Code Generation Chapter 9 1 Compiler Construction Code Generation - - PowerPoint PPT Presentation

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Code Generation Chapter 9 1 Compiler Construction Code Generation - - PowerPoint PPT Presentation

Apr 05, 2023 304 likes •506 views

Code Generation Chapter 9 1 Compiler Construction Code Generation Issues in Code Generation Target language Relocatable machine code Commercial compilers produce this Absolute machine code OS code must start at a particular memory

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SLIDE 1

Code Generation

Chapter 9

Compiler Construction Code Generation

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SLIDE 2

Issues in Code Generation

Target language

  • Relocatable machine code

Commercial compilers produce this

  • Absolute machine code

OS code must start at a particular memory address

  • Assembly language

– Easier, assembler does the linking – Commerical compilers don’t do this normally

  • High-level language

E.g., cfront

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SLIDE 3

Issues in Code Generation

  • Instruction selection

ADD R0, 1 STR R0, a LDR R0, a a = a + 1; INC a (If a memory increment instruction exists)

  • Registers

– Maximize register hit ratios – NP-complete problem, heuristics are helpful

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SLIDE 4

Issues in Code Generation

Code optimization

  • By analyzing various aspects of the code it may be possible to transform

the program into an equivalent but more efficient form

  • Example, code motion:

Code motion while ( a < b ) { c = 10 * d; a = a + c; } c = 10 * d;

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SLIDE 5

Basic Blocks

  • A sequence of consecutive statements in which flow of control enters

at the beginning and leaves at the end without any branching

  • A basic block always ends at a branch or at end of code

L: print c goto M a = a * 2 c = c + b M: if (c < a ) goto L b = c + b a = 0

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SLIDE 6

Dividing Code into Basic Blocks L: print c goto M a = a * 2 c = c + b M: if (c < a ) goto L b = c + b a = 0 2 3 1 4

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SLIDE 7

Variables and Basic Blocks

  • Variable definition

A statement that assigns a value to a variable

  • Variable use

A statement that accesses the value of a variable Example:

a = b + c

  • a is defined
  • b is used

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SLIDE 8

Flow graph

  • A directed graph that shows the

flow of control among basic blocks

  • There will be a directed edge from

B1 to B2 if either of the following

two conditions are satisfied:

  • 1. there is a conditional or unconditional

jump from B1 to B2

  • 2. B2 immediately follows B1 and B1 does

not have an unconditional jump

B1 B2 B3 B4

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SLIDE 9

Basic Blocks and Flow Graphs

  • Within a basic

block we can compute uses of variables

  • Basic blocks allow

us to do local

  • ptimizations
  • Flow graphs allow

us to global

  • ptimizations

L: print c goto M a = a * 2 c = c + b M: if (c < a ) goto L b = c + b a = 0 2 3 1 4

B1 B2 B3 B4

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SLIDE 10

Partitioning Algorithm

Program −

→ basic blocks

  • Input: A sequence of 3-address statements
  • Output: A list of basic blocks

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SLIDE 11

Partitioning Algorithm

  • 1. Determine the set of leaders, the first statement in a basic block
  • The first statement in a program is a leader
  • Any statement that is the target of a conditional or unconditional jump

is a leader (can’t branch into the middle of a basic block)

  • Any statement that immediately follows a conditional or unconditional

jump is a leader

  • 2. For each leader, its basic block consists of the leader and all statements

up to, but not including, the next leader or the end of the program

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SLIDE 12

After Partitioning into Basic Blocks

Now we can

  • construct flow graphs for global optimization
  • do local optimizations on basic blocks

Here local and global mean relative to basic blocks

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SLIDE 13

Potential Optimizations on Basic Blocks

Potential optimizations include

  • Common subexpression elimination
  • Dead code elimination
  • Algebraic optimizations

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SLIDE 14

Common Subexpression Elimination

a = b + c b = a - d c = b + c d = a - d

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SLIDE 15

Dead Code Elimination

a = b + c b = d + e e = b + f a = e + x

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SLIDE 16

Algebraic Optimizations

Use algebraic identities to eliminate code

x = x + 0; y = 1 * y;

Can happen more often than you think

/* In C/C++ */ #define OFFSET 0 #define FACTOR 1 . . . x = x + OFFSET; y = FACTOR * y;

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SLIDE 17

Next Use Information

Allows us to determine if a variable is “live”

  • A live variable is used subsequently in the basic block
  • Applies only locally
  • Need to perform data flow analysis on the flow graph for global
  • ptimization

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SLIDE 18

Next Use Information

Let the statements within a basic block be numbered sequentially from 1 to n; for ( i ← n; i > 0; i ← i−1 ) { Scan backwards if ( Statement i has the form x = y op z ) { Attach to Statement i the information found in the symbol table about the next use and liveness of x, y, and z; In the symbol table, set x to “not live” and “no next use” In the symbol table, set y and z to “live” and their next use to i;

} }

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SLIDE 19

Example

  • 1. t1 = a∗b
  • 2. t2 = a∗a
  • 3. t3 = 2∗t2
  • 4. t4 = t1 +t3
  • 5. t5 = b∗b
  • 6. t6 = t4 +t5

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