JIT: consider rewrite `movs` logic for struct copies

[markples]

See discussion in #80086

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Issue Details

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See discussion in #80086

Author:	markples
Assignees:	-
Labels:	area-CodeGen-coreclr
Milestone:	9.0.0

[jakobbotsch]

@EgorBo Did we ever end up fixing this? I recall you had some PRs around this, but that block copies perhaps still aren't handled.

[EgorBo]

@EgorBo Did we ever end up fixing this? I recall you had some PRs around this, but that block copies perhaps still aren't handled.

Yep, I only optimized it for inits and only for cases where we can emit SIMD. One thing I remember that rep movs is well aligned with write barriers by the registers they use, so if we switch to normal copy instructions, then we might see spills before/after write barriers