compiler construction dr. naveed ejaz lecture 4. 2 the back end register allocation: have each...

Compiler Compiler ConstructionConstruction

Compiler Compiler ConstructionConstruction

Dr. Naveed Ejaz

Lecture 4

2

The Back EndThe Back EndThe Back EndThe Back End

Register Allocation: Have each value in a register

when it is used.

Instructionselection

IR machine code

errors

Registerallocation

Instructionscheduling

IR IR

3

The Back EndThe Back EndThe Back EndThe Back End

Register Allocation: Manage a limited set of

resources – register file.

Instructionselection

IR machine code

errors

Registerallocation

Instructionscheduling

IR IR

4

The Back EndThe Back EndThe Back EndThe Back End

Register Allocation: Can change instruction choices

and insert LOADs and STOREs.

Instructionselection

IR machine code

errors

Registerallocation

Instructionscheduling

IR IR

5

The Back EndThe Back EndThe Back EndThe Back End

Register Allocation: Optimal register allocation is

NP-Complete.

Instructionselection

IR machine code

errors

Registerallocation

Instructionscheduling

IR IR

6

The Back EndThe Back EndThe Back EndThe Back End

Instruction Scheduling: Avoid hardware stalls and

interlocks.

Instructionselection

IR machine code

errors

Registerallocation

Instructionscheduling

IR IR

7

The Back EndThe Back EndThe Back EndThe Back End

Instruction Scheduling: Use all functional units

productively.

Instructionselection

IR machine code

errors

Registerallocation

Instructionscheduling

IR IR

8

The Back EndThe Back EndThe Back EndThe Back End

Instruction Scheduling: Optimal scheduling is

NP-Complete in nearly all cases.

Instructionselection

IR machine code

errors

Registerallocation

Instructionscheduling

IR IR

9

Three-pass CompilerThree-pass CompilerThree-pass CompilerThree-pass Compiler

Intermediate stage for code improvement or optimization

FrontEnd

Sourcecode

machine code

errors

MiddleEnd

BackEnd

IR IR

10

Three-pass CompilerThree-pass CompilerThree-pass CompilerThree-pass Compiler

Analyzes IR and rewrites (or transforms) IR

FrontEnd

Sourcecode

machine code

errors

MiddleEnd

BackEnd

IR IR

11

Three-pass CompilerThree-pass CompilerThree-pass CompilerThree-pass Compiler

Primary goal is to reduce running time of the compiled code

FrontEnd

Sourcecode

machine code

errors

MiddleEnd

BackEnd

IR IR

12

Three-pass CompilerThree-pass CompilerThree-pass CompilerThree-pass Compiler

May also improve space usage, power consumption, ...

FrontEnd

Sourcecode

machine code

errors

MiddleEnd

BackEnd

IR IR

13

Three-pass CompilerThree-pass CompilerThree-pass CompilerThree-pass Compiler

Must preserve “meaning” of the code.

Measured by values of named variables

FrontEnd

Sourcecode

machine code

errors

MiddleEnd

BackEnd

IR IR

14

OptimizerOptimizerOptimizerOptimizer

Modern optimizers are structured as a series of passes

Opt1

IR IR

errors

Opt2

Optn

IR IR Opt3

IR

15

OptimizerOptimizerOptimizerOptimizer

Typical transformations Discover & propagate some

constant value

Opt1

IR IR

errors

Opt2

Optn

IR IR Opt3

IR

16

OptimizerOptimizerOptimizerOptimizer

Typical transformations Move a computation to a less

frequently executed place

Opt1

IR IR

errors

Opt2

Optn

IR IR Opt3

IR

17

OptimizerOptimizerOptimizerOptimizer

Typical transformations Specialize some computation

based on context

Opt1

IR IR

errors

Opt2

Optn

IR IR Opt3

IR

18

OptimizerOptimizerOptimizerOptimizer

Typical transformations Discover a redundant

computation & remove it

Opt1

IR IR

errors

Opt2

Optn

IR IR Opt3

IR

19

OptimizerOptimizerOptimizerOptimizer

Typical transformations Remove useless or

unreachable code

Opt1

IR IR

errors

Opt2

Optn

IR IR Opt3

IR

20

OptimizerOptimizerOptimizerOptimizer

Typical transformations Encode an idiom in some

particularly efficient form

Opt1

IR IR

errors

Opt2

Optn

IR IR Opt3

IR

21

Related to CompilersRelated to CompilersRelated to CompilersRelated to Compilers Interpreters (direct execution) Assemblers Preprocessors Text formatters (non-

WYSIWYG) Analysis tools