Method and apparatus for optimizing away a null-check

• US 20050149588A1
• Filed: 01/05/2004
• Published: 07/07/2005
• Est. Priority Date: 01/05/2004
• Status: Abandoned Application

• Current Assignee: International Business Machines Corporation
• Original Assignee: International Business Machines Corporation
• Inventors: Bacon, David Francis, Cheng, Perry, Rajan, Vadakkedathu Thomas
• Application Number:
  US10/751,775
• Publication Number:
  US 20050149588A1
• Time in Patent Office: Days
• US Class Current: 1/1
• CPC Class Codes: G06F 12/0269 Incremental or concurrent garbage collection, e.g. in real-time systems G06F12/0261 takes precedence

Abstract

A garbage collection system that needs to meet real-time requirements combines a null-check required for an application with a null check required by its read barrier. The null-check for the application may be the null-check required for a Java object dereference and may be performed via explicit comparisons or implicit traps. To facilitate the combining of the null-checks, the read barrier is broken into portions and sunk to its point of use. Thus, any exception due to a null-pointer in the read barrier will occur at the same location as the exception occurs in the application. Combining the null-checks reduces the overhead costs associated with the read barrier and improves the performance of the real-time application.

Claims

1. A method of using a read barrier in connection with a garbage collection process for an application program, the method comprising:

combining a null-check required for the application with a null check for the read barrier.

2. The method of claim 1 wherein the null-check for the application is a null-check for a Java object dereference.

3. The method of claim 1 wherein the null-checks are performed via explicit comparisons.

4. The method of claim 1 wherein the null-checks are performed via implicit traps.

5. The method of claim 1 comprising sinking the read barrier to its point of use.

6. The method of claim 1 wherein the application is a real-time application.

7. The method of claim 1 comprising insuring that any exception due to a null pointer in the read barrier occurs at a same location as the exception occurs in the application.

8. A method of utilizing a read barrier to access and move objects in connection with an application comprising:

folding a null-check required for the read barrier into a null-check required by the application.

9. The method of claim 8 comprising sinking the read barrier to its point of use.

10. The method of claim 8 comprising insuring that any exception due to a null pointer in the read barrier occurs at a same location as the exception occurs in the application.

11. The method of claim 8 wherein the null-check for the application is a null-check for a Java object dereference.

12. The method of claim 8 wherein the read barrier uses a forwarding pointer located in a header of an object.

13. The method of claim 8 wherein the null-checks are performed via explicit comparisons or implicit traps.

14. A computer-readable medium comprising instructions for performing a garbage collection process for an application program, the medium comprising;

null-check optimizing software for combining a null-check for the garbage collection process with a null-check required by semantics of the application program.

15. The medium of claim 14 wherein the null-check for the application is a Java object dereference null-check.

16. The medium of claim 14 wherein the null-check required by the garbage collection process is a null-check for a read barrier.

17. The medium of claim 14 wherein the null-checks are performed via explicit comparisons or implicit traps.

18. The medium of claim 14 comprising a read barrier for accessing and moving objects.

19. The medium of claim 18 comprising instructions for optimizing the read barrier using a compiler optimization.

20. The medium of claim 18 wherein the read barrier comprises a forwarding pointer located in objects.

21. The medium of claim 19 wherein the instructions for optimizing the read barrier comprise sinking the read barrier to its point of use.

22. The medium of claim 19 wherein the instructions for optimizing the read barrier comprise using common sub-expression optimization.

23. An information processing system for performing a garbage collection process for an application program, comprising:

a processor configured for combining a null-check for the garbage collection process with a null-check required by semantics of the application program.

24. The information processing system of claim 23 wherein the null-check for the application is a Java object dereference null-check.

25. The information processing system of claim 23 wherein the null-check for the garbage collection process is a null-check for a read barrier.

26. The information processing system of claim 23 wherein null-checks are performed via explicit comparisons or implicit traps.