MachineLevel Representation of Programs IV

1
Machine-Level Representation of Programs IV
2
Outline

• Translate Control constructs in C to assembly
• Suggested reading
• Chap 3.6,

3
Control

• Two of the most important parts of program
  execution
• Data flow (Accessing and operating data)
• Control flow (control the sequence of operations)

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Control

• Sequential execution is default
• The statements in C and
• the instructions in assembly code
• are executed in the order they appear in the
  program
• Chang the control flow
• Control constructs in C
• Jump in assembly

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Goto Constructs in C

• goto L
• L
• break
• continue

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Jump Instructions

• Under normal execution
• instructions follow each other in the order they
  are listed
• A jump instruction can cause
• the execution to switch to a completely new
  position in the program.
• Label
• Jump destinations

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Jump Instructions

• 1 xorl eax, eax Set eax to 0
• 2 jmp .L1 Goto .L1
• 3 movl (eax), edx Null pointer
  dereference
• 4 .L1
• 5 popl edx

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Unconditional jump

• Jumps unconditionally
• Direct jump jmp label
• jmp .L
• Indirect jump jmp Operand
• jmp eax
• jmp (eax)

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Other Control Constructs in C

• if () else
• while ()
• do while ()
• for (init test incr)
• switch ()
• What are the counterparts in assembly?
• Conditional jump

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Assembly Programmers View
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Condition codes

• Condition codes
• A set of single-bit
• Maintained in a condition code register
• Describe attributes of the most recently
  arithmetic or logical operation

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Condition codes

• EFLAGS
• CF Carry Flag
• The most recent operation generated a carry out
  of the most significant bit
• Used to detect overflow for unsigned operations
• OF Overflow Flag
• The most recent operation caused a twos
  complement overflow either negative or positive

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Condition codes

• EFLAGS
• ZF Zero Flag
• The most recent operation yielded zero
• SF Sign Flag
• The most recent operation yielded a negative value

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Setting Conditional Codes

• Implicit Setting By Arithmetic Operations
• addl Src,Dest
• C analog t ab
• CF set if carry out from most significant bit
• Used to detect unsigned overflow
• ZF set if t 0
• SF set if t lt 0
• OF set if twos complement overflow
• (agt0 bgt0 tlt0) (alt0 blt0 tgt0)

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Conditional Code

• lea instruction
• has no effect on condition codes
• Xorl instruction
• The carry and overflow flags are set to 0
• Shift instruction
• carry flag is set to the last bit shifted out
• Overflow flag is set to 0

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Setting Conditional Codes

• Explicit Setting by Compare Instruction
• cmpl Src2,Src1
• cmpl b,a like computing a-b without setting
  destination
• CF set if carry out from most significant bit
• Used for unsigned comparisons
• ZF set if a b
• SF set if (a-b) lt 0
• OF set if twos complement overflow
• (agt0 blt0 (a-b)lt0)
• (alt0 bgt0 (a-b)gt0)

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Setting Conditional Codes

• Explicit Setting by Test instruction
• testl Src2,Src1
• Sets condition codes based on value of Src1
  Src2
• Useful to have one of the operands be a mask
• testl b,a like computing ab without setting
  destination
• ZF set when ab 0
• SF set when ab lt 0

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Accessing Conditional Codes

• The condition codes cannot be read directly
• One of the most common methods of accessing them
  is
• setting an integer register based on some
  combination of condition codes
• Set commands

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Accessing Conditional Codes

• After each set command is executed
• A single byte to 0 or to 1 is obtained
• The descriptions of the different set commands
  apply to the case
• where a comparison instruction has been executed

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Accessing Conditional Codes
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Accessing Conditional Codes

• The destination operand is either
• one of the eight single-byte register elements or
  
• a memory location where the single byte is to be
  stored
• To generate a 32-bit result
• we must also clear the high-order 24 bits

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Accessing Conditional Codes

• Initially a is in edx, b is in eax
• 1 cmpl eax, edx compare ab
• 2 setl al set low order by to 0 or 1
• 3 movzbl al, eax
• set remaining bytes of eax to 0

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Conditional jump

• Either jump or continue executing at the next
  instruction in the code sequence
• Depending on some combination of the condition
  codes
• All direct jump

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Jump Instructions

• 1 jle .L4
• 2 .p2align 4,,7 align next instruction to
  multiple of 8
• 3 .L5
• movl edx, eax
• sarl 1, eax
• subl eax, edx
• testl edx, edx
• jg .L5
• 9 .L4
• 10 movl edx, eax

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Translating Conditional Branches
t test-expr if ( t ) goto true
else-statement goto done true then-statement
done
if ( test-expr ) then-statement
else else-statement
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Translating Conditional Branches
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Jump Instructions

• movl 8(ebp), edx get x
• movl 12(ebp), eax get y
• cmpl eax, edx cal x - y
• jl .L3 if x lt y goto less
• subl eax, edx compute x - y
• movl edx, eax set return val
• jmp .L5 goto done
• .L3 less
• subl edx, eax compute y x
• .L5 done Begin Completion code

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Do-while Translation

• do
• body-statement
• while (test-expr)
• loop
• body-statement
• t test-expr
• if ( t )
• goto loop

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Do-while Translation
.L6 lea (ebx, edx), eax movl edx,
ebx movl eax, edx incl ecx cmpl
esi, ecx jl .L6 movl ebx, eax

• int fib_dw(int n)
• int i 0
• int val 0
• int nval 1
• do
• int t val nval
• val nval
• nval t
• i
• while ( iltn)
• return val

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While Loop Translation

• while (test-expr)
• body-statement
• loop if ( !test-expr)
• t test-expr goto done
• if ( !t ) do
• goto done body-statement
• body-statement while(test-expr)
• goto loop done
• done

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While Loop Translation

• int fib_w(int n)
• int i1
• int val1
• int nval1
• while ( iltn )
• int tvalnval
• val nval
• nval t
• return val

int fib_w_got0(int n) int val1 int
nval1 int nmi, t if ( val gt n ) goto
done nmi n-1 loop tvalnval val
nval nval t nmi-- if ( nmi ) goto
loop done return val
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While Loop Translation

• movl 8(ebp), eax
• movl 1, ebx
• movl 1, ecx
• cmpl eax, ebx
• jge .L9
• lea 1(eax), edx
• .L10
• lea (ecx, ebx), eax
• movl ecx, ebx
• movl eax, ecx
• decl edx
• jnz .L10
• .L9