RFR: 8366145: Help diagnose ubsan division by zero in computing pause time ratios

[Albert Mingkun Yang]

On Tue, 26 Aug 2025 11:22:30 GMT, Leo Korinth <lkorinth at openjdk.org> wrote:

> This pull request is created to help diagnose the bug report https://bugs.openjdk.org/browse/JDK-8354430.
> 
> The problem is that we get a ubsan runtime error when dividing by zero. Ubsan is not normally running, so I will add an assert making it easier to catch the problem and printing the value of the two variables that when subtracted will create a zero; it will hopefully help with diagnosis.
> 
> Some comments: 
> 1) the result of `most_recent_gc_end_time_sec()` will not change within the function, thus I will not clutter the code with a new variable, is that okay?
> 2) I added two calls to `fabs` which are most probably not needed (ieee floating point rules seems to say that positive and negative zero equals). However I think my proposed code makes it clear that I have thought about it. 
> 
> I am now running tier1-3

I suggest adding a `G1: ` prefix in the title.

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PR Comment: https://git.openjdk.org/jdk/pull/26939#issuecomment-3228709050