There's a "branch hint" in PowerPC, I wonder if that's acted on by the Xenon CPU in question.
edit: it's discussed in the comments as well but they don't know either. The author responds: "I can’t remember how PowerPC branch hints work but if the branch hint overrode the branch predictor then it could have avoided the bug."
No, because the branch predictor is just "on" or "off". You'd be wondering if you can make it not speculatively execute specific instructions. I'm going to infer from the article that there wasn't a way to do that and it was far too late to spin a new CPU revision to prohibit speculatively executed xdcbt.
I assume one could have put a serializing instruction inside the "dangerous" path, which should then lead to the speculative execution path being rolled back before reaching the dangerous instruction. Obviously it'd also be expensive...
I wonder why they didn't just make two separate routines without any branch except for loop.
Then you would just make another safety offset to assume for the last loop iteration prediction.
There's a "branch hint" in PowerPC, I wonder if that's acted on by the Xenon CPU in question.
edit: it's discussed in the comments as well but they don't know either. The author responds: "I can’t remember how PowerPC branch hints work but if the branch hint overrode the branch predictor then it could have avoided the bug."
No, because the branch predictor is just "on" or "off". You'd be wondering if you can make it not speculatively execute specific instructions. I'm going to infer from the article that there wasn't a way to do that and it was far too late to spin a new CPU revision to prohibit speculatively executed xdcbt.
I assume one could have put a serializing instruction inside the "dangerous" path, which should then lead to the speculative execution path being rolled back before reaching the dangerous instruction. Obviously it'd also be expensive...
I wonder why they didn't just make two separate routines without any branch except for loop. Then you would just make another safety offset to assume for the last loop iteration prediction.
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You probably could have done the equivalent if it were IA-64, but I suspect you can't do this with plain old branch-prediction mechanisms.