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Code Optimization

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Outline

• Machine-Independent Optimization– Code motion– Memory optimization

• Suggested reading– 5.2 ~ 5.6

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Motivation

• Constant factors matter too!– easily see 10:1 performance range depending on

how code is written– must optimize at multiple levels

• algorithm, data representations, procedures, and loops

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Motivation

• Must understand system to optimize performance– how programs are compiled and executed– how to measure program performance and

identify bottlenecks– how to improve performance without destroying

code modularity and generality

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5.3 Program Example

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Vector ADT P384

typedef struct { int len ;

data_t *data ; } vec_rec, *vec_ptr ; typedef int data_t ;

length

data

0 1 2 length–1

Figure 5.3 P385

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Procedures P386

• vec_ptr new_vec(int len)– Create vector of specified length

• data_t *get_vec_start(vec_ptr v)– Return pointer to start of vector data

P392

P386

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Procedures

• int get_vec_element(vec_ptr v, int index, int *dest)– Retrieve vector element, store at *dest– Return 0 if out of bounds, 1 if successful

• Similar to array implementations in Pascal, Java– E.g., always do bounds checking

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Vector ADT

vec_ptr new_vec(int len) {

/* allocate header structure */vec_ptr result = (vec_ptr) malloc(sizeof(vec_rec)) ;if ( !result )

return NULL ;

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Vector ADT

/* allocate array */if ( len > 0 ) {

data_t *data = (data_t *)calloc(len, sizeof(data_t)) ;

if ( !data ) { free( (void *)result ) ; return NULL ; /* couldn’t allocte stroage

*/}result->data = data

} else result->data = NULL

return result ;}

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Vector ADT

/** Retrieve vector element and store at dest.* Return 0 (out of bounds) or 1 (successful)*/ int get_vec_element(vec_ptr v, int index, data_t *dest)

{ if ( index < 0 || index >= v->len)

return 0 ;*dest = v->data[index] ;return 1;

}

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Vector ADT

/* Return length of vector */ int vec_length(vec_ptr) {return v->len ;

}

/* Return pointer to start of vector data */data_t *get_vec_start(vec_ptr v){return v->data ;

}

P392

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Optimization Example P385

#ifdef ADD #define IDENT 0#define OPER +

#else#define IDENT 1#define OPER *

#endif

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Optimization Example P387

void combine1(vec_ptr v, data_t *dest){ int i; *dest = IDENT; for (i = 0; i < vec_length(v); i++) { int val; get_vec_element(v, i, &val); *dest = *dest OPER val; }}

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Optimization Example

• Procedure– Compute sum (product) of all elements of

vector– Store result at destination location

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5.2 Expressing Program Performance

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Time Scales P382

• Absolute Time– Typically use nanoseconds

• 10–9 seconds

– Time scale of computer instructions

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Time Scales

• Clock Cycles– Most computers controlled by high frequency

clock signal– Typical Range

• 100 MHz – 108 cycles per second– Clock period = 10ns

• 2 GHz – 2 X 109 cycles per second– Clock period = 0.5ns

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CPE P383

1 void vsum1(int n)2 {3 int i;45 for (i = 0; i < n; i++)6 c[i] = a[i] + b[i];7 }8

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CPE P383

9 /* Sum vector of n elements (n must be even) */10 void vsum2(int n)11 {12 int i;1314 for (i = 0; i < n; i+=2) {15 /* Compute two elements per iteration */16 c[i] = a[i] + b[i];17 c[i+1] = a[i+1] + b[i+1];18 }19 }

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Cycles Per Element

• Convenient way to express performance of program that operators on vectors or lists

• Length = n• T = CPE*n + Overhead

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Cycles Per Element Figure 5.2 P383

0

100

200

300

400

500

600

700

800

900

1000

0 50 100 150 200

Elements

Cyc

les

vsum1Slope = 4.0

vsum2Slope = 3.5

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5.3 Program Example5.4 Eliminating Loop Inefficiencies5.5 Reducing Procedure Calls5.6 Eliminating Unneeded Memory References

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Time Scales P387

void combine1(vec_ptr v, int *dest){ int i; *dest = 0; for (i = 0; i < vec_length(v); i++) { int val; get_vec_element(v, i, &val); *dest += val; }}

5.3 Program Example

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Time Scales P385

• Procedure– Compute sum of all elements of integer vector– Store result at destination location– Vector data structure and operations defined

via abstract data type• Pentium II/III Performance: CPE

– 42.06 (Compiled -g) 31.25 (Compiled -O2)

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Understanding Loop

void combine1-goto(vec_ptr v, int *dest){ int i = 0; int val; *dest = 0; if (i >= vec_length(v)) goto done; loop: get_vec_element(v, i, &val); *dest += val; i++; if (i < vec_length(v)) goto loop done:}

1 iteration

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Inefficiency

• Procedure vec_length called every iteration• Even though result always the same

5.4 Eliminating Loop Inefficiencies

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Code Motion P388

void combine2(vec_ptr v, int *dest){ int i; int length = vec_length(v); *dest = 0; for (i = 0; i < length; i++) { int val; get_vec_element(v, i, &val); *dest += val; }}

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Code Motion P388

• Optimization– Move call to vec_length out of inner loop

• Value does not change from one iteration to next• Code motion

– CPE: 22.61 (Compiled -O2)• vec_length requires only constant time, but

significant overhead

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Reduction in Strength P392

void combine3(vec_ptr v, int *dest){ int i; int length = vec_length(v); int *data = get_vec_start(v); *dest = 0; for (i = 0; i < length; i++) { *dest += data[i];}

5.5 Reducing Procedure Calls

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Reduction in Strength

• Optimization– Avoid procedure call to retrieve each vector

element• Get pointer to start of array before loop• Within loop just do pointer reference• Not as clean in terms of data abstraction

– CPE: 6.00 (Compiled -O2)• Procedure calls are expensive!• Bounds checking is expensive

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Eliminate Unneeded Memory References P394void combine4(vec_ptr v, int *dest){ int i; int length = vec_length(v); int *data = get_vec_start(v); int sum = 0; for (i = 0; i < length; i++) sum += data[i]; *dest = sum;}

5.6 Eliminating Unneeded Memory References

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Eliminate Unneeded Memory References

• Optimization– Don’t need to store in destination until end– Local variable sum held in register– Avoids 1 memory read, 1 memory write per

cycle– CPE: 2.00 (Compiled -O2)

• Memory references are expensive!

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Detecting Unneeded Memory References

.L18:movl (%ecx,%edx,4),%eaxaddl %eax,(%edi)

incl %edxcmpl %esi,%edxjl .L18

.L24:addl (%eax,%edx,4),%ecx

incl %edxcmpl %esi,%edxjl .L24

Combine3 Combine4

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Detecting Unneeded Memory References

• Performance– Combine3

• 5 instructions in 6 clock cycles• addl must read and write memory

– Combine4• 4 instructions in 2 clock cyles