Selective use of branch prediction hints

1. An apparatus comprising:

an instruction decoder to decode a branch instruction having a hint; and

a branch predictor to provide a prediction and a hint-override indicator, wherein the hint-override indicator is to indicate whether the prediction is based on stored information about the branch instruction, and wherein the prediction is to override the hint if the hint-override indicator indicates that the prediction is based on stored information about the branch instruction.

2. The apparatus of claim 1 , further comprising a tagged structure to provide the stored information.

3. The apparatus of claim 1 , wherein the instruction decoder is to associate a branch hint override indicator to the decoded branch instruction.

4. The apparatus of claim 3 , wherein the branch hint override indicator is to be passed down an execution core pipeline from the instruction decoder.

5. The apparatus of claim 4 , wherein override the hint includes flushing of the execution pipeline of instructions following a hint path.

6. The apparatus of claim 1 , further comprising a counter to count conditional branch instructions for which the branch predictor has no stored information and for which the prediction is to not take a branch.

7. The apparatus of claim 1 , further comprising a counter to count correct predictions for conditional branch instructions for which the branch predictor has no stored information and for which the prediction is to not take a branch.

8. The apparatus of claim 1 , further comprising a counter to count mispredictions for conditional branch instructions for which the branch predictor has no stored information and for which the prediction is to not take a branch.

9. The apparatus of claim 1 , further comprising a counter to count conditional branch instructions for which the branch predictor has no stored information and for which the prediction is to take a branch.

10. The apparatus of claim 1 , further comprising a counter to count correct predictions for conditional branch instructions for which the branch predictor has no stored information and for which the prediction is to take a branch.

11. The apparatus of claim 1 , further comprising a counter to count mispredictions for conditional branch instructions for which the branch predictor has no stored information and for which the prediction is to take a branch.

12. A method comprising:

receiving, by a branch predictor, a branch instruction with a hint;

making a prediction;

determining whether the prediction was based on stored information about the branch instruction;

setting a hint-override indicator if the prediction was based on stored information; and

overriding the hint if the hint-override indicator indicates that the prediction was based on stored information.

13. The method of claim 12 , further comprising using the hint if the hint-override indicator indicates that the prediction was not based on stored information.

14. The method of claim 12 , wherein overriding the hint includes flushing an execution pipeline of instructions following a hint path.

15. The method of claim 14 , further comprising counting conditional branch instructions for which the branch predictor had no stored information and for which the prediction was to not take a branch.

16. The method of claim 14 , further comprising counting correct predictions for conditional branch instructions for which the branch predictor had no stored information and for which the prediction was to not take a branch.

17. The method of claim 14 , further comprising counting mispredictions for conditional branch instructions for which the branch predictor had no stored information and for which the prediction was to not take a branch.

18. A method comprising:

executing code including branch instructions having hints on a processor having a branch predictor to provide a prediction and a hint-override indicator, wherein the hint-override indicator is to indicate whether the prediction is based on stored information about the branch instruction;

overriding the hints if the hint-override indicator indicates that the prediction is based on stored information about the corresponding branch instruction; and

counting mispredictions for conditional branch instructions for which the branch predictor had no stored information and for which the prediction was to not take a branch.

19. The method of claim 18 , further comprising profiling the code based on the count of mispredictions.

20. The method of claim 19 , wherein profiling includes finding branch instructions to which to add hints.