GCStress: ensure every object moves on a GC

[BruceForstall]

At least, every object that is legal to move should move.

One problem with GCStress today is that when a GC occurs, most of the time (in some measurement I've done) objects don't actually move. This means that bad GC info doesn't get noticed.

[Maoni0]

related https://github.com/dotnet/coreclr/issues/10403

[Maoni0]

@BruceForstall

most of the time (in some measurement I've done) objects don't actually move

can you please tell me which tests you used to do your measurements? I just want to establish a baseline before we put in fixes for this.

[BruceForstall]

I can't remember what I was doing at the time. I was probably looking at the CLR logging (or maybe GC logging?) for a GCStress 4 or 8 or 0xC run of some relatively long-running test with allocations, and noticing that the output indicated objects didn't move.

[Maoni0]

ok... what do you recommend that we use to see the effect of the fixes? just run jit tests with GCStress 4/8/C?

[BruceForstall]

If you want to verify that objects move, you'll obviously need a test that uses some kind of internal logging with before/after addresses for every object. Or maybe a COMPlus variable that asserts when they are the same after a GC.

In terms of correctness, we run lots of GCStress 0xC jobs in the CI that can be triggered.

[Maoni0]

I was not asking how to verify whether an object moves during a GC. I was asking for a set of tests your team normally runs so I can say "without these fixes x% objects moved for your scenarios whereas with them y% objects moved and y >> x, success!".

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[BruceForstall]

@davidwrighton We were chatting about this issue recently.