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Examples of Basic Blocks

Examples of Basic Blocks. t7. +. t5. t4. *. +. t1. t3. *. *. b. b. t2. 2. *. a. a. a. b. a b. 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

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Examples of Basic Blocks

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  1. Examples of Basic Blocks t7 + t5 t4 * + t1 t3 * * b b t2 2 * a a a b ab 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: a*a + 2*a*b + b*b Given 3-AS x := y+z: -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 t6

  2. 3 Address Code. • i = m-1; j = n ; v = a[n]; • (1) i := m-l; j:=n; t1 := 4*n; v := a[t1]; • while(1) { • do i = i + 1; while(a[i]<v) ; • (5) i := i+1; t2 := 4*i; t3 := a[t2]; • (8) if t3<v goto(5) • do j = j-1; while (a[j] > v); • j:=j+1; t4:=4*j ; t5:=a[t4]; • (12) if t5 >v goto (9) • (13) if( i >=j) goto (23) • x = a[i]; a[i] = a[n]; a[n]=x • (14) t6 := 4*i; x:=a[t6]; • (16) t7 := 4*i; x:=a[t6]; • (18) t9:= a[t8]; a[t7]:=t9; • (20) t10:= 4*j; a[t10]:= x • (22) goto(5) • (23) ... Examples of 3 address code translation C-code void quicksort(m,n) int m,n { int i,j; int v,x; if (n<=m) return; /* fragment begins */ i:=m-1; j=n ; v=a[n]; while(1) { do i=i+1; while(a[i]<v) do j=j-1; while(a[j]>v) if (i>=j) break; x=a[i];a[i]=a[n];a[n]=x;} x=a[i];a[i]=a[n];a[n]=x; /* fragment ends */ quicksort( m ,j); quicksort(l+1,n); }

  3. i:=m-1 j:=n t1=4*n v:= a[t1] B1 Example of Flow graphs I=i+1 t2:=4*i t3:=a[t2] if( t3<v) goto B2 B2 j:=j-1 t4:= 4 * j t5:= a[t4] if( t5 >v ) goto B3 B3 B4 if(i>=j) goto B6 t11:= 4* i X= a[t11] t12:= 4 * i t13:= 4*n t14 := a[t13] a[t12]:= t14 t15 := 4 * n a[t15]:= x B6 t6:=4*i x:= a[t6] t7 := 4 * i t8 := 4 * j t9:= a[t8] a[t7]:= t9 t10:= 4 * j a[t10]:=x 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 : = a[t1] (5) t3 : = 4 * i (6) t4 : = b[t3] (7) t5 : = t2 * t4 (8) t6 : = prod + t5 (9) prod : = t6 (10) t7 : = i+1 (11) i : = t7 (12) if( i <=20) 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 : = a[t1] (5) t3 : = 4 * i (6) t4 : = b[t3] (7) t5 : = t2 * t4 (8) t6 : = prod + t5 (9) prod : = t6 (10) t7 : = i+1 (11) i : = t7 (12) if( i <=20) goto (3) B1 B2

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