[release/7.0] fix div by zero in gen size calculation

[github-actions]

Backport of #76294 to release/7.0

/cc @Maoni0

Customer Impact

This was reported with #76257. It's a bug that only shows up with regions. With segments, because we always allocate a min object in each generation to indicate the generation start, the size can never be 0. With regions we no long need to but this also means a generation size can be 0. The workaround is unacceptable to the customer -

it is not easy to disable floating point exception around garbage collector because it may be called everywhere. Net 6 and older versions (and the mono runtime) don't need such a workaround. In the HPC world it is a common practice to trap FPE to detect false numerical results. I agree that some .Net libraries may not support that but if the runtime doesn't have this new limitation it is better.

Testing

I tested this under the debugger. I can see the div zero exception and with the fix I no longer see it.

Risk

Not for a servicing release.

IMPORTANT: Is this backport for a servicing release? If so and this change touches code that ships in a NuGet package, please make certain that you have added any necessary package authoring and gotten it explicitly reviewed.

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Issue Details

---

Backport of #76294 to release/7.0

/cc @Maoni0

Customer Impact

Testing

Risk

IMPORTANT: Is this backport for a servicing release? If so and this change touches code that ships in a NuGet package, please make certain that you have added any necessary package authoring and gotten it explicitly reviewed.

Author:	github-actions[bot]
Assignees:	-
Labels:	area-GC-coreclr
Milestone:	-

[carlossanlop]

@Maoni0 if this is ready, can you please add the servicing-consider label and send an email to Tactics requesting approval?

[carlossanlop]

Approved by Tactics via email. Signed off.
CI failure was a timeout cancellation in OSX NativeAOT.
Ready to merge.