Cache debugger patches to speed up x64 stackwalk epilogue/prologue scanning

[hoyosjs]

Second partial fix for #122459

Caches the list of debugger breakpoint patches in the DAC so that x64 stack unwinding doesn't re-scan the patch hash table on every frame. During mini dump collection, each stack frame triggers DacReplacePatchesInHostMemory to restore original opcodes before reading memory — even though there are typically zero active patches during a dump. The patch hash table has 1,000 fixed buckets, so each call walked all of them regardless. The cache is populated once on first access and invalidated only on Flush().

Measured minidump collection against the same repro app with 10,000 iterations across 10 threads. The baseline was 55s, this change alone brings it to ~7s

[dotnet-policy-service]

Tagging subscribers to this area: @steveisok, @tommcdon, @dotnet/dotnet-diag
See info in area-owners.md if you want to be subscribed.

[Copilot]

Pull request overview

This PR optimizes DAC-side x64 stackwalk instruction scanning by caching debugger breakpoint patches so the unwinder can quickly replace patched opcodes without repeatedly iterating the target’s patch hash table.

Changes:

• Introduces a DacPatchCache that snapshots patch addresses/opcodes on first use and reuses them for subsequent patch replacement.
• Updates DacReplacePatchesInHostMemory to use the cached patch list instead of enumerating the patch table each time.
• Flushes the patch cache from ClrDataAccess::Flush() to invalidate cached entries when DAC caches are reset.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
src/coreclr/debug/daccess/dacimpl.h	Adds DacPatchCache/entry types and stores the cache on ClrDataAccess.
src/coreclr/debug/daccess/dacfn.cpp	Uses the cache in DacReplacePatchesInHostMemory and implements cache population.
src/coreclr/debug/daccess/daccess.cpp	Flushes the patch cache during ClrDataAccess::Flush().

---

You can also share your feedback on Copilot code review. Take the survey.

[hoyosjs]

/backport to release/8.0

[github-actions]

Started backporting to release/8.0 (link to workflow run)

[hoyosjs]

/backport to release/10.0

[github-actions]

Started backporting to release/10.0 (link to workflow run)

[github-actions]

@hoyosjs backporting to release/8.0 failed, the patch most likely resulted in conflicts. Please backport manually!

git am output

$ git am --3way --empty=keep --ignore-whitespace --keep-non-patch changes.patchApplying: Cache debugger patches to speed up x64 stackwalk epilogue/prologue scanningUsing index info to reconstruct a base tree...M	src/coreclr/debug/daccess/daccess.cppM	src/coreclr/debug/daccess/dacfn.cppM	src/coreclr/debug/daccess/dacimpl.hFalling back to patching base and 3-way merge...Auto-merging src/coreclr/debug/daccess/daccess.cppAuto-merging src/coreclr/debug/daccess/dacfn.cppAuto-merging src/coreclr/debug/daccess/dacimpl.hCONFLICT (content): Merge conflict in src/coreclr/debug/daccess/dacimpl.herror: Failed to merge in the changes.hint: Use 'git am --show-current-patch=diff' to see the failed patchhint: When you have resolved this problem, run "git am --continue".hint: If you prefer to skip this patch, run "git am --skip" instead.hint: To restore the original branch and stop patching, run "git am --abort".hint: Disable this message with "git config set advice.mergeConflict false"Patch failed at 0001 Cache debugger patches to speed up x64 stackwalk epilogue/prologue scanningError: The process '/usr/bin/git' failed with exit code 128

Link to workflow output