Specializing write-barriers for objects in a garbage collected heap

• US 7,089,272 B1
• Filed: 06/18/2003
• Issued: 08/08/2006
• Est. Priority Date: 06/18/2003
• Status: Active Grant

• Current Assignee: Oracle America, Inc. (Oracle Corporation)
• Original Assignee: Sun Microsystems Incorporated (Oracle Corporation)
• Inventors: Garthwaite, Alexander T., Detlefs, David L.
• Primary Examiner(s): Robinson, Greta
• Assistant Examiner(s): LEWIS, CHERYL RENEA
• Application Number:
  US10/464,371
• Time in Patent Office: 1,147 Days
• Field of Search: 707/203, 707/205, 707/206, 707/100, 707/103.R
• US Class Current: 1/1
• CPC Class Codes: G06F 12/0276 Generational garbage collection
• CPC Class Codes: Y10S 707/99944 Object-oriented database structure
• CPC Class Codes: Y10S 707/99957 Garbage collection

Abstract

A technique is provided for reducing the number of write barriers executed in mutator code without compromising garbage collector performance. Advantageously, a compiler generates two forms of a mutator code—a first version with write barriers and a second version substantially without write barriers. In operation, the first version of the code may be accessed by a vtable in a “mature” near-class and the second version may be accessed by a vtable in a “nascent” near-class. According to the invention, mapping of functionally equivalent points in the first and second versions of the mutator code may be facilitated by an associated pcmap. Further, each of the first and second versions may also be associated with a respective nr_map that facilitates mapping functionally equivalent points within different branches of guard code sequences corresponding to reference-writes to non-receiver objects.

Claims

1. In a computer system, a method for controlling the execution of write-barrier program code when a mutator program calls a reference-modifying method of a receiver object via a class pointer field, the method comprising:

A) including in the receiver object, both a first method version with first write-barrier code and a second method version with second write-barrier code that differs from the first write-barrier code;

B) before a predetermined condition is met, storing a first pointer in the class pointer field so that a subsequent call to the receiver object method follows the first pointer and executes the first method version; and

C) after the predetermined condition is met, storing a second pointer in the class pointer field so that a subsequent call to the receiver object method follows the second pointer and executes the second method version.

2. the method according to claim 1, wherein the first and second versions contain different numbers of write barriers.

3. The method according to claim 2, wherein the first version contains no write barriers.

4. The method according to claim 1, further comprising:

generating a pcmap associated with the class-member method, the pcmap defining a correspondence between program-counter values of instructions in the first version of the class-member method and program-counter values of instructions in the second version of the class-member method.

5. The method according to claim 4, further comprising:

using the generated pcmap to update a program-counter value in a stack frame that corresponds to a call of the class-member method on the receiver object.

6. The method according to claim 4, further comprising:

storing the pcmap in a metadata data structure associated with the first or second version of the class-member method.

7. The method according to claim 1, further comprising:

generating an nr_map for each of the first and second versions of the class-member method, the generated nr_maps being used to map program-counter values of instructions located in different branches of guard code implemented in the first and second versions of the class-member method to each other.

8. The method according to claim 7, further comprising:

using one of the generated nr_maps to update a program-counter value that corresponds to the next instruction that will be executed by the mutator.

9. The method according to claim 7, further comprising:

storing each generated nr_map in a metadata data structure associated with the nr_map'"'"'s corresponding version of the class-member method.

10. The method according to claim 1, wherein the predetermined condition is based on the object'"'"'s longevity in a region of the memory.

11. The method according to claim 1, wherein the predetermined condition is based on a runtime condition.

12. The method according to claim 1, wherein the predetermined condition is based on information generated at compile-time.

13. The method according to claim 12, wherein the information generated at compile-time is based on the object'"'"'s expected lifetime in the heap.

14. The method according to claim 12, wherein the information generated at compile time is based on the number of expected reference modifications made in the object.

