Backpatching II

1
Backpatching II

• Prepared by
• Shreyas Ashok Karkhedkar
• Roll Number 04CS3013

2
Backpatching II

• Consider the following grammar
• E ? E1 or ME2
• E ? E1 and ME2
• E ? not E1
• E ? (E1)
• E ? id1 relop id2
• E ? false
• E ? true
• M ? e

3
Backpatching II

• The corresponding semantic rules are given by
• E ? E1 or ME2
• Backpatch( E1.falselist,M.quad)
• E.truelistmerge(E2.truelist,E1.truelist)
• E.falselistE2.falselist

4
Backpatching II

• E ? E1 and ME2
• Backpatch(E1.truelist,M.quad)
• E.falselistmerge(E1.falselist,E2.falselist)
• E.truelistE2.truelist

5
Backpatching II

• E ? not E1
• E.truelist E1.falselist
• E.falselist E1.truelist

6
Backpatching II

• E ? (E1)
• E.truelist E1.truelist

7
Backpatching II

• E ? id1 relop id2
• E.truelist makelist(nextquad)
• E.falselist makelist(nextquad 1)
• emit( if id1 relop id2 goto ____)
• emit( goto ____)

8
Backpatching II

• E ? false
• E.falselist makelist(nextquad)
• emit( goto ____)
• E ? true
• E.truelist makelist(nextquad)
• emit( goto ____)

9
Backpatching II

• M ? e
• M.quad nextquad

10
Backpatching II

• Example
• Consider the string
• altb or cltd and eltf
• (Assuming that the grammar is left associative)

11
Backpatching II

• The corresponding intermediate code is
• 100 if ( altb ) goto L3
• goto L1
• 102 L1 if ( cltd ) goto L2
• goto L4
• 104 L2 if ( eltf ) goto L3
• goto L4
• L3 ..
• ..
• L4 ..
• ..

12
Backpatching II

• The code after backpatching becomes
• 100 if ( altb ) goto ____
• goto 102
• 102 L1 if ( cltd ) goto 104
• goto ____
• 104 L2 if ( eltf ) goto ____
• goto ____
• L3 ..
• ..
• L4 ..
• ..
• This code is more similar to the machine code

13
Backpatching II

• We define the function Backpatch (X , Y) as
  follows
• Backpatch (X , Y) replace the line numbers
  listed in X with the value Y
• Note We try to design the function Backpatch
  in such a way that Y is unique for every X