[mono][aot] Avoid generating rgctx fetch trampolines in generic sharing

[kotlarmilos]

Description

For a generic sharing methods, size of variable types is only known at runtime and AOT compiler cannot allocate static stack slots for them. Instead, a stack area is allocated at runtime and address is dynamically computed. The information required for this is either stored in the vtable or in a MonoMethodRuntimeGenericContext structure. In the structure, the offsets are known at compile time so it can generate inline code. However, in the vtable, they are assigned at runtime, so rgctx fetch trampolines are used to read it.

In order to improve size of an AOT image, the idea is to pass the structure to all gshared methods, and thus avoid generating rgctx fetch trampolines. As it requires a lot of changes, an experimental data on size and speed should be retrieved before considering the implementation.

[ghost]

Tagging subscribers to 'size-reduction': @eerhardt, @SamMonoRT, @marek-safar
See info in area-owners.md if you want to be subscribed.

Issue Details

---

Description

For a generic sharing methods, size of variable types is only known at runtime and AOT compiler cannot allocate static stack slots for them. Instead, a stack area is allocated at runtime and address is dynamically computed. The information required for this is either stored in the vtable or in a MonoMethodRuntimeGenericContext structure. In the structure, the offsets are known at compile time so it can generate inline code. However, in the vtable, they are assigned at runtime, so rgctx fetch trampolines to read it.

In order to improve size of an AOT image, the idea is to pass the structure to all gshared methods, and thus avoid generating rgctx fetch trampolines. As it requires a lot of changes, an experimental data on size and speed should be retrieved before considering the implementation.

Author:	kotlarmilos
Assignees:	kotlarmilos
Labels:	area-Codegen-AOT-mono, size-reduction
Milestone:	Future

[vargaz]

This PR can be used to test this:
#82981

[kotlarmilos]

@vargaz I've tested the change on HelloiOS app and size reduction could be about 700kb. About 1k rgctx fetch trampolines are gone, but there are still some rgctx fetch calls. Symbols from binaries before and after the change are attached.

bloaty-symbol-compare.txt
bloaty-symbols-before-change.txt
bloaty-symbols-after-change.txt

[kotlarmilos]

@vargaz I repeated the measurements but didn't get any size difference. What could be the case?