15. A computer system for controlling the execution of write-barrier program code when a mutator program calls a reference-modifying method of a receiver object via a class pointer field, the system comprising:

a processor; and

a computer-readable memory that stores program instructions executable by the processor for

A) including in the receiver object, both a first method version with first write-barrier code and a second method version with second write-barrier code that differs from the first write-barrier code;

B) before a predetermined condition is met, storing a first pointer in the class pointer field so that a subsequent call to the receiver object method follows the first pointer and executes the first method version; and

C) after the predetermined condition is met, storing a second pointer in the class pointer field so that a subsequent call to the receiver object method follows the second pointer and executes the second method version.

16. The system according to claim 15, wherein the first and second versions contain a different number of write barriers.

17. The system according to claim 16, wherein the first version contains no write barriers.

18. The system according to claim 15, wherein the computer-readable memory further stores instructions executable by the processor for:

generating a pcmap associated with the class-member method, the pcmap defining, a correspondence between program-counter values of instructions in the first version of the class-member method and program-counter values of instructions in the second version of the class-member method.

19. The system according to claim 18, wherein the computer-readable memory further stores instructions executable by the processor for:

using the generated pcmap to update a program-counter value in a stack frame that corresponds to a call of the class-member method on the receiver object.

20. The system according to claim 18, wherein the computer-readable memory further stores instructions executable by the processor for:

storing the pcmap in a metadata data structure associated with the first or second version of the class-member method.

21. The system according to claim 15, wherein the computer-readable memory further stores instructions executable by the processor for:

generating an nr_map for each of the first and second versions of the class-member method, the generated nr_maps being used to map program-counter values of instructions located in different branches of guard code implemented in the first and second versions of the class-member method to each other.

22. The system according to claim 21, wherein the computer-readable memory further stores instructions executable by the processor for:

using one of the generated nr_maps to update a program-counter value that corresponds to the next instruction that will be executed by the mutator.

23. The system according to claim 21, wherein the computer-readable memory further stores instructions executable by the processor for:

storing each generated nr_map in a metadata data structure associated with the nr_map'"'"'s corresponding version of the class-member method.

24. The system according to claim 15, wherein the predetermined condition is based on the object'"'"'s longevity in a region of the memory.

25. The system according to claim 15, wherein the predetermined condition is based on a runtime condition.

26. The system according to claim 15, wherein the predetermined condition is based on information generated at compile-time.

27. The system according to claim 26, wherein the information generated at compile-time is based on the object'"'"'s expected lifetime in the heap.

28. The system according to claim 26, wherein the information generated at compile-time is based on the number of expected reference modifications made in the object.

29. An apparatus operable in a computer system having a memory for controlling the execution of write-barrier program code when a mutator program executing in the memory calls a reference-modifying method of a receiver object via a class pointer field, the apparatus comprising:

means for inserting a first method version into the receiver object with first write-barrier code and for inserting a second method version into the receiver object with second write-barrier code that differs from the first write-barrier code;

means, operable before a predetermined condition is met, for storing a first pointer in the class pointer field so that a subsequent call to the receiver object method follows the first pointer and executes the first method version; and

means, operable after the predetermined condition is met, for storing a second pointer in the class pointer field so that a subsequent call to the receiver object method follows the second pointer and executes the second method version.

30. The apparatus according to claim 29, further comprising:

means for generating a pcmap associated with the class-member method, the pcmap defining a correspondence between program-counter values of instructions in the first version of the class-member method and program-counter values of instructions in the second version of the class-member method.

31. The apparatus according to claim 30, further comprising:

means for using the generated pcmap to update a program-counter value in a stack frame that corresponds to a call of the class-member method on the receiver object.

32. The apparatus according to claim 30, further comprising:

means for storing the pcmap in a metadata data structure associated with the first or second version of the class-member method.

33. The apparatus according to claim 29, further comprising:

means for generating an nr_map for each of the first and second versions of the class-member method the generated nr_maps being used to map program-counter values of instructions located in different branches of guard code implemented in the first and second versions of the class-member method to each other.

34. The apparatus according to claim 33, further comprising:

means for using one of the generated nr_maps to update a program-counter value that corresponds to the next instruction that will be executed by the mutator.

35. The apparatus according to claim 33, further comprising:

means for storing each generated nr_map in a metadata data structure associated with the nr_map'"'"'s corresponding version of the class-member method.