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Conditional execution examples

if (r0 == 0)

C source code

CMP r0, #0

ARM instructionsunconditional

CMP r0, #0conditional

if (r0 0){r1 = r1 + 1;

}

CMP r0, #0BNE elseADD r1, r1, #1B end

CMP r0, #0ADDEQ r1, r1, #1ADDNE r2, r2, #1...}

else{r2 = r2 + 1;

elseADD r2, r2, #1

end} ...

C il ll th d Compile all three cases and measure Instruction count Byte count Cycle count Cycle count

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Loop InterchangeLoop Interchange

Loop Interchange: change nesting of loops to access data in order Loop Interchange: change nesting of loops to access data in order stored in memory

/* Before */for (j = 0; j < 100; j = j+1)

for (i = 0; i < 5000; i = i+1)x[i][j] = 2 * x[i][j];

100

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Loop InterchangeLoop Interchange

Loop Interchange: change nesting of loops to access data in order Loop Interchange: change nesting of loops to access data in order stored in memory

/* After */for (i = 0; i < 5000; i = i+1)for (j = 0; j < 100; j = j+1)

x[i][j] = 2 * x[i][j]; , .

cache block

spatial locality

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Loop FusionLoop Fusion

Loop Fusion: Combine 2 independent loops that have same looping Loop Fusion: Combine 2 independent loops that have same looping and some variables overlap

/* Before */for (i = 0; i < N; i = i+1)

for (j = 0; j < N; j = j+1)a[i][j] = 1/b[i][j] * c[i][j];

for (i = 0; i < N; i = i+1)for (j = 0; j < N; j = j+1)

d[i][j] = a[i][j] + c[i][j];

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Loop FusionLoop Fusion

Loop Fusion: Combine 2 independent loops that have same looping Loop Fusion: Combine 2 independent loops that have same looping and some variables overlap

/* After */for (i = 0; i < N; i = i+1)

for (j = 0; j < N; j = j+1){ a[i][j] = 1/b[i][j] * c[i][j];

d[i][j] = a[i][j] + c[i][j];}

improve spatial localityp p y

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Reduce costly computations Reduce costly computations

Before Beforefor(int i = 0; i < N; i++)

{z += x/y;}

Afterdouble denom = 1/y;ffor(int i = 0; i < N; i++) {z += x*denom;z += x*denom;

}

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Remove Branch overheadsRemove Branch overheads

float *data = ...; float *data = ...;float data ...;int length = ...;// Slow version

;int length = ...;// Fast version

float total = 0.0f;int i;

float total1, total2, total3, total4;for (i=0; i < length-3; i += 4) {

for (i=0; i < length; i++) {total += data[i]

total1 += data[i];total2 += data[i+1];t t l3 + d t [i+2]} total3 += data[i+2];total4 += data[i+3];

}}total += (total1 + total2) +

(total3 + total4);`

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Eliminate branchesEliminate branches

Eliminating branches (if/elses) by using boolean math: Eliminating branches (if/elses) by using boolean math:

if(x == 0) x = 5; ( ) ; // becomes: x += (x == 0) * 5;

// if '5' was a base 2 number, let's say 4: x += (x == 0)

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Using array indicesUsing array indices

switch ( queue ) { switch ( queue ) { case 0 : letter = 'W'; break; case 1 : letter = 'S'; break; ; ;case 2 : letter = 'U'; break; }

if ( queue == 0 ) letter = 'W'; else if ( queue == 1 ) letter = 'S'; else letter = 'U';

static char *classes="WSU"; letter = classes[queue];

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Global VariablesGlobal Variables

Global variables are never allocated to registers. Global variables are never allocated to registers. int f(void); int g(void); g( );int errs; void test1(void) { errs += f(); errs += g(); } void test2(void) { int localerrs = errs; localerrs += f(); localerrs += g(); errs = localerrs; }

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Local variablesLocal variables

Avoid char and short as local variables. Avoid sign-extending Avoid char and short as local variables. Avoid sign extending for signed variables and zero extending for unsigned variables.

int wordinc (int a) { return a + 1; }

char charinc (char a) { return a + 1; }

The results will be identical, but the first code segment will run faster than othersfaster than others.

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Loop terminationLoop termination

Use count-down-to-zero loops and use simple termination Use count down to zero loops and use simple termination conditions.

int fact1_func (int n) { int i, fact = 1; for (i = 1; i

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More unrollingMore unrolling

//Example - 1 //Example - 2//Example 1int countbit1(uint n) { int bits = 0;

// pint countbit2(uint n) { int bits = 0; ;

while (n != 0) { if (n & 1) bits++;

while (n != 0) { if (n & 1) bits++;

n >>= 1; } return bits;

if (n & 2) bits++; if (n & 4) bits++;

} if (n & 8) bits++; n >>= 4; } return bits; }