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Bug 1429764 details a test failure that was asserting that the performance navigation timers were strictly increasing (or equal). fetchStart should have a timestamp before domainLookupStart. But it didn't. The problem is two-fold. This corrects the test and the issue by addressing one part of the problem, the second part of the problem needs to be written up in a new bug and addressed there. (That bug is not yet filed at writing, but see dependencies of 1429764 in the future to find it.) The second, and underlying, problem is that calling ReduceTimerPrecision with the same value multiple times may continually reduce it. Meaning that the first you call it with, say, .75, (and a precision of .20), it will be reduced to .6. The second time you call it (with .6), instead of staying at .6 it will be reduced to .4. This is because floats are fuzzy. Inside ReduceTimerPrecision we are multiplying a decimal by a decimal, so while floor(.6 / .20) should equal 3, sometimes it's actually 2.999... which gets floors to 2, gets multiplied again by .2, and which results in .4 If that's the underlying problem, the first, and surface, problem is - why are we calling ReduceTimerPrecision multiple times? We shouldn't be. That's what this patch fixes. TimeStampToDOMHighResOrFetchStart will return either TimeStampToDOMHighRes() or FetchStartHighRes(). FetchStartHighRes() internally calls TimeStampToDOMHighRes and then ReduceTimerPrecision - this is where (some of) the two reduction calls happen - because TimeStampToDOMHighRes itself calls ReduceTimerPrecision also. I remove the ReduceTimerPrecision from TimeStampToDOMHighRes. FetchStartHighRes will now only call ReduceTimerPrecision once, at the end of the return. But we have to fix places we call TimeStampToDOMHighResOrFetchStart, because the callers of that function also call ReduceTimerPrecision. So if TimeStampToDOMHighResOrFetchStart returned FetchStartHighRes, we'd be calling ReduceTimerPrecision twice for those callers. So inside first off, we remove the outer call to ReduceTimerPrecision. that surrounds the 5 or so callsites of TimeStampToDOMHighResOrFetchStart. Then inside of TimeStampToDOMHighResOrFetchStart we return either FetchStartHighRes (which is has already called ReduceTimerPrecision) or we call ReduceTimerPrecision with the value. Now. TimeStampToDOMHighRes was used in more places than just FetchStartHighRes - there were several other places where we were doing double rounding, and this fixed those as well. AsyncOpenHighRes, WorkerStartHighRes, DomainLookupEndHighRes, ConnectStartHighRes, SecureConnectionStartHighRes, ConnectEndHighRes, and ResponseEndHighRes. MozReview-Commit-ID: K5nHql135rb --HG-- extra : rebase_source : e06785203f0f8b01fc7b694ce840f07dc09bc4a1