1. Code Generation for Control Flow Mooly Sagiv html://www.math.tau.ac.il/~msagiv/courses/wcc03.html Chapter 6.4

2. Outline Local flow of control –Conditionals –Switch –Loops Routine Invocation Non-local gotos Runtime errors Handling Exceptions Summary

3. Machine Code Assumptions InstructionMeaning GOTO LabelJump to Label GOTO label registerIndirect jump IF condition register then GOTO LabelConditional Jump IF not condition register then GOTO Label

4. Boolean Expressions In principle behave like arithmetic expressions But are treated specially –Different machine instructions –Shortcut computations Code for a < b yielding a condition value Conversion condition value into Boolean Conversion from Boolean in condition value Jump to l on condition value if (a < b) goto l

5. Shortcut computations Languages such as C define shortcut computation rules for Boolean Incorrect translation of e1 && e2 Code to compute e1 in loc1 Code to compute e2 in loc2 Code for && operator on loc1 and loc2

6. Code for Booleans (Location Computation) Top-Down tree traversal Generate code sequences instructions Jump to a designated ‘true’ label when the Boolean expression evaluates to 1 Jump to a designated ‘false’ label when the Boolean expression evaluates to 0 The true and the false labels are passed as parameters

7. Example if ((a==0) && (b > 5)) x = ((7 * a) + b) if &&:= x+ *b 7a == a0 > b5

8. if &&:= x+ *b 7a == a0 > b5 No label Lf Lt Lf Load_Local -8(FP), R0 Cmp_Constant R0, 0 IF NOT EQ THEN GOTO Lf Lt Lf Load_Local -12(FP), R0 Cmp_Constant R0, 5 IF GT THEN GOTO Lt GOTO Lf Lt: Code for := Lf:

9. Location Computation for Booleans

10. Code generation for IF if Boolean expressiontrue sequencefalse sequence Allocate two new labels Lf, Lend Generate code for Boolean(left, 0, Lf) Code for Boolean with jumps to Lf Code for true sequence Code for false sequence GOTO Lend Lf: Lend:

11. Code generation for IF (no-else) if Boolean expressiontrue sequence Allocate new label Lend Generate code for Boolean(left, 0, Lend) Code for Boolean with jumps to Lend Code for true sequence Lend:

12. Coercions into value computations := xa > b Generate new label Lf Load_Constant R0, 0; Generate code for Boolean(right, 0, Lf) Load_Local -8(FP), R1; CMP R1, -12(FP) ; IF <= GOTO Lf; Load_Constant R0, 1 Lf: Store_Local R0, -20(FP)

13. Effects on performance Number of executed instructions Unconditional vs. conditional branches Instruction cache Branch prediction Target look-ahead

14. Code for case statements Three possibilities –Sequence of IFs O(n) comparisons –Jump table O(1) comparisons –Balanced binary tree O(log n) comparisons Performance depends on n Need to handle runtime errors

15. Simple Translation

16. Jump Table Generate a table of L high -L low +1 entries –Filled at ?time Each entry contains the start location of the corresponding case or a special label Generated code tmp_case_value:= case expression; if tmp_case_value L high GOTO label_else; GOTO table[tmp_case_value –L low ];

17. Balanced trees The jump table may be inefficient –Space consumption –Cache performance Organize the case labels in a balanced tree –Left subtrees smaller labels –Right subtrees larger labels Code generated for node_k label_k: IF tmp_case_value l k THEN GOTO label of right branch; code for statement sequence; GOTO label_next;

18. Repetition Statements (loops) Similar to IFs Preserve language semantics Performance can be affected by different instruction orderings Some work can be shifted to compile-time –Loop invariant –Strength reduction –Loop unrolling

19. while statements Boolean expression while statement Sequence test_label: Generate new labels test_label, L end Generate code for Boolean(left, 0, L end ) Code for Boolean with jumps to L end Code for statement sequence GOTO test_label; L end :

20. while statements(2) Boolean expression while statement Sequence GOTO test_label: Ls: Generate labels test_label, Ls Generate code for Boolean(left, Ls, 0) Code for Boolean with jumps to L S Code for statement sequence test_label:

