NativeAOT: Optimize static fields of gc types

[EgorBo]

Follow up to #79709 but for gc fields now, e.g.:

static string field;

static string GetField() => field;

Was:

; Method ConsoleApp1.Program:GetField():System.String
       4883EC28             sub      rsp, 40
       E800000000           call     CORINFO_HELP_READYTORUN_GCSTATIC_BASE
       488B4008             mov      rax, gword ptr [rax+08H]
       4883C428             add      rsp, 40
       C3                   ret      
; Total bytes of code: 18

Now:

; Method ConsoleApp1.Program:GetField():System.String
       488B0500000000       mov      rax, bword ptr [(reloc 0x40000000004225c0)]
       488B4008             mov      rax, gword ptr [rax+08H]
       C3                   ret      
; Total bytes of code: 12

based on #63620 and @SingleAccretion's suggestions

[ghost]

Tagging subscribers to this area: @JulieLeeMSFT, @jakobbotsch, @kunalspathak
See info in area-owners.md if you want to be subscribed.

Issue Details

---

Follow up to #79709 but for gc fields now, e.g.:

static string field;

static string GetField() => field;

Was:

; Method ConsoleApp1.Program:GetField():System.String
       4883EC28             sub      rsp, 40
       E800000000           call     CORINFO_HELP_READYTORUN_GCSTATIC_BASE
       488B4008             mov      rax, gword ptr [rax+08H]
       4883C428             add      rsp, 40
       C3                   ret      
; Total bytes of code: 18

Now:

; Method ConsoleApp1.Program:GetField():System.String
       488B0500000000       mov      rax, bword ptr [(reloc 0x40000000004225c0)]
       488B4008             mov      rax, gword ptr [rax+08H]
       C3                   ret      
; Total bytes of code: 12

based on #63620 and @SingleAccretion's suggestions

Author:	EgorBo
Assignees:	EgorBo
Labels:	area-CodeGen-coreclr
Milestone:	-

[EgorBo]

/azp run runtime-extra-platforms

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[EgorBo]

@SingleAccretion PTAL when you have time 🙂 CI looks good so far

[EgorBo]

Failures are all Mono-related in runtime-extra-platforms: #80976 and likely dotnet/arcade#12266

[SingleAccretion]

Jit part LGTM!

[EgorBo]

@MichalStrehovsky PTAL NAOT side. runtime-extra-platforms NativeAOT tests all are green

[MichalStrehovsky]

Awesome, thank you!

[MichalStrehovsky]

Just to double check - typing this a TYP_BYREF and retyping it as TYP_I_IMPL when creating the GT_ADD below won't cause any issues?

Could we just type it as TYP_I_IMPL here? If I'm reading it right we're assuming it's pinned anyway.

[EgorBo]

Ah, right, TYP_I_IMPL works here to for the static base address (and as we discussed inhttps://github.com//pull/79709 it doesn't lead to constant folding) and won't generate unnecessary gc info. The root GT_IND node has TYP_REF for gc refs so everything should be fine.