Basic blocks - compiler design
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The document introduces basic blocks in programming, explaining them as sequences of instructions executed without branching. It covers the construction of basic blocks, various transformations like common-subexpression elimination and dead code elimination, and the significance of these transformations for optimizing code. Additionally, it highlights the representation of basic blocks and their relationship to flow graphs for better code generation and optimization.
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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](https://image.slidesharecdn.com/basicblocks-191026130154/85/Basic-blocks-compiler-design-8-320.jpg)
Basic blocks - compiler design
- 1. G r e a t M a t e s
- 2. W E L C O M E ! Thank you for coming today!
- 5. 5 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.
- 6. 6 Type of Basic Blocks Basic Blocks Transformations on Basic Blocks Structure preserving Transformations Algebraic transformations Representation of Basic Blocks
- 7. 7 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.
- 8. 8 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
- 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.
- 11. 11 Basic Blocks Find the leaders
- 12. Basic Blocks
- 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
- 16. 16 Transformations on Basic Blocks LocalTransformation Some local transformation are: Common- Subexpression Elimination Dead Code Elimination Renaming Temporary Variables Interchange of Statements Algebraic Transformations
- 17. 17 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
- 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
- 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
- 24. 24 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’.
- 25. 25 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.
- 26. 26 • Google.com • Wikipedia . • Slide Shares.com • Online.Visual-paradiagram.com • Youtube.com Reference & Tools
- 27. T H A N K Y O U ! A N Y Q U E S T I O N S ?