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Machine-Level Programming II:
Control Flow
Sept. 12, 2002
Machine-Level Programming II:
Control Flow
Sept. 12, 2002
Topics Topics
n Condition Codes
l Setting l Testing
n Control Flow
l If-then-else l Varieties of Loops l Switch Statements class06.ppt
“The course that gives CMU its Zip!”
Condition Codes Condition Codes
Single Bit Registers Single Bit Registers
CF Carry Flag SF Sign Flag
ZF Zero Flag OF Overflow Flag
Implicitly Set By Arithmetic Operations Implicitly Set By Arithmetic Operations
addl Src , Dest
C analog: t = a + b
n CF set if carry out from most significant bit
l Used to detect unsigned overflow
n ZF set if t == 0
n SF set if t < 0
n OF set if two’s complement overflow
(a>0 && b>0 && t<0) || (a<0 && b<0 && t>=0)
Not Not Set bySet by lealleal instructioninstruction
Setting Condition Codes (cont.) Setting Condition Codes (cont.)
Explicit Setting by Compare InstructionExplicit Setting by Compare Instruction
cmpl Src2 , Src
n cmpl b,a like computing a-b without setting destination
n CF set if carry out from most significant bit
l Used for unsigned comparisons
n ZF set if a == b
n SF set if (a-b) < 0
n OF set if two’s complement overflow
(a>0 && b<0 && (a-b)<0) || (a<0 && b>0 && (a-b)>0)
Setting Condition Codes (cont.) Setting Condition Codes (cont.)
Explicit Setting by Test instruction Explicit Setting by Test instruction
testl Src2 , Src
n Sets condition codes based on value of Src1 & Src
l Useful to have one of the operands be a mask
n testl b,a like computing a&b without setting destination
n ZF set when a&b == 0
n SF set when a&b < 0
Reading Condition Codes Reading Condition Codes
SetX Condition Description
sete ZF Equal / Zero
setne ~ZF Not Equal / Not Zero
sets SF Negative
setns ~SF Nonnegative
setg ~(SF^OF)&~ZF Greater (Signed)
setge ~(SF^OF) Greater or Equal (Signed)
setl (SF^OF) Less (Signed)
setle (SF^OF)|ZF Less or Equal (Signed)
seta ~CF&~ZF Above (unsigned)
setb CF Below (unsigned)
SetXSetX InstructionsInstructions
n Set single byte based on combinations of condition codes
Reading Condition Codes (Cont.) Reading Condition Codes (Cont.)
SetX SetX InstructionsInstructions
n Set single byte based on
combinations of condition codes
n One of 8 addressable byte registers
l Embedded within first 4 integer registers l Does not alter remaining 3 bytes l Typically use movzbl to finish job
%eax
%edx
%ecx
%ebx
%esi
%edi
%esp
%ebp
%ah %al
%dh %dl
%ch %cl
%bh %bl
int gt (int x, int y) { return x > y; }
movl 12(%ebp),%eax # eax = y cmpl %eax,8(%ebp) # Compare x : y setg %al # al = x > y movzbl %al,%eax # Zero rest of %eax
Note inverted ordering!
Body
Jumping Jumping
jX Condition Description
jmp 1 Unconditional
je ZF Equal / Zero
jne ~ZF Not Equal / Not Zero
js SF Negative
jns ~SF Nonnegative
jg ~(SF^OF)&~ZF Greater (Signed)
jge ~(SF^OF) Greater or Equal (Signed)
jl (SF^OF) Less (Signed)
jle (SF^OF)|ZF Less or Equal (Signed)
ja ~CF&~ZF Above (unsigned)
jb CF Below (unsigned)
jXjX InstructionsInstructions
n Jump to different part of code depending on condition codes
Conditional Branch Example Conditional Branch Example
int max(int x, int y) { if (x > y) return x; else return y; }
_max: pushl %ebp movl %esp,%ebp
movl 8(%ebp),%edx movl 12(%ebp),%eax cmpl %eax,%edx jle L movl %edx,%eax L9:
movl %ebp,%esp popl %ebp ret
Body
Set Up
Finish
C Code
int fact_while (int x) { int result = 1; while (x > 1) { result *= x; x = x-1; }; return result; }
First Goto Version
int fact_while_goto (int x) { int result = 1; loop: if (!(x > 1)) goto done ; result *= x; x = x-1; goto loop ; done: return result; }
“While” Loop Example #1 “While” Loop Example
n Is this code equivalent to the do-while version?
