JIT: ARR_LENGTH(new T[CNS]) --> CNS

[EgorBo]

Quick 5 lines-of-code change to enable ARR_LENGTH(JitNewArr) constant folding in VN:

int Test()
{
    var a = new int[100];
    return a.Length;
}

Current codegen:

; Method Proga:Test():int:this
       sub      rsp, 40
       mov      rcx, 0xD1FFAB1E      ; int[]
       mov      edx, 100
       call     CORINFO_HELP_NEWARR_1_VC
       mov      eax, dword ptr [rax+08H]
       add      rsp, 40
       ret      
; Total bytes of code: 32

New codegen:

; Method Proga:Test():int:this
       mov      eax, 100
       ret      
; Total bytes of code: 6

I recommend enabling "hide whitespaces" for code review since most of the changes are just extra { } block

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Issue Details

---

3 lines-of-code change to enable ARR_LENGTH(JitNewArr) in VN:

int Test()
{
    var a = new int[100];
    return a.Length;
}

Current codegen:

; Method Proga:Test():int:this
       sub      rsp, 40
       mov      rcx, 0xD1FFAB1E      ; int[]
       mov      edx, 100
       call     CORINFO_HELP_NEWARR_1_VC
       mov      eax, dword ptr [rax+08H]
       add      rsp, 40
       ret      
; Total bytes of code: 32

New codegen:

; Method Proga:Test():int:this
       mov      eax, 100
       ret      
; Total bytes of code: 6

I recommend enabling "ignore whitespaces" since most of the changes are just extra { } block

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

[EgorBo]

@dotnet/jit-contrib PTAL, simple change (if you ignore whitespace changes). diffs

[SingleAccretion]

There is also VNF_JitReadyToRunNewArr.

There is ValueNumStore::GetNewArrSize, it would be ideal to extract a part from it that deals with an already-queried VNFuncApp (e. g. as bool TryGetNewArrSize(VNFuncApp* pFunc, int* pSize)).

[EgorBo]

yeah, my first impl used GetNewArrSize but for TP reasons I inlined it here, forgot about R2R

[EgorBo]

Simplified the impl, GetVNFunc is still here, if TP report is fine with that then I don't think I need to complicate it more.

[EgorBo]

[EgorBo]