05 more about conditionals 09 49 tủ tài liệu bách khoa

University
 of
 Washington
 

Condi3onal
 Branch
 Example
 
int absdiff(int x, int y)
{
int result;
if (x > y) {
result = x-y;
} else {
result = y-x;
}
return result;
}

absdiff:
pushl
movl
movl
movl
cmpl
jle
subl
movl
.L8:
leave

ret
.L7:
subl
jmp

x86
 

%ebp
%esp, %ebp
8(%ebp), %edx
12(%ebp), %eax
%eax, %edx
.L7
%eax, %edx
%edx, %eax

Setup
 

Body1
 

Finish
 
%edx, %eax
.L8

Body2
 

University
 of
 Washington
 

Condi3onal
 Branch
 Example
 (Cont.)
 
int goto_ad(int x, int y)
{
int result;
if (x <= y) goto Else;
result = x-y;
Exit:
return result;
Else:
result = y-x;
goto Exit;
}

int absdiff(int x, int y)
{
int result;
if (x > y) {
result = x-y;
} else {

result = y-x;
}
return result;
}

¢ 

¢ 

x86
 

C
 allows
 “goto”
 as
 means
 of
 
transferring
 control
 
§  Closer
 to
 machine-­‐level
 
programming
 style
 
Generally

 considered
 bad
 coding
 
style
 


University
 of
 Washington
 

Condi3onal
 Branch
 Example
 (Cont.)
 
absdiff:
pushl
movl
movl
movl
cmpl
jle
subl
movl
.L8:
leave
ret

.L7:
subl
jmp

int goto_ad(int x, int y)
{
int result;
if (x <= y) goto Else;
result = x-y;
Exit:
return result;
Else:
result = y-x;
goto Exit;
}

int x
int y

%edx
%eax

x86
 

%ebp
%esp, %ebp
8(%ebp), %edx
12(%ebp), %eax
%eax, %edx

.L7
%eax, %edx
%edx, %eax

%edx, %eax
.L8


University
 of
 Washington
 

Condi3onal
 Branch
 Example
 (Cont.)
 
absdiff:
pushl
movl
movl
movl
cmpl
jle
subl
movl
.L8:
leave
ret

.L7:
subl
jmp

int goto_ad(int x, int y)
{
int result;
if (x <= y) goto Else;
result = x-y;
Exit:
return result;
Else:
result = y-x;
goto Exit;
}

int x
int y

%edx
%eax

x86
 

%ebp
%esp, %ebp
8(%ebp), %edx
12(%ebp), %eax
%eax, %edx

.L7
%eax, %edx
%edx, %eax

%edx, %eax
.L8


University
 of
 Washington
 

Condi3onal
 Branch
 Example
 (Cont.)
 
absdiff:
pushl
movl
movl
movl
cmpl
jle
subl
movl
.L8:
leave
ret

.L7:
subl
jmp

int goto_ad(int x, int y)
{
int result;
if (x <= y) goto Else;
result = x-y;
Exit:
return result;
Else:
result = y-x;
goto Exit;
}

int x
int y

%edx
%eax

x86
 

%ebp
%esp, %ebp
8(%ebp), %edx
12(%ebp), %eax
%eax, %edx

.L7
%eax, %edx
%edx, %eax

%edx, %eax
.L8


University
 of
 Washington
 

Condi3onal
 Branch
 Example
 (Cont.)
 
absdiff:
pushl
movl
movl
movl
cmpl
jle
subl
movl
.L8:
leave
ret

.L7:
subl
jmp

int goto_ad(int x, int y)
{
int result;
if (x <= y) goto Else;
result = x-y;
Exit:
return result;
Else:
result = y-x;
goto Exit;
}

int x
int y

%edx
%eax

x86
 

%ebp
%esp, %ebp
8(%ebp), %edx
12(%ebp), %eax
%eax, %edx

.L7
%eax, %edx
%edx, %eax

%edx, %eax
.L8


University
 of
 Washington
 

Condi3onal
 Branch
 Example
 (Cont.)
 
absdiff:
pushl
movl
movl
movl
cmpl
jle
subl
movl
.L8:
leave
ret

.L7:
subl
jmp

int goto_ad(int x, int y)
{
int result;
if (x <= y) goto Else;
result = x-y;
Exit:
return result;
Else:
result = y-x;
goto Exit;
}

int x
int y

%edx
%eax

x86
 

%ebp
%esp, %ebp
8(%ebp), %edx
12(%ebp), %eax
%eax, %edx

.L7
%eax, %edx
%edx, %eax

%edx, %eax
.L8


University
 of
 Washington
 

General
 Condi3onal
 Expression
 Transla3on
 
C
 Code
 
val = Test ? Then-­‐Expr : Else-­‐Expr;
result = x>y ? x-y : y-x;

if (Test)
val = Then-­‐Expr;
else
val = Else-­‐Expr;

§  Test

 is
 expression
 returning
 integer
 

Goto
 Version
 

=
 0
 interpreted
 as
 false
 
≠0
 interpreted
 as
 true
 
§  Create
 separate
 code
 regions
 for
 
then
 &
 else

 expressions
 
§  Execute
 appropriate
 one
 

nt = !Test;
if (nt) goto Else;
val = Then-­‐Expr;
Done:
. . .
Else:
val = Else-­‐Expr;
goto Done;

§  How
 might
 you
 make
 this
 more
 
efficient?
 

x86
 

University
 of
 Washington
 

Condi3onals:
 x86-­‐64
 
int absdiff(
int x, int y)
{
int result;
if (x > y) {
result = x-y;
} else {
result = y-x;
}
return result;
}
¢ 

absdiff:
movl
movl
subl
subl
cmpl
cmovle
ret

# x in %edi, y in %esi
%edi, %eax # eax = x
%esi, %edx # edx = y
%esi, %eax # eax = x-y
%edi, %edx # edx = y-x
%esi, %edi # x:y
%edx, %eax # eax=edx if <=

Condi3onal
 move
 instruc3on
 
§ 
§ 
§ 
§ 

cmovC
 
 src,
 dest
 
Move
 value
 from
 src
 to
 dest
 if
 condiBon

 C
 holds
 
More
 efficient
 than
 condiBonal
 branching
 (simple
 control
 flow)
 
But
 overhead:
 both
 branches
 are
 evaluated
 
x86
 


University
 of
 Washington
 

PC
 Rela3ve

 Addressing
 
0x100
0x102
0x104
…
0x172
l 
l 

cmp
je
…
…
add

r2, r3
0x70
r3, r4

PC
 rela3ve
 branches
 are
 relocatable
 
Absolute
 branches
 are
 not

 

x86
 

0x1000
0x1002
0x1004
…
0x1072