n Must jump out of loop if test fails
C Code
int fact_while(int x) { int result = 1; while (x > 1) { result *= x; x = x-1; }; return result; }
Second Goto Version
int fact_while_goto (int x) { int result = 1; if (!(x > 1)) goto done ; loop: result *= x; x = x-1; if (x > 1) goto loop ; done: return result; }
Actual “While” Loop Translation Actual “While” Loop Translation
n Uses same inner loop
as do-while version
n Guards loop entry with
extra test
C Code
while ( Test ) Body
Do-While Version
if (! Test ) goto done ; do Body while( Test ); done :
General “While” Translation General “While” Translation
Goto Version
if (! Test ) goto done ; loop : Body if ( Test ) goto loop ; done :
“For” Loop Example “For” Loop Example
Algorithm Algorithm
n Exploit property that p = p 0 + 2 p 1 + 4 p 2 + … 2 n –1 pn –
n Gives: xp^ = z 0 · z 1 2 · ( z 2 2 ) 2 · … · (…(( zn –1^2 ) 2 )…) 2
zi = 1 when pi = 0 zi = x when pi = 1
n Complexity O(log p )
/* Compute x raised to nonnegative power p */ int ipwr_for(int x, unsigned p) { int result; for (result = 1; p != 0; p = p>>1) { if (p & 0x1) result = x; x = xx; } return result; }
n–1 times
Example
310 = 3^2 *** 3**^8
= 3^2 *** ((3**^2 ) 2 ) 2
ipwr ipwr ComputationComputation
/* Compute x raised to nonnegative power p */ int ipwr_for(int x, unsigned p) { int result; for (result = 1; p != 0; p = p>>1) { if (p & 0x1) result = x; x = xx; } return result; }
result x p 1 3 10 1 9 5 9 81 2 9 6561 1 531441 43046721 0
“For” Loop Example “For” Loop Example
for ( Init ; Test ; Update )
Body
int result; for (result = 1; p != 0; p = p>>1) { if (p & 0x1) result = x; x = xx; }
General Form
Init
result = 1
Test
p != 0
Update
p = p >> 1
Body
if (p & 0x1) result = x; x = xx; }
“For” “For” →→ “While”“While”
for ( Init ; Test ; Update )
Body
Init ; while ( Test ) { Body Update ; }
Goto Version
Init ; if (! Test ) goto done ; loop: Body Update ; if ( Test ) goto loop ; done:
For Version While Version
Do-While Version
Init ; if (! Test ) goto done ; do { Body Update ; } while ( Test ) done :
“For” Loop Compilation “For” Loop Compilation
Init
result = 1
Test
p != 0
Update
p = p >> 1
Body
if (p & 0x1) result = x; x = xx; }
Goto Version
Init ; if (! Test ) goto done ; loop : Body Update ; if ( Test ) goto loop ; done :
result = 1; if (p == 0) goto done ; loop : if (p & 0x1) result = x; x = xx; p = p >> 1; if (p != 0) goto loop ; done :
Jump Table Jump Table
Enumerated Values
ADD 0
MULT 1
MINUS 2
DIV 3
MOD 4
BAD 5
.section .rodata .align 4 .L57: .long .L51 #Op = 0 .long .L52 #Op = 1 .long .L53 #Op = 2 .long .L54 #Op = 3 .long .L55 #Op = 4 .long .L56 #Op = 5
Table Contents
.L51:
movl $43,%eax # ’+’ jmp .L .L52: movl $42,%eax # ’*’ jmp .L .L53: movl $45,%eax # ’-’ jmp .L .L54: movl $47,%eax # ’/’ jmp .L .L55: movl $37,%eax # ’%’ jmp .L .L56: movl $63,%eax # ’?’
