This is our compiler design presentation. The presentation topic is Basic Blocks. Hope you enjoy it...

1. G r e a t
M a t e s

2. W E L C O M E !
Thank you for coming today!

3. Fazle Rabbi Khan
161-15-6917

4. 4
BASIC BLOCKS ANDFLOW
GRAPHS

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What is Basic Blocks?
A basic block is a sequence of consecutive instructions
which are always executed in sequence without halt or
possibility of branching.

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Type of Basic Blocks
Basic Blocks
Transformations on Basic Blocks
Structure preserving Transformations
Algebraic transformations
Representation of Basic Blocks

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Steps of Basic Blocks
Introduce a graph representation of three address statement that is helpful
for discussing code generation.
Nodes in flow graph – Computation
Edges in flow graph – Control Flow
Register Assignment to find inner loop flow control is used.
 Partition the intermediate code into basic blocks
 The basic blocks become the nodes of a flow graph, whose edges indicate which blocks can follow
which other blocks.

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Basic Blocks
B2:
The flow graph for the above three address code is givenbelow:
B1:
PROD = 0
I = 1
T2=addr(A)-4
T4 =addr(B)-4
T1 =4*I
T3= T2[T1]
T5 = T4[T1]
T6 =T3*T5
PROD =PROD + T6
I =I+1
IF I<= 20 GOTO B2

9. Abdur Rahman
162-15-7749

10. 10
Basic Blocks ConstructionAlgorithm.
Algorithm: Partitioning three-address instructions into basic
blocks.
INPUT: A sequence of three-address instructions.
OUTPUT: A list of the basic blocks for that sequence in which
each instruction is assigned to exactly one basic block.

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Basic Blocks
Find the leaders

12. Basic Blocks

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Follow Diagram

14. 14
H.M Nasim
161-15-7365

15. 15
Transformations on Basic Blocks
■ A code-improving transformation is a code optimization to improve speed or reduce code size.
■ Global transformations are performed across basic blocks.
■ Local transformations are only performed on single basic blocks.
■ Transformations must be safe and preserve the meaning of the code .
■ A local transformation is safe if the transformed basic block is guaranteed to be equivalent to its
original form

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Transformations on Basic Blocks
LocalTransformation
Some local transformation are:
Common-
Subexpression
Elimination
Dead Code Elimination
Renaming Temporary
Variables
Interchange of
Statements
Algebraic
Transformations

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Common-Subexpression Elimination
■ Remove redundant computations
■ 2nd and 4th:compute same expression in fig:1(a)
■ Look at 1st and 3rd in fig:1(b) :b is redefine in 2nd therefore different in3rd, not the same
expression
fig:1(a) fig:1(b)
a := b + c b
:= a - d c := b
+ c d := a - d
a := b + c b
:= a - d c := b
+ c d := b
t1 := b * c t2 :=
a - t1 t3 := b * c
t4 := t2 + t3
t1 := b * c t2 :=
a - t1 t4 := t2 +
t1

18. 18
Sukanta Dey
161-15-7259

19. Dead Code Elimination
■ Remove unused statements
Assuming a is dead (not used)
Remove unreachable code
b := a + 1
a := b + c
…
b := a + 1
…
goto L2
b := x + y
…

20. Renaming Temporary Variables
■ Temporary variables that are dead at the end of a block can be safely
renamed.
■ The basic block is transforms into an equivalent block in which each statement
that
defines a temporary defines a new temporary.
■ Such a basic block is called normal-form block or simple block.
Normal-form block
■ Note that normal-form blocks permit all statement interchanges
that arepossible.
t1 := b + c
t2 := a - t1
t1 := t1 * d
d := t2 + t1
t1 := b + c
t2 := a - t1
t3 := t1 * d
d := t2 + t3

21. Interchange of Statements
■ Independent statements can be reordered without effecting the value of block to
make its optimal use.
■ Note that normal-form blocks permit all statement interchanges that are
possible
t1 := b + c
t2 := a - t1
t3 := t1 * d
d := t2 + t3
t1 := b + c
t3 := t1 * d
t2 := a - t1
d := t2 + t3

22. 22
OVI modak
162-15-7795

23. Algebraic Transformations
■ Change arithmetic operations to transform blocks to algebraic equivalent
forms.
■ Simplify expression or replace expensive expressions by cheaper ones.
■ In statement 3,usually require a function call
■ Transforms to simple and equivalent statement
t1 := a - a
t2 := b + t1
t3 := t2 **2
t1 := 0
t2 := b
t3 := t2 * t2

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Representation of Basic Blocks
Variety of data structures.
Quadruples are used with pointers.
Jumping from one block to another block with conditional and
unconditional jumps.
Block B finishes the code then B’ is continued
Block B’ reaches last line then control goes back to the beginning of B’.

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L O O P S o f D i a g r a mN o . 1
N o . 2
N o . 3
Loop contains no other loop is
called inner loop.
The collection of node has unique entry
All node in the collection are strongly connected.

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27. T H A N K Y O U !
A N Y Q U E S T I O N S ?