1. Carnegie Mellon
Ithaca College
Machine-Level Programming IV Control Comp 21000: Introduction to Computer Organization & Systems Systems book chapter 3*

2. Today Control: Condition codes Conditional branches Loops
Ithaca College
Today
Control: Condition codes
Conditional branches
If-then-else statements
Conditional moves
Loops
Switch Statements

3. Conditional Branch Example (Old Style)
Ithaca College
Conditional Branch Example (Old Style)
Generation
barr> gcc –Og -S –fno-if-conversion control.c
absdiff:
cmpq %rsi, %rdi # x:y
jle .L4
movq %rdi, %rax
subq %rsi, %rax
ret
.L4: # x <= y
movq %rsi, %rax
subq %rdi, %rax
long absdiff
(long x, long y) {
long result;
if (x > y)
result = x-y;
else
result = y-x;
return result; }
Register
Use(s)
%rdi
Argument x
%rsi
Argument y
%rax
Return value

4. Expressing with Goto Code
Ithaca College
Expressing with Goto Code
C allows goto statement
Jump to position designated by label
long absdiff
(long x, long y) {
long result;
if (x > y)
result = x-y;
else
result = y-x;
return result; }
long absdiff_j
(long x, long y) {
long result;
int ntest = (x <= y);
if (ntest) goto else;
result = x-y;
goto Done;
else:
result = y-x;
Done:
return result; }
Generally considered bad coding style

5. Expressing with Goto Code
Ithaca College
Expressing with Goto Code
C allows goto statement
Jump to position designated by label
absdiff:
cmpq %rsi, %rdi # x:y
jle .L4
movq %rdi, %rax
subq %rsi, %rax
ret
.L4: # x <= y
movq %rsi, %rax
subq %rdi, %rax
long absdiff_j
(long x, long y) {
long result;
int ntest = (x <= y);
if (ntest) goto else;
result = x-y;
goto Done;
else:
result = y-x;
Done:
return result; }
Register
Use(s)
%rdi
Argument x
%rsi
Argument y
%rax
Return value

6. Expressing with Goto Code
Ithaca College
Expressing with Goto Code
C allows goto statement
Jump to position designated by label
absdiff:
cmpq %rsi, %rdi # x:y
jle .L4
movq %rdi, %rax
subq %rsi, %rax
ret
.L4: # x <= y
movq %rsi, %rax
subq %rdi, %rax
long absdiff_j
(long x, long y) {
long result;
int ntest = (x <= y);
if (ntest) goto else;
result = x-y;
goto Done;
else:
result = y-x;
Done:
return result; }
Register
Use(s)
%rdi
Argument x
%rsi
Argument y
%rax
Return value

7. Expressing with Goto Code
Ithaca College
Expressing with Goto Code
C allows goto statement
Jump to position designated by label
absdiff:
cmpq %rsi, %rdi # x:y
jle .L4
movq %rdi, %rax
subq %rsi, %rax
ret
.L4: # x <= y
movq %rsi, %rax
subq %rdi, %rax
long absdiff_j
(long x, long y) {
long result;
int ntest = (x <= y);
if (ntest) goto else;
result = x-y;
goto Done;
else:
result = y-x;
Done:
return result; }
Register
Use(s)
%rdi
Argument x
%rsi
Argument y
%rax
Return value

8. Expressing with Goto Code
Ithaca College
Expressing with Goto Code
C allows goto statement
Jump to position designated by label
absdiff:
cmpq %rsi, %rdi # x:y
jle .L4
movq %rdi, %rax
subq %rsi, %rax
ret
.L4: # x <= y
movq %rsi, %rax
subq %rdi, %rax
long absdiff_j
(long x, long y) {
long result;
int ntest = (x <= y);
if (ntest) goto else;
result = x-y;
goto Done;
else:
result = y-x;
Done:
return result; }
Register
Use(s)
%rdi
Argument x
%rsi
Argument y
%rax
Return value

9. Example 2 Copy ifStmt.c from home/barr/Student/comp210/examples/
gcc –O0 –o ifStmt –fno-if-conversion ifStmt.c
gdb ifStmt
In gdb: disass testIf1
int testIf1(int x, int y) {
int result;
int ntest = (x >= y);
if (ntest) goto else;
result = x * 4;
goto Done;
else:
result = y * 4;
Done:
return result; }
int testIf1(int x, int y) {
int result;
if (x < y)
result = x * 4;
else
result = y * 4;
return result; }

10. Example 2 continued… Register Use(s) %rdi Argument x %rsi Argument y
%rax
Return value
pushq %rbp movq %rsp, %rbp movq %rdi, -18(%rbp) movq %rsi, -20(%rbp) movq -18(%rbp), %rax cmpq -20(%rbp), %rax jge LBBO_2 shlq $2, %rax movq %rax, -8(%rbp) jmp LBB0_3 LBB0_2: movq -20(%rbp), %rax LBB0_3: movq -8(%rbp), %rax popq %rbp retq
int testIf1
(int x, int y) {
int result;
int ntest = (x >= y);
if (ntest) goto else;
result = x * 4;
goto Done;
else:
result = y * 4;
Done:
return result; }
LBBO_2 is
0x40054a <testIf1+36>
LBBO_3 is
0x <testIf1+48>

11. Setting up the runtime stack
Example 2 continued…
Register
Use(s)
%rdi
Argument x
%rsi
Argument y
%rax
Return value
pushq %rbp movq %rsp, %rbp movq %rdi, -18(%rbp) movq %rsi, -20(%rbp) movq -18(%rbp), %rax cmpq -20(%rbp), %rax jge LBBO_2 shlq $2, %rax movq %rax, -8(%rbp) jmp LBB0_3 LBB0_2: movq -20(%rbp), %rax LBB0_3: movq -8(%rbp), %rax popq %rbp retq
Setting up the runtime stack
int testIf1
(int x, int y) {
int result;
int ntest = (x >= y);
if (ntest) goto else;
result = x * 4;
goto Done;
else:
result = y * 4;
Done:
return result; }
if x - y >= 0
jump
calculate x=x*4
and jump
Calculate y=y*4
LBBO_2 is
0x40054a <testIf1+36>
LBBO_3 is
0x <testIf1+48>
Set up the return value

12. General Conditional Expression Translation (Using Branches)
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General Conditional Expression Translation (Using Branches)
C Code
val = Test ? Then_Expr : Else_Expr;
val = x>y ? x-y : y-x;
Test Is an expression returning an integer
= 0 interpreted as false
≠ 0 interpreted as true
Create separate code regions for then & else expressions
Execute appropriate one
Goto Version
ntest = !Test;
if (ntest) goto Else;
val = Then_Expr;
goto Done;
Else:
val = Else_Expr;
Done:
. . .