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## PowerPoint Slideshow about ' Branches' - kirima

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Branches

- if statements in C
- if (condition)

clause

- if (condition)

clause1

else

clause2

- Rearrange 2nd if into the following:

if (condition) branch to L1;

clause2;

branch to L2;

L1: clause1;

…

L2:

Description

Flags tested

EQ

Equal

Z=1

NE

Not equal

Z=0

CS/HS

Unsigned higher or same

C=1

CC/LO

Unsigned lower

C=0

MI

Minus

N=1

PL

Positive or Zero

N=0

VS

Overflow

V=1

VC

No overflow

V=0

HI

Unsigned higher

C=1 & Z=0

LS

Unsigned lower or same

C=0 or Z=1

GE

Greater or equal

N=V

LT

Less than

N!=V

GT

Greater than

Z=0 & N=V

LE

Less than or equal

Z=1 or N=!V

AL

Always

Condition Codes- The possible condition codes are listed below
- Note AL is the default and does not need to be specified

ARM Branch Instructions

Unconditional branch

B (or BAL) branch always

testing individual condition codes:

bmi – branch on negative (N==1)

bpl – branch on positive or zero (N==0)

bvs– branch on overflow set (V==1)

bvc – branch on overflow clear (V==0)

bcs – branch on carry set (C==1)

bcc – branch on carry clear (C==0)

ARM Branch Instructions

testing result of compare or other operation (signed arithmetic):

be –branch on equal (Z==1)

bne–branch on not equal (Z==0)

bls–branch on less than ((N xor V)==1)

ble–branch on less than or equal ((Z or (N xor V))==1)

bge–branch on greater than or equal ((N xor V)==0)

bgt–branch on greater than ((Z or (N xor V))==0)

Comparison Instructions

- CMP – Compare: subtracts a register or an immediate value from a register value and updates condition codes
- Examples:
- CMP r3, #0 ; set Z flag if r3 == 0
- CMP r3, r4 ; set Z flag if r3 == r4

All flags are set as a result of this operation, not just Z.

a+b > c?

(false)

a += b

Exit

C if statement in ARMif((a+b)>c)

a+=b;

- Use this mapping:

a: r0, b: r1, c: r2, sum:r3

- ARM code:

add r3,r0,r1

cmp r3, r2

blefalse @ branch to false when((a+b)>c) is false

add r0, r0, r1 @ a = a+bfalse:

(true) i == j

i == j?

f=g+h

f=g-h

Exit

C if statement in ARMif(i == j)

f=g+h;

else

f=g-h;

- Use this mapping:

f: r0, g: r1, h: r2, i: r3, j: r4

(true) i == j

i == j?

f=g+h

f=g-h

Exit

C if-else in ARM- if (i == j)
- f=g+h; else
- f=g-h;

- ARM code:

cmp r3, r4 @ Z = 1 if i==j

beq true @ branch to true when i==j sub r0,r1,r2 @ f = g-h (false) b done @ branch to done true: add r0,r1,r2 @ f = g+h (true) done:

Conditional execution in ARM

An unusual ARM feature is that all instructions may be conditional:

CMP r0, #5 @ if (r0 != 5)

{

ADDNE r1, r1, r0 @ r1 := r1 + r0 - r2

SUBNE r1, r1, r2

}

- this removes the need for some short branches, improving performance and code density

Loops in C/Assembly

while loop assume this mapping:

@ a: r0, b: r1

while ( a <= 17 ) test:

{ cmp r0, #17

a = a + b; bgtdone

} add r0, r0, r1

b test

done:

Loops in C/Assembly

for loop @ a: r0, b: r1, c: r2

for ( a = 1; a <= b; a++ ) mov r0, #1

{ test:

c *= a; cmp r0, r1

} bgt done

mul r2, r2, r0

rewritten as while loop add r0, r0, #1

a = 1; b test

while ( a <= b ) done:

{

c *= a;

a++;

}

Loops in C/Assembly

Three ways to do a counted loop:

for loop, counted up from 0 to n-1 @ i: r0, n: r1

for ( i = 0; i < n; i++ ) { clr r0

<body> test:

} cmp r0, r1

bge done

rewritten as while loop <body>

i = 0; add r0, r0, #1

while ( i < n ) b test

{ done:

<body>

i++;

}

for loop, counted up from 1 to n

for ( i = 1; i <= n; i++ ) { mov 1, %i_r

<body> test:

} cmp %i_r, n

bg done

nop

rewritten as while loop

<body>

i = 1;

while ( i <= n ) { inc %i_r

<body> ba test

i++; nop

} done:

for loop, counted down from to n to 1

for ( i = n; i > 0; i-- ) { mov n, %i_r

<body> test:

} cmp %i_r, 0

ble done

nop

rewritten as while loop

<body>

i = n;

while ( i > 0 ) { dec %i_r

<body> ba test

i--; nop

} done:

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