Patch vtable slots and similar when tiering is enabled #21292

@jkotas

For a method eligible for tiered compilation and vtable slot backpatching: - The entry point to the final code is versionable, as for any method eligible for tiering - 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 tiering 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 `MethodDescVirtualInfo::AddDependentLoaderAllocatorsWithSlotsToBackpatch_Locked()` - At the end of a `LoaderAllocator`'s lifetime, the `LoaderAllocator` is unregistered from dependency `LoaderAllocators`, see `LoaderAllocator::ClearDependencyMethodDescEntryPointSlotsToBackpatchHash()` - 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 tiered vtable method 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 tiered vtable methods and that in turn means that recording slots is necessary for versionability. @jkotas @davidwrighton @noahfalk @AndyAyersMS

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Patch vtable slots and similar when tiering is enabled #21292

Patch vtable slots and similar when tiering is enabled #21292

Commits on Jan 11, 2019