] += 10;
b[i] += 20;
}
}
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10. Dead Code Elimination
int global;
void f()
{
int i ;
i = 1; /* dead store */
global = 1; /* dead store */
global = 2;
return;
global = 3; /* unreachable*/
}
int global;
void f()
{
global = 2;
return;
}
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11. Expression Simplification
void f(int i)
{
a[0] = i + 0;
a[1] = i * 0;
a[2] = i - i;
a[3] = 1 + i + 1;
}
void f(int i)
{
a[0] = i;
a[1] = 0;
a[2] = 0;
a[3] = 2 + i;
}
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12. Forward Store
int sum;
void f()
{
int i;
sum = 0;
for( i = 0; i < 100; i++ )
{
sum += a[i];
}
}
int sum;
void f()
{
int i;
register int t;
t = 0;
for( i = 0; i < 100; i++ )
{
t += a[i];
}
sum = t;
}
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13. If Optimization (1)
void f(int *p)
{
if( p )
g(1);
if( p )
g(2);
}
void f(int *p)
{
if( p )
{
g(1);
g(2);
}
}
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14. If Optimization (2)
void f(int *p)
{
if( p )
{
g(1);
if( p )
g(2);
g(3);
}
}
void f(int *p)
{
if( p )
{
g(1);
g(2);
g(3);
}
}
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15. New Expression Optimization
{
int a[];
a = new int[100];
}
{
/*
a not used, so not allocated
*/
}
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16. Try Catch Block Optimization
try
{
a = (int) 5;
}
Catch(Exception e)
{
//………
}
a = 5;
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17. Loop Unrolling
for( i = 0; i < 100; i++ )
{
g();
}
for( i = 0; i < 100; i += 2 )
{
g();
g();
}
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18. Unswitching
for( i = 0; i < 100; i++ )
{
if( x )
a[i] = i;
else
b[i] = i;
}
if( x )
{
for( i = 0; i < 100; i++ )
a[i] = i;
}
else
{
for( i = 0; i < 100; i++ )
b[i] = i;
}
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19. Why we need compiler optimization ?
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20. Reference
http://www.compileroptimizations.com/index.html
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21. Any questions ?
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22. Thank you.
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