RyuJIT: String.Empty vs "" #42694

It looks like some of the jit optimizations aren't String.Empty friendly and prefer "" instead, e.g.:

Inliner

class Tests
{
    static int GetLengthWithLimit(string str) => str.Length > 10 ? str.Length : 0;

    // I expect both Case1 and Case2 to have the same codegen:
    int Case1() => GetLengthWithLimit("");
    int Case2() => GetLengthWithLimit(string.Empty);
}

Current codegen for Case1 and Case2:

; Method Case1():int:this
       33C0                 xor      eax, eax    ;; <--- inlined & folded into `return 0;`
       C3                   ret      
; Total bytes of code: 3


; Method Case2():int:this
       48B9603000E9D7010000 mov      rcx, 0x1D7E9003060
       488B09               mov      rcx, gword ptr [rcx]
       E95E81FFFF           jmp      Tests:GetLengthWithLimit(System.String):int    ;; <--- not even inlined!
; Total bytes of code: 18

JIT imports string.Empty as a GT_IND + <address> node whether "" is emitted as GT_CNS_STR (which is then transformed into GT_IND + <address> later but the optimizations expect GT_CNS_STR). Since GT_CNS_STR node is know to be a const (has GTK_CONST flag), the inliner for the second case (Case2) has:

Inline candidate has **const arg** that feeds a conditional.  Multiplier increased to 5.

and doesn't have it for Case1.

String.get_Length optimization

This optimization also supports only GT_CNS_STR nodes, e.g:

int Case1() => "".Length;
int Case2() => string.Empty.Length;

Codgen:

; Method Case1():int:this
       33C0                 xor      eax, eax
       C3                   ret      


; Method Case2():int:this
       488B00               mov      rax, gword ptr [rax]
       8B4008               mov      eax, dword ptr [rax+8]
       C3                   ret   

I wonder if we can easily fix it by importing String.Empty as a fake (or real) GT_CNS_STR node like this: EgorBo@09f81b1 (this prototype fixes both issues above ^)

Jit-diff: https://gist.github.com/EgorBo/c6c12e79b7b90ff7aaee58f3fb5a49fb (diff is a "regression" because the fix allows to inline more stuff)
/cc @dotnet/jit-contrib

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