Examples of Basic Blocks

1
Examples of Basic Blocks

• Following sequence of three-address statements
  forms a basic block
• t1 a a
• t2 a b
• t3 2 t2
• t4 t1 t3
• t5 b b
• t6 t4 t5
• It computes aa 2ab bb
• Given 3-AS x yz
• -it defines X
• -it uses (reference) y and z
• Live name A name is a basic block is said to be
  live at a given point if its value is used after
  that point in the program, perhaps in another
  basic block.

t7

t5
t4


t1
t3


b
b
t2

2
a
a
a
b
a b
t7
t6
2
Examples of 3 address code translation

• C-code
• void quicksort(m,n)
• int m,n
• int i,j
• int v,x
• if (nltm) return
• / fragment begins /
• im-1 jn van
• while(1)
• do ii1 while(ailtv)
• do jj-1 while(ajgtv)
• if (igtj) break
• xaiaiananx
• xaiaiananx
• / fragment ends /
• quicksort( m ,j) quicksort(l1,n)

• 3 Address Code.
• i m-1 j n v an
• (1) i m-l jn t1 4n v at1
• while(1)
• do i i 1 while(ailtv)
• (5) i i1 t2 4i t3 at2
• (8) if t3ltv goto(5)
• do j j-1 while (aj gt v)
• jj1 t44j t5at4
• (12) if t5 gtv goto (9)
• (13) if( i gtj) goto (23)
• x ai ai an anx
• (14) t6 4i xat6
• (16) t7 4i xat6
• (18) t9 at8 at7t9
• (20) t10 4j at10 x
• (22) goto(5)
• (23) ...

3
Example of Flow graphs
im-1 jn t14n v at1
B1
Ii1 t24i t3at2 if( t3ltv) goto B2
B2
jj-1 t4 4 j t5 at4 if( t5 gtv ) goto B3
B3
B4
if(igtj) goto B6
t11 4 i X at11 t12 4 i t13 4n t14
at13 at12 t14 t15 4 n at15 x
B6
t64i x at6 t7 4 i t8 4 j t9
at8 at7 t9 t10 4 j at10x goto B2
B5
4
Partition into Basic Blocks algorithm

• Input A sequence of 3 address statements
• Output A sequence of basic blocks with each 3A
  Statement in exactly
• one block.
• Method
• (1) First determine a set of leaders, the 1st
  statement of basic blocks
• a) The first statement is a leader
• b) Any statement that is a target of a
  conditional or
• unconditional goto is a
  leader
• c) Any statement that immediately follows a
  goto, or conditional
• goto statement is a leader.
• (2) For each leader its basic block consists
  of
• a) The leader
• b) All statements upto but not including the
  next leader or the
• end of the program.

5
Example of Partition into Basic Blocks

• (1) prod 0
• (2) i 1
• (3) t1 4 i
• (4) t2 at1
• (5) t3 4 i
• (6) t4 bt3
• (7) t5 t2 t4
• (8) t6 prod t5
• (9) prod t6
• (10) t7 i1
• (11) i t7
• (12) if( i lt20) goto (3)

6
Example of Partition into Basic Blocks
A leader by rule 1.a A block by rule 2 A leader
by rule 1.b A block by rule 2 A leader by
rule 1.c
(1) prod 0 (2) i 1 (3) t1 4
i (4) t2 at1 (5) t3 4 i (6)
t4 bt3 (7) t5 t2 t4 (8) t6
prod t5 (9) prod t6 (10) t7 i1 (11)
i t7 (12) if( i lt20) goto (3)
B1
B2