Patch vtable slots and similar when tiering is enabled (#21292)

Patch vtable slots and similar when tiering is enabled

For a method eligible for code versioning and vtable slot backpatch:
  - It does not have a precode (`HasPrecode()` returns false)
  - It does not have a stable entry point (`HasStableEntryPoint()` returns false)
  - A call to the method may be:
    - An indirect call through the `MethodTable`'s backpatchable vtable slot
    - A direct call to a backpatchable `FuncPtrStub`, perhaps through a `JumpStub`
    - For interface methods, an indirect call through the virtual stub dispatch (VSD) indirection cell to a backpatchable `DispatchStub` or a `ResolveStub` that refers to a backpatchable `ResolveCacheEntry`
  - The purpose is that typical calls to the method have no additional overhead when code versioning is enabled

Recording and backpatching slots:
  - In order for all vtable slots for the method to be backpatchable:
    - A vtable slot initially points to the `MethodDesc`'s temporary entry point, even when the method is inherited by a derived type (the slot's value is not copied from the parent)
    - The temporary entry point always points to the prestub and is never backpatched, in order to be able to discover new vtable slots through which the method may be called
    - The prestub, as part of `DoBackpatch()`, records any slots that are transitioned from the temporary entry point to the method's at-the-time current, non-prestub entry point
    - Any further changes to the method's entry point cause recorded slots to be backpatched in `BackpatchEntryPointSlots()`
  - In order for the `FuncPtrStub` to be backpatchable:
    - After the `FuncPtrStub` is created and exposed, it is patched to point to the method's at-the-time current entry point if necessary
    - Any further changes to the method's entry point cause the `FuncPtrStub` to be backpatched in `BackpatchEntryPointSlots()`
  - In order for VSD entities to be backpatchable:
    - A `DispatchStub`'s entry point target is aligned and recorded for backpatching in `BackpatchEntryPointSlots()`
      - The `DispatchStub` was modified on x86 and x64 such that the entry point target is aligned to a pointer to make it backpatchable
    - A `ResolveCacheEntry`'s entry point target is recorded for backpatching in `BackpatchEntryPointSlots()`

Slot lifetime and management of recorded slots:
  - A slot is recorded in the `LoaderAllocator` in which the slot is allocated, see `RecordAndBackpatchEntryPointSlot()`
  - An inherited slot that has a shorter lifetime than the `MethodDesc`, when recorded, needs to be accessible by the `MethodDesc` for backpatching, so the dependent `LoaderAllocator` with the slot to backpatch is also recorded in the `MethodDesc`'s `LoaderAllocator`, see `MethodDescBackpatchInfo::AddDependentLoaderAllocator_Locked()`
  - At the end of a `LoaderAllocator`'s lifetime, the `LoaderAllocator` is unregistered from dependency `LoaderAllocators`, see `MethodDescBackpatchInfoTracker::ClearDependencyMethodDescEntryPointSlots()`
  - When a `MethodDesc`'s entry point changes, backpatching also includes iterating over recorded dependent `LoaderAllocators` to backpatch the relevant slots recorded there, see `BackpatchEntryPointSlots()`

Synchronization between entry point changes and backpatching slots
  - A global lock is used to ensure that all recorded backpatchable slots corresponding to a `MethodDesc` point to the same entry point, see `DoBackpatch()` and `BackpatchEntryPointSlots()` for examples

Due to startup time perf issues:
  - `IsEligibleForTieredCompilation()` is called more frequently with this change and in hotter paths. I chose to use a `MethodDesc` flag to store that information for fast retreival. The flag is initialized by `DetermineAndSetIsEligibleForTieredCompilation()`.
  - Initially, I experimented with allowing a method versionable with vtable slot backpatch to have a precode, and allocated a new precode that would also be the stable entry point when a direct call is necessary. That also allows recording a new slot to be optional - in the event of an OOM, the slot may just point to the stable entry point. There are a large number of such methods and the allocations were slowing down startup perf. So, I had to eliminate precodes for methods versionable with vtable slot backpatch and that in turn means that recording slots is necessary for versionability.

1 files changed, 1 insertions, 8 deletions

diff --git a/Documentation/design-docs/code-versioning.md b/Documentation/design-docs/code-versioning.md
index fdf8413871..928f4a5512 100644
--- a/Documentation/design-docs/code-versioning.md
+++ b/Documentation/design-docs/code-versioning.md
@@ -327,14 +327,7 @@ to update the active child at either of those levels (ReJIT uses SetActiveILCode
 2. Recalculate the active code version for each entrypoint
 3. Update the published code version for each entrypoint to match the active code version
 
-In order to do step 3 the CodeVersionManager relies on one of two different mechanisms, either a FixupPrecode or a JumpStamp. Both techniques roughly involve using a jmp instruction as the method entrypoint and then updating that jmp to point at whatever code version should be published. In the FixupPrecode case this is memory that was allocated dynamically for the explicit purpose of being the method entrypoint. In the JumpStamp this is memory that was initially used as the prolog of the default code version and then repurposed. JumpStamp is required for AOT compiled images that use direct calls from method to method, however changing between prolog instructions and a jmp instruction requires EE suspension to ensure that threads have been evacuated from the region. FixupPrecode can be updated with only an Interlocked operation which offers lower overhead updates when it can be used.
-
-All methods have been classified to use at most one of the techniques, based on:
-
-```
-MethodDesc::IsVersionableWithPrecode()
-MethodDesc::IsVersionableWithJumpStamp()
-```
+In order to do step 3 the `CodeVersionManager` relies on one of three different mechanisms, a `FixupPrecode`, a `JumpStamp`, or backpatching entry point slots. In [method.hpp](https://github.com/dotnet/coreclr/blob/master/src/vm/method.hpp) these mechanisms are described in the `MethodDesc::IsVersionableWith*()` functions, and all methods have been classified to use at most one of the techniques, based on the `MethodDesc::IsVersionableWith*()` functions.
 
 ### Thread-safety ###
 CodeVersionManager is designed for use in a free-threaded environment, in many cases by requiring the caller to acquire a lock before calling. This lock can be acquired by constructing an instance of the