RFR: 8344414: ZGC: Another division by zero in rule_major_allocation_rate

[Erik Österlund]

On Tue, 19 Nov 2024 07:18:20 GMT, Axel Boldt-Christmas <aboldtch at openjdk.org> wrote:

> This specific issue was known since #20888.  As well as a more serious issue in `calculate_extra_young_gc_time` which may introduce a `NaN`. This specific issue is sane as long as we have IEEE 754 sans the C++ standard making division by zero UB.
> 
> As discussed in #21304 it is probably better to try and tackle the division by zero issue by making sure the input is never zero. This patch introduces a small offset to the average which will effectively leave the value unchanged unless it is zero, and behave as almost zero in calculations without causing actual division by zero.
> 
> There is still the issue with `NaN`, this patch adds a short circuit when this can occur and returns the analytical result of the calculation.

Looks good.

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Marked as reviewed by eosterlund (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/22228#pullrequestreview-2461361841