1 machine-level programming iv: control: loops comp 21000: introduction to computer organization...

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Machine-Level Programming IV: Control: loops

Comp 21000: Introduction to Computer Organization & SystemsMarch 2015

Systems book chapter 3*

* Modified slides from the book “Computer Systems: a Programmer’s Perspective”, Randy Bryant & David O’Hallaron, 2011

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Today Complete addressing mode, address

computation (leal) Arithmetic operations x86-64 Control: Condition codes Conditional branches and moves Loops

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C Codeint pcount_do(unsigned x) { int result = 0; do { result += x & 0x1; x >>= 1; } while (x); return result;}

int pcount_do(unsigned x) { int result = 0; do { result += x & 0x1; x >>= 1; } while (x); return result;}

Goto Versionint pcount_do(unsigned x){ int result = 0;loop: result += x & 0x1; x >>= 1; if (x) goto loop; return result;}

int pcount_do(unsigned x){ int result = 0;loop: result += x & 0x1; x >>= 1; if (x) goto loop; return result;}

“Do-While” Loop Example

Count number of 1’s in argument x (“popcount”)

Use conditional branch to either continue looping or to exit loop

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Goto Version“Do-While” Loop Compilation

Registers:%edx x%ecx result

movl $0, %ecx # result = 0.L2: # loop:

movl %edx, %eaxandl $1, %eax # t = x & 1addl %eax, %ecx # result += tshrl %edx # x >>= 1jne .L2 # If !0, goto loop

int pcount_do(unsigned x) { int result = 0;loop: result += x & 0x1; x >>= 1; if (x) goto loop; return result;}

int pcount_do(unsigned x) { int result = 0;loop: result += x & 0x1; x >>= 1; if (x) goto loop; return result;}

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C Code

do Body while (Test);

do Body while (Test);

Goto Version

loop: Body if (Test) goto loop

loop: Body if (Test) goto loop

General “Do-While” Translation

Body:

Test returns integer = 0 interpreted as false ≠ 0 interpreted as true

{ Statement1; Statement2; … Statementn;}

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C Code Goto Version

“While” Loop Example

Is this code equivalent to the do-while version? Must jump out of loop if test fails

int pcount_while(unsigned x) { int result = 0; while (x) { result += x & 0x1; x >>= 1; } return result;}

int pcount_while(unsigned x) { int result = 0; while (x) { result += x & 0x1; x >>= 1; } return result;}

int pcount_do(unsigned x) { int result = 0; if (!x) goto done;loop: result += x & 0x1; x >>= 1; if (x) goto loop;done: return result;}


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AssemblyGoto Version

“While” Loop Example

Note that the jmp instruction goes to the test, it doesn’t implement the initial if statement itself.

_pcount_do:00001d30 pushl %ebp00001d31 movl %esp, %ebp00001d33 subl $16, %esp00001d36 movl 8(%ebp), %eax00001d39 movl %eax, -4(%ebp)00001d3c movl $0, -16(%ebp)00001d43 jmp 0x1d5e00001d45 movl -4(%ebp), %eax00001d48 andl $1, %eax00001d4b movl -16(%ebp), %ecx00001d4e addl %ecx, %eax00001d50 movl %eax, -16(%ebp)00001d53 movl -4(%ebp), %eax00001d56 shrl %eax00001d5e cmpl $0, %eax00001d61 jne 0x1d4500001d63 movl -16(%ebp), %eax00001d72 addl $16, %esp00001d75 popl %ebp00001d76 ret

_pcount_do:00001d30 pushl %ebp00001d31 movl %esp, %ebp00001d33 subl $16, %esp00001d36 movl 8(%ebp), %eax00001d39 movl %eax, -4(%ebp)00001d3c movl $0, -16(%ebp)00001d43 jmp 0x1d5e00001d45 movl -4(%ebp), %eax00001d48 andl $1, %eax00001d4b movl -16(%ebp), %ecx00001d4e addl %ecx, %eax00001d50 movl %eax, -16(%ebp)00001d53 movl -4(%ebp), %eax00001d56 shrl %eax00001d5e cmpl $0, %eax00001d61 jne 0x1d4500001d63 movl -16(%ebp), %eax00001d72 addl $16, %esp00001d75 popl %ebp00001d76 ret



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While version

while (Test) Bodywhile (Test) Body

Do-While Version

if (!Test) goto done; do Body while(Test);done:

if (!Test) goto done; do Body while(Test);done:

General “While” Translation

Goto Version

if (!Test) goto done;loop: Body if (Test) goto loop;done:

if (!Test) goto done;loop: Body if (Test) goto loop;done:

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C Code

“For” Loop Example

Is this code equivalent to other versions?