21. For-Statements Special case of while Tricky semantics –Number of executions –Effect on induction variables –Overflow

22. Simple-minded translation FOR i in lower bound.. upper bound DO statement sequence END for  i := lower_bound; tmp_ub := upper_bound; WHILE I <= tmp_ub DO code for statement sequence i := i + 1; END WHILE

23. Correct Translation FOR i in lower bound.. upper bound DO statement sequence END for  i := lower_bound; tmp_ub := upper_bound; IF i >tmp_ub THEN GOTO end_label; loop_label: code for statement sequence if (i==tmp_ub) GOTO end_label; i := i + 1; GOTO loop_label; end_label:

24. Tricky question

25. Loop unrolling FOR i := 1 to n DO sum := sum + a[i]; END FOR;

26. Summary Handling control flow statements is usually simple Complicated aspects –Routine invocation –Non local gotos –Runtime errors Runtime profiling can help

27. Routine Invocation Identify the called routine Generate calling sequence –Some instructions are executed by the callee Filling the activation record –Actual parameters (caller) –Administrative part –The local variable area –The working stack

28. Parameter Passing Mechanisms By value By reference By result By value-result Pass the R-value of the parameter Pass the L-value of the parameter The callee creates a temporary Stores the temporary upon return Can use registers Pass the L-value of the parameter The callee creates a temporary Store the temporary upon return

29. Caller Sequence Save caller-save registers Pass actual parameters –In stack –In register Pass lexical pointer Generate code for the call –Store return address –Pass flow of control

30. Callee Sequence Allocate the frame Store callee-save registers Perform the procedure code Return function result Restore callee-save registers Deallocate the frame Transfer the control back to the caller

31. Two activation records on the stack

32. Non-Local goto in C syntax

33. Non-local gotos Close activation records Restore callee-save registers Main p: l: q: goto l; code Memory before p r q Main Memory After p

34. Runtime errors The smartest compiler cannot catch all potential errors at compile-time –Missing information –Undecidability Compiler need to generate code to identify runtime errors –Non-trivial

35. Common runtime errors Overflow –Integers –Stack frame Limited resources Division by zero Null pointer dereferences Dangling pointer dereferences Buffer overrun (array out of bound)

36. Runtime Errors C –No support for runtime errors –Situations with “undefined” ANSI C semantics Leads to security vulnerabilities Pascal –Runtime errors abort execution Java –Runtime errors result in raised exceptions –Can be handled by the programmer code

37. Detecting a runtime error Array bound-check foo() { int a[100], i, j; scanf(“%d%d”, &i, &j); while (i < j) { a[i] = i ; i++; } if (“i” =100) THROW range_error;

38. Detecting a runtime error(2) Array bound-check foo() { int a[100], i, j; scanf(“%d%d”, &i, &j); if ((i >=0) && j <=100) { while (i < j) { a[i] = i ; i++; } else … }

39. Handling Runtime Errors Abort Statically assigned error handlers (signal) Exceptions

40. Signal Handlers Binds errors to handler routines Invoke a specific routine when runtime error occurs Report an error –Close open resources and exit –Resume immediately after the error –Resume in some “synchronization” point

41. Signal Example

42. Exceptions Flexible mechanism for handling runtime errors Available in modern programming languages Useful programming paradigm But are hard to compile void f() { { … g() … catch(error1) { … } } void g() { … h() … } void h() { … throw error1 … }

43. Why are exceptions hard to compile? Dynamic addresses –Not always known when at compile time –Non local goto –Register state The handler may change in the execution of a routine The handler code assumes sequential execution

44. Handling Exceptions At compile time store mappings from exceptions to handlers Store a pointer to the table in the activation record When an exception is raised scan the stack frames to locate the most recent handler Perform a non-goto

45. Generate code for handler Terminate with a jump to the end of block/routine A unique label where the handler code begins Generate a table Store a pointer to the table at the activation record Handler Code Block/Routine exceptionhandler

46. Code for raising exception Extract the pointer to the table from the activation record Search for a handler for the exception raised If not found pop the stack and repeat If found perform a non-local goto Usually combine the search and the goto

47. Summary Non local transfer of control can be expensive –Hard to understand = Hard to implement But are necessary Challenging optimization problems