Code Generation Techniques and Examples in Compilers

1. Compilers 7. Code Generation Chih-Hung Wang References 1. C. N. Fischer, R. K. Cytron and R. J. LeBlanc. Crafting a Compiler. Pearson Education Inc., 2010. 2. D. Grune, H. Bal, C. Jacobs, and K. Langendoen. Modern Compiler Design. John Wiley & Sons, 2000. 3. Alfred V. Aho, Ravi Sethi, and Jeffrey D. Ullman. Compilers: Principles, Techniques, and Tools. Addison-Wesley, 1986. (2nd Ed. 2006)

2. Overview

3. Interpretation • An interpreter is a program that consider the nodes of the AST in the correct order and performs the actions prescribed for those nodes by the semantics of the language. • Two varieties • Recursive • Iterative

4. Interpretation • Recursive interpretation • operates directly on the AST [attribute grammar] • simple to write • thorough error checks • very slow: 1000x speed of compiled code • Iterative interpretation • operates on intermediate code • good error checking • slow: 100x speed of compiled code

5. Recursive Interpretation

6. Self-identifying data • must handle user-defined data types • value = pointer to type descriptor + array of subvalues • example: complex number 3.0 re: 4.0 im:

7. Complex number representation

8. IF condition THEN ELSE FI Iterative interpretation • Operates on threaded AST • Active node pointer • Flat loop over a case statement

9. Sketch of the main loop

10. Example for demo compiler

11. Code Generation • Compilation produces object code from the intermediate code tree through a process called code generation • Tree rewriting • Replace nodes and subtrees of the AST by target code segments • Produce a linear sequence of instructions from the rewritten AST

12. Example of code generation a:=(b[4*c+d]*2)+9;

13. Machine instructions • Load_Addr M[Ri], C, Rd • Loads the address of the Ri-th element of the array at M into Rd, where the size of the elements of M is C bytes • Load_Byte (M+Ro)[Ri], C, Rd • Loads the byte contents of the Ri-th element of the array at M plus offset Ro into Rd, where the other parameters have the same meanings as above

14. Two sample instructions with their ASTs

15. Code generation Main issues: • Code selection – which template? • Register allocation – too few! • Instruction ordering Optimal code generation is NP-complete • Consider small parts of the AST • Simplify target machine • Use conventions

16. Object code sequence Load_Byte (b+Rd)[Rc], 4, Rt Load_Addr 9[Rt], 2, Ra

17. Trivial code generation

18. Code for (7*(1+5))

19. Partial evaluation

20. New Code

21. frame Simple code generation • Consider one AST node at a time • Two simplistic target machines • Pure register machine • Pure stack machine stack SP vars BP

22. Pure stack machine • Instructions

23. Example of p:=p+5 Push_Local #p Push_Const 5 Add_Top2 Store_Local #p

24. Pure register machine • Instructions

25. Example of p:=p+5 Load_Mem p, R1 Load_Const 5, R2 Add_Reg R2, R1 Store_Reg R1, p

26. Simple code generation for a stack machine • The AST for b*b – 4 *(a*c)

27. The ASTs for the stack machine instructions

28. The AST for b*b - 4*(a*c) rewritten

29. Simple code generationfor a stack machine (demo) • example: b*b – 4*a*c • threaded AST - * * b b 4 * a c

30. Simple code generationfor a stack machine (demo) • example: b*b – 4*a*c • threaded AST Sub_Top2 - Mul_Top2 Mul_Top2 * * Push_Local #b b Push_Local #b b Push_Const 4 4 Mul_Top2 * Push_Local #a a Push_Local #c c

31. Sub_Top2 - Mul_Top2 Mul_Top2 * * Push_Local #b b Push_Local #b b Push_Const 4 4 Mul_Top2 * Push_Local #a a Push_Local #c c Simple code generationfor a stack machine (demo) Push_Local #b Push_Local #b Mul_Top2 Push_Const 4 Push_Local #a Push_Local #c Mul_Top2 Mul_Top2 Sub_Top2 • example: b*b – 4*a*c • rewritten AST

32. Depth-first code generation

33. Stack configurations

34. Simple code generation for a register machine • The ASTs for the register machine instructions

35. Code generation with register allocation

36. Code generation with register numbering

37. Register machine code for b*b - 4*(a*c)

38. Register contents

39. Weighted register allocation • It is advantageous to generate the code for the child that requires the most registers first • Weight: • The number of registers required by a node

40. Register weight of a node

41. AST for b*b-4*(a*c) with register weights

42. Weighted register machine code

43. Example Parameter number N 2 3 1 Stored weight 4 2 1 Registers occupied when 0 1 2 starting parameter N Maximum per parameter 4 3 3 Overall maximum 4

44. Example: Tree representation

45. - * * T1 1 b b 4 * a c Register spilling Too few registers? • Spill registers in memory, to be retrieved later • Heuristic: select subtree that uses all registers, and replace it by a temporary example: • b*b – 4*a*c • 2 registers 3 2 2 2 2 1 1 1 1 1

46. - 3 2 T1 * * 2 2 1 b b 4 * 2 1 1 1 a c 1 1 Register spilling Load_Mem b, R1 Load_Mem b, R2 Mul_Reg R2, R1 Store_Mem R1, T1 Load_Mem a, R1 Load_Mem c, R2 Mul_Reg R2, R1 Load_Const 4, R2 Mul_Reg R1, R2 Load_Mem T1, R1 Sub_Reg R2, R1

47. - * * T1 1 b b 4 * a c Another example 3 2 2 2 2 1 1 1

48. Algorithm

49. Machines with register-memory operations • An instruction: • Add_Mem X, R1 • Adding the contents of memory location X to R1

50. Register-weighted tree for a memory-register machine