Method and apparatus for a stew-based loop predictor
A method and apparatus for a loop predictor for predicting the end of a loop is disclosed. In one embodiment, the loop predictor may have a predict counter to hold a predict count representing the expected number of times that a predictor stew value will repeat during the execution of a given loop. The loop predictor may also have one or more running counters to hold a count of the times that the stew value has repeated during the execution of the present loop. When the counter values match the predictor may issue a prediction that the loop will end.
1. An apparatus, comprising:
a source of instructions to store a set of instructions;
a loop predictor to predict when a branch of said set of instructions will be not loop direction, to include a predict counter to hold a predict count corresponding to a number of times a stew value was repeated during a previous execution of a loop bounded by said branch, and to include a running counter to hold a running count of a number of times said stew value has been repeated during a current execution of said loop.
2. The apparatus of claim 1 , further comprising a compare circuitry to issue a prediction when said running count matches said predict count.
3. The apparatus of claim 1 , wherein said predict count is trained by results from a pipeline retirement stage.
4. The apparatus of claim 1 , wherein said running counter includes a retired counter to hold a retired count.
5. The apparatus of claim 4 , wherein said retired count is incremented responsive to when a value of said stew repeats.
6. The apparatus of claim 4 , wherein said predict count is set equal to said retired count after said branch is not loop direction when said retired count is not zero.
7. The apparatus of claim 4 , wherein said running counter further includes a speculative counter to hold a speculative count incremented when instructions of said set of instructions are issued from said source of instructions and a value of said stew repeats.
8. The apparatus of claim 7 , wherein said speculative count is set to said retired count when said speculative count is not valid.
9. The apparatus of claim 7 , further including a switch to select between said speculative count and said retired count responsive to a speculative valid signal.
10. The apparatus of claim 1 , further including a confidence counter to hold a confidence count incremented when said predict count equals said running count and when said branch is not loop direction.
11. A method, comprising:
storing a predict count of the number of times a stew value repeats between a first occurrence of a branch not loop direction and a second occurrence of said branch not loop direction;
maintaining a running count of the number of times a stew value repeats subsequent to a third occurrence of a branch not loop direction; and
predicting a fourth branch not loop direction when said running count matches said predict count.
12. The method of claim 11 , further comprising training said predict count by results from a pipeline retirement stage.
13. The method of claim 11 , wherein said maintaining said running count includes maintaining a retired count.
14. The method of claim 13 , further comprising incrementing said retired count responsive to when a value of said stew repeats.
15. The method of claim 14 , further comprising setting said predict count to said retired count at said second occurrence when said retired count is not zero.
16. The method of claim 14 , wherein said maintaining said running count includes maintaining a speculative count incremented when instructions are issued from a source of instructions and a value of said stew repeats.
17. The method of claim 16 , wherein speculative count is set to said retired count when said speculative is not valid.
18. The method of claim 16 , further comprising selecting said running count from said retired count and said speculative count responsive to whether said speculative count is valid.
19. The method of claim 11 , further comprising incrementing a confidence count when said predict count equals said running count and when said branch is not loop direction.
20. A system, comprising:
a processor including a source of instructions to store a set of instructions, and a loop predictor to predict when a branch of said set of instructions will be not loop direction, to include a predict counter to hold a predict count corresponding to a number of times a stew value was repeated during a previous execution of a loop bounded by said branch, and to include a running counter to hold a running count of a number of times said stew value has been repeated during a current execution of said loop;
an interface coupled to said processor; and
an audio input/output coupled to said interface and said processor.
21. The system of claim 20 , further comprising a compare circuitry to issue a prediction when said running count matches said predict count.
22. The system of claim 21 , wherein said predict count is trained by results from a pipeline retirement stage.
23. The system of claim 21 , wherein said running counter includes a retired counter to hold a retired count.
24. The system of claim 23 , wherein said retired count is incremented responsive to when a value of said stew repeats.
25. The system of claim 24 , wherein said predict count is set equal to said retired count after said branch is not loop direction when said retired count is not zero.
26. An apparatus, comprising:
means for storing a predict count of the number of times a stew value repeats between a first occurrence of a branch not loop direction and a second occurrence of said branch not loop direction;
means for maintaining a running count of the number of times a stew value repeats subsequent to a third occurrence of a branch not loop direction; and
means for predicting a fourth branch not loop direction when said running count matches said predict count.
27. The apparatus of claim 26 , further comprising means for training said predict count by results from a pipeline retirement stage.
28. The apparatus of claim 27 , wherein said means for maintaining said running count includes means for maintaining a retired count.
29. The apparatus of claim 28 , further comprising means for incrementing said retired count responsive to when a value of said stew repeats.
30. The apparatus of claim 28 , further comprising means for setting said predict count to said retired count at said second occurrence when said retired count is not zero.
31. The apparatus of claim 28 , wherein said means for maintaining said running count includes means for maintaining a speculative count incremented when instructions are issued from a source of instructions and a value of said stew repeats.
32. The apparatus of claim 31 , wherein said speculative count is set to said retired count when said speculative is not valid.
33. The apparatus of claim 31 , further comprising means for selecting said running count from said retired count and said speculative count responsive to whether said speculative count is valid.
34. The apparatus of claim 26 , further comprising means for incrementing a confidence count when said predict count equals said running count and when said branch is not loop direction.
35. A processor, comprising: a loop predictor to produce a loop prediction; a global predictor to produce a global prediction; and a prediction resolution circuit to give priority to said loop prediction over said global prediction; wherein said loop predictor predicts when a branch of said set of instructions will be not loop direction, includes a predict counter to hold a predict count corresponding to a number of times a stew value was repeated during a previous execution of a loop bounded by said branch, and includes a running counter to hold a running count of a number of times said stew value has been repeated during a current execution of said loop.
36. The processor of claim 35 , further comprising a bimodal predictor to produce a bimodal prediction, wherein said prediction resolution circuit gives priority to said global prediction over said bimodal prediction.