1. Compiler Construction Dr. Naveed Ejaz Lecture 4

2. 2 The Back End Register Allocation:  Have each value in a register when it is used. Instruction selection IR machine code errors Register allocation Instruction scheduling IR

3. 3 The Back End Register Allocation:  Manage a limited set of resources – register file. Instruction selection IR machine code errors Register allocation Instruction scheduling IR

4. 4 The Back End Register Allocation:  Can change instruction choices and insert LOADs and STOREs. Instruction selection IR machine code errors Register allocation Instruction scheduling IR

5. 5 The Back End Register Allocation:  Optimal register allocation is NP-Complete. Instruction selection IR machine code errors Register allocation Instruction scheduling IR

6. 6 The Back End Instruction Scheduling:  Avoid hardware stalls and interlocks. Instruction selection IR machine code errors Register allocation Instruction scheduling IR

7. 7 The Back End Instruction Scheduling:  Use all functional units productively. Instruction selection IR machine code errors Register allocation Instruction scheduling IR

8. 8 The Back End Instruction Scheduling:  Optimal scheduling is NP-Complete in nearly all cases. Instruction selection IR machine code errors Register allocation Instruction scheduling IR

9. 9 Three-pass Compiler  Intermediate stage for code improvement or optimization Front End Source code machine code errors Middle End Back End IR

10. 10 Three-pass Compiler  Analyzes IR and rewrites (or transforms) IR Front End Source code machine code errors Middle End Back End IR

11. 11 Three-pass Compiler  Primary goal is to reduce running time of the compiled code Front End Source code machine code errors Middle End Back End IR

12. 12 Three-pass Compiler  May also improve space usage, power consumption,... Front End Source code machine code errors Middle End Back End IR

13. 13 Three-pass Compiler  Must preserve “meaning” of the code.  Measured by values of named variables Front End Source code machine code errors Middle End Back End IR

14. 14 OptimizerOptimizer  Modern optimizers are structured as a series of passes Opt 1 IR errors Opt 2 Opt n IR Opt 3 IR

15. 15 OptimizerOptimizer Typical transformations  Discover & propagate some constant value Opt 1 IR errors Opt 2 Opt n IR Opt 3 IR

16. 16 OptimizerOptimizer Typical transformations  Move a computation to a less frequently executed place Opt 1 IR errors Opt 2 Opt n IR Opt 3 IR

17. 17 OptimizerOptimizer Typical transformations  Specialize some computation based on context Opt 1 IR errors Opt 2 Opt n IR Opt 3 IR

18. 18 OptimizerOptimizer Typical transformations  Discover a redundant computation & remove it Opt 1 IR errors Opt 2 Opt n IR Opt 3 IR

19. 19 OptimizerOptimizer Typical transformations  Remove useless or unreachable code Opt 1 IR errors Opt 2 Opt n IR Opt 3 IR

20. 20 OptimizerOptimizer Typical transformations  Encode an idiom in some particularly efficient form Opt 1 IR errors Opt 2 Opt n IR Opt 3 IR

21. 21 Related to Compilers  Interpreters (direct execution)  Assemblers  Preprocessors  Text formatters (non- WYSIWYG)  Analysis tools