1. Machine-Level Programming V: Control: loops Comp 21000: Introduction to Computer Organization & Systems Systems book chapter 3*
* Modified slides from the book “Computer Systems: a Programmer’s Perspective, 3rd ed.”, Randy Bryant & David O’Hallaron, 2015

2. Today Complete addressing mode, address computation (leal)
Arithmetic operations
x86-64
Control: Condition codes
Conditional branches and moves
Loops

3. “Do-While” Loop Example
C Code
Goto Version
int 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;
loop:
result += x & 0x1;
x >>= 1;
if (x)
goto loop;
return result; }
Count number of 1’s in argument x (“popcount”)
Use conditional branch to either continue looping or to exit loop

4. “Do-While” Loop Compilation
Goto Version
long pcount_goto
(unsigned long x) {
long result = 0;
loop:
result += x & 0x1;
x >>= 1;
if(x) goto loop;
return result; }
Register
Use(s)
%rdi
Argument x
%rax
result
movl $0, %eax # result = 0
.L2: # loop:
movq %rdi, %rdx
andl $1, %edx # t = x & 0x1
addq %rdx, %rax # result += t
shrq %rdi # x >>= 1
jne .L2 # if (x) goto loop
rep; ret

5. General “Do-While” Translation
C Code
Goto Version do
Body
while (Test);
loop:
Body
if (Test)
goto loop
Body:
Test returns integer
= 0 interpreted as false
≠ 0 interpreted as true {
Statement1;
Statement2; …
Statementn; }

6. General “While” Translation #1
“Jump-to-middle” translation
Used with –O0 or -Og
Goto Version
goto test;
loop:
Body
test:
if (Test)
goto loop;
done:
While version
while (Test)
Body

7. While Loop Example #1 C Code Jump to Middle Version
long pcount_while
(unsigned long x) {
long result = 0;
while (x) {
result += x & 0x1;
x >>= 1; }
return result;
long pcount_goto_jtm
(unsigned long x) {
long result = 0;
goto test;
loop:
result += x & 0x1;
x >>= 1;
test:
if(x) goto loop;
return result; }
Compare to do-while version of function
Initial goto starts loop at test

8. General “While” Translation #2
While version
“Do-while” conversion
Used with –O1
while (Test)
Body
Goto Version
Do-While Version
if (!Test)
goto done;
loop:
Body
if (Test)
goto loop;
done:
if (!Test)
goto done; do
Body
while(Test);
done:

9. While Loop Example #2 C Code Do-While Version
long pcount_while
(unsigned long x) {
long result = 0;
while (x) {
result += x & 0x1;
x >>= 1; }
return result;
long pcount_goto_dw
(unsigned long x) {
long result = 0;
if (!x) goto done;
loop:
result += x & 0x1;
x >>= 1;
if(x) goto loop;
done:
return result; }
Compare to do-while version of function
Initial conditional guards entrance to loop

10. “For” Loop Example C Code Is this code equivalent to other versions?
#define WSIZE 8*sizeof(int)
int pcount_for(unsigned x) {
int i;
long result = 0;
for (i = 0; i < WSIZE; i++) {
unsigned bit = (x >> i) & 0x1;
result += bit; }
return result;
Is this code equivalent to other versions?

11. for (Init; Test; Update )
“For” Loop Form
Init
i = 0
General Form
Test
for (Init; Test; Update )
Body
i < WSIZE
Update
i++
for (i = 0; i < WSIZE; i++) {
unsigned bit = (x >> i) & 0x1;
result += bit; }
Body {
unsigned bit = (x >> i) & 0x1;
result += bit;}

12. for (Init; Test; Update )
“For” Loop  While Loop
For Version
for (Init; Test; Update )
Body
While Version
Init;
while (Test ) {
Body
Update; }

13. For-While Conversion Init Test Update Body long pcount_for_while
(unsigned long x) {
size_t i;
long result = 0;
i = 0;
while (i < WSIZE)
unsigned bit =
(x >> i) & 0x1;
result += bit;
i++; }
return result;
Init
i = 0
Test
i < WSIZE
Update
i++
Body {
unsigned bit =
(x >> i) & 0x1;
result += bit; }

14. “For” Loop  …  Goto For Version goto Version While Version
Init;
if (!Test)
goto done;
loop:
Body
Update
if (Test)
goto loop;
done:
For Version
goto Version
for (Init; Test; Update )
Body
While Version
Do-While Version
Init;
while (Test ) {
Body
Update; }
Init;
if (!Test)
goto done; do
Body
Update
while(Test);
done:

15. “For” Loop Do-While Conversion
Goto Version
C Code
long pcount_for_goto_dw
(unsigned long x) {
size_t i;
long result = 0;
i = 0;
if (!(i < WSIZE))
goto done;
loop: {
unsigned bit =
(x >> i) & 0x1;
result += bit; }
i++;
if (i < WSIZE)
goto loop;
done:
return result;
long pcount_for
(unsigned long x) {
size_t i;
long result = 0;
for (i = 0; i < WSIZE; i++)
unsigned bit =
(x >> i) & 0x1;
result += bit; }
return result;
Init
!Test
Body
Update
Test
Initial test can be optimized away; i always < WSIZE since WSIZE is a constant

16. “For” Loop Do-While Conversion
Goto Version
C Code
long pcount_for_goto_dw
(unsigned long x) {
size_t i;
long result = 0;
i = 0;
if (!(i < WSIZE))
goto done;
loop: {
unsigned bit =
(x >> i) & 0x1;
result += bit; }
i++;
if (i < WSIZE)
goto loop;
done:
return result;
long pcount_for
(unsigned long x) {
size_t i;
long result = 0;
for (i = 0; i < WSIZE; i++)
unsigned bit =
(x >> i) & 0x1;
result += bit; }
return result;
Init
!Test
Body
Update
Test
Initial test can be optimized away
What if WSIZE < 0?
Then compiler will recognize that the loop will never run and will eliminate code

17. “For” Loop Conversion Example
Assembly Version
_pcount_for:
0x040052d <+0>: push %rbp
0x040052e <+1>: mov %rsp,%rbp
0x <+4>: mov %rdi,-0x28(%rbp)
0x <+8>: movq $0x0,-0x8(%rbp)
0x040053d <+16>: movq $0x0,-0x10(%rbp)
0x <+24>: jmp 0x <pcount_for+60>
0x <+26>: mov -0x10(%rbp),%rax
0x040054b <+30>: mov -0x28(%rbp),%rdx
0x040054f <+34>: mov %eax,%ecx
0x <+36>: shr %cl,%rdx
0x <+39>: mov %rdx,%rax
0x <+42>: and $0x1,%eax
0x040055a <+45>: mov %eax,-0x14(%rbp)
0x040055d <+48>: mov -0x14(%rbp),%eax
0x <+51>: add %rax,-0x8(%rbp)
0x <+55>: addq $0x1,-0x10(%rbp)
0x <+60>: cmpq $0x3f,-0x10(%rbp)
0x040056e <+65>: jbe 0x <pcount_for+26>
0x <+67>: mov -0x8(%rbp),%rax
0x <+71>: pop %rbp
0x <+72>: retq
Note that the compiler changed 64 to 63 and < to <=

18. Summary Today Next Time
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