Fall Through to .L
Targets & Completion
Switch Statement Completion Switch Statement Completion
PuzzlePuzzle
n What value returned when op is invalid?
AnswerAnswer
n Register %eax set to op at beginning of procedure
n This becomes the returned value
Advantage of Jump TableAdvantage of Jump Table
n Can do k -way branch in O ( 1 ) operations
.L49: # Done: movl %ebp,%esp # Finish popl %ebp # Finish ret # Finish
Object Code Object Code
Setup Setup
n Label .L49 becomes address 0x804875c
n Label .L57 becomes address 0x8048bc
08048718 : 8048718: 55 pushl %ebp 8048719: 89 e5 movl %esp,%ebp 804871b: 8b 45 08 movl 0x8(%ebp),%eax 804871e: 83 f8 05 cmpl $0x5,%eax 8048721: 77 39 ja 804875c 8048723: ff 24 85 c0 8b jmp *0x8048bc0(,%eax,4)
Object Code (cont.) Object Code (cont.)
Jump TableJump Table
n Doesn’t show up in disassembled code
n Can inspect using GDB
gdb code-examples
(gdb) x/6xw 0x8048bc
l E x amine 6 he x adecimal format “ w ords” (4-bytes each) l Use command “ help x ” to get format documentation
0x8048bc0 <_fini+32>:
0x
0x
0x
0x
0x
0x
Extracting Jump Table from Binary Extracting Jump Table from Binary
Jump Table Stored in Read Only Data Segment (. Jump Table Stored in Read Only Data Segment (.rodatarodata))
n Various fixed values needed by your code
Can examine with Can examine with objdumpobjdump
objdump code-examples –s –-section=.rodata
n Show everything in indicated segment.
Hard to read Hard to read
n Jump table entries shown with reversed byte ordering
n E.g., 30870408 really means 0x
Contents of section .rodata: 8048bc0 30870408 37870408 40870408 47870408 [email protected]... 8048bd0 50870408 57870408 46616374 28256429 P...W...Fact(%d) 8048be0 203d2025 6c640a00 43686172 203d2025 = %ld..Char = % …
Disassembled Targets Disassembled Targets
n movl %esi,%esi does nothing
n Inserted to align instructions for better cache performance
8048730: b8 2b 00 00 00 movl $0x2b,%eax 8048735: eb 25 jmp 804875c 8048737: b8 2a 00 00 00 movl $0x2a,%eax 804873c: eb 1e jmp 804875c 804873e: 89 f6 movl %esi,%esi 8048740: b8 2d 00 00 00 movl $0x2d,%eax 8048745: eb 15 jmp 804875c 8048747: b8 2f 00 00 00 movl $0x2f,%eax 804874c: eb 0e jmp 804875c 804874e: 89 f6 movl %esi,%esi 8048750: b8 25 00 00 00 movl $0x25,%eax 8048755: eb 05 jmp 804875c 8048757: b8 3f 00 00 00 movl $0x3f,%eax
Matching Disassembled Targets Matching Disassembled Targets
8048730: b8 2b 00 00 00 movl 8048735: eb 25 jmp 8048737: b8 2a 00 00 00 movl 804873c: eb 1e jmp 804873e: 89 f6 movl 8048740: b8 2d 00 00 00 movl 8048745: eb 15 jmp 8048747: b8 2f 00 00 00 movl 804874c: eb 0e jmp 804874e: 89 f6 movl 8048750: b8 25 00 00 00 movl 8048755: eb 05 jmp 8048757: b8 3f 00 00 00 movl
Entry
0x 0x 0x 0x 0x 0x
Sparse Switch Example Sparse Switch Example
n Not practical to use
jump table
l Would require 1000 entries
n Obvious translation into
if-then-else would have
max. of 9 tests
/* Return x/111 if x is multiple && <= 999. -1 otherwise */ int div111(int x) { switch(x) { case 0: return 0; case 111: return 1; case 222: return 2; case 333: return 3; case 444: return 4; case 555: return 5; case 666: return 6; case 777: return 7; case 888: return 8; case 999: return 9; default: return -1; } }