#define WSIZE 8*sizeof(int)int pcount_for(unsigned x) { int i; int result = 0; for (i = 0; i < WSIZE; i++) { unsigned mask = 1 << i; result += (x & mask) != 0; } return result;}


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“For” Loop Form

for (Init; Test; Update )

Body

General Form

for (i = 0; i < WSIZE; i++) { unsigned mask = 1 << i; result += (x & mask) != 0; }

for (i = 0; i < WSIZE; i++) { unsigned mask = 1 << i; result += (x & mask) != 0; }

i = 0i = 0

i < WSIZEi < WSIZE

i++i++

{ unsigned mask = 1 << i; result += (x & mask) != 0;}

{ unsigned mask = 1 << i; result += (x & mask) != 0;}

Init

Test

Update

Body

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“For” Loop While Loop


Body

For Version

Init;

while (Test ) {

Body

Update;

}

While Version

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“For” Loop … Goto


Body

For Version

Init;

while (Test ) {

Body

Update;

}

While Version

Init; if (!Test) goto done; do Body Update while(Test);done:

Init; if (!Test) goto done; do Body Update while(Test);done:

Init; if (!Test) goto done;loop: Body Update if (Test) goto loop;done:

Init; if (!Test) goto done;loop: Body Update if (Test) goto loop;done:

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C Code

“For” Loop Conversion Example

Initial test can be optimized away



Goto Version

int pcount_for_gt(unsigned x) { int i; int result = 0; i = 0; if (!(i < WSIZE)) goto done; loop: { unsigned mask = 1 << i; result += (x & mask) != 0; } i++; if (i < WSIZE) goto loop; done: return result;}

int pcount_for_gt(unsigned x) { int i; int result = 0; i = 0; if (!(i < WSIZE)) goto done; loop: { unsigned mask = 1 << i; result += (x & mask) != 0; } i++; if (i < WSIZE) goto loop; done: return result;}

Init

!Test

Body

UpdateTest

What if WSIZE < 0?Then compiler will recognize that the loop will never run and will eliminate code

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“For” Loop Conversion Example

Redundant instructions have been removed, so addresses are not correct.

Assembly Version00001d43 cmpl $0, %eax

00001d46 setne %al00001d49 andb $1, %al00001d4b movzbl %al, %eax00001d4e movl -20(%ebp), %edx00001d51 addl %edx, %eax00001d53 movl %eax, -20(%ebp)00001d56 movl -16(%ebp), %eax00001d59 addl $1, %eax00001d62 cmpl $31, %eax00001d65 jbe 0x1d2c00001d67 movl -20(%ebp), %eax00001d76 addl $24, %esp00001d79 popl %ebp00001d7a ret

00001d43 cmpl $0, %eax00001d46 setne %al00001d49 andb $1, %al00001d4b movzbl %al, %eax00001d4e movl -20(%ebp), %edx00001d51 addl %edx, %eax00001d53 movl %eax, -20(%ebp)00001d56 movl -16(%ebp), %eax00001d59 addl $1, %eax00001d62 cmpl $31, %eax00001d65 jbe 0x1d2c00001d67 movl -20(%ebp), %eax00001d76 addl $24, %esp00001d79 popl %ebp00001d7a ret

_pcount_for:00001d10 pushl %ebp00001d11 movl %esp, %ebp00001d13 subl $24, %esp00001d16 movl 8(%ebp), %eax00001d19 movl %eax, -4(%ebp)00001d1c movl $0, -20(%ebp)00001d23 movl $0, -16(%ebp)00001d2a jmp 0x1d5f00001d2c movl -16(%ebp), %eax00001d2f movl %eax, %ecx00001d31 movl $1, %eax00001d36 shll %cl, %eax00001d38 movl %eax, -24(%ebp)00001d3b movl -4(%ebp), %eax00001d3e movl -24(%ebp), %edx00001d41 andl %edx, %eax

_pcount_for:00001d10 pushl %ebp00001d11 movl %esp, %ebp00001d13 subl $24, %esp00001d16 movl 8(%ebp), %eax00001d19 movl %eax, -4(%ebp)00001d1c movl $0, -20(%ebp)00001d23 movl $0, -16(%ebp)00001d2a jmp 0x1d5f00001d2c movl -16(%ebp), %eax00001d2f movl %eax, %ecx00001d31 movl $1, %eax00001d36 shll %cl, %eax00001d38 movl %eax, -24(%ebp)00001d3b movl -4(%ebp), %eax00001d3e movl -24(%ebp), %edx00001d41 andl %edx, %eax

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Summary Today

Complete addressing mode, address computation (leal) Arithmetic operations Control: Condition codes Conditional branches & conditional moves Loops

Next Time Switch statements Stack Call / return Procedure call discipline

1 machine-level programming iv: control: loops comp 21000: introduction to computer organization...

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