[release/8.0-staging] GT_STORE_BLK - do not call memset for blocks containg gc pointers on heap

[github-actions]

Backport of #95530 to release/8.0-staging

/cc @EgorBo

Customer Impact

When jit needed to zero a struct with gc references it used to emit a call to an internal helper JIT_MemSet that didn't have atomicity guarantees and could lead to torn values if GC happened during that memset. This issue was present from .NET core 3.

Example:

void Test(ref LargeStructWithGC s)
{
    s = default; // GC may stop during this zeroing and read `a` field which might be half-zeroed.
}

struct LargeStructWithGC
{
    long b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, r, s, t, u, v, w, z;
    string a;
}

The PR fixes that issue by inlining memset so we can 100% guarantee atomicity by using correct instructions.
The original issue was found via internal escalation (service crashing 3x per day).

Testing

Local testing + stress/outerloop jobs

Risk

Low. It is not a common case. It happens for a big struct with GC types. Even our SPMI has very few of these cases.

[ghost]

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Issue Details

---

Backport of #95530 to release/8.0-staging

/cc @EgorBo

Customer Impact

Testing

Risk

IMPORTANT: If this backport is for a servicing release, please verify that:

• The PR target branch is release/X.0-staging, not release/X.0.

• If the change touches code that ships in a NuGet package, you have added the necessary package authoring and gotten it explicitly reviewed.

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

[BruceForstall]

LGTM

You should probably run outerloop and jitstress jobs to ensure the 8.0 sources haven't diverged in important ways from main.

[EgorBo]

/azp run runtime-coreclr outerloop, runtime-coreclr jitstress

[azure-pipelines]

Azure Pipelines successfully started running 2 pipeline(s).

[carlossanlop]

@EgorBo Friendly reminder that Tuesday January 16th 4pm is the Code Complete deadline for the February Release. If all requirements are met, please merge your PR before that date and time to ensure this fix gets included in that Release.

[JulieLeeMSFT]

@jeffschwMSFT, please consider this for approval.

[jeffschwMSFT]

approved. we will take for consideration in 8.0.x