Eliminate bound checks for "arr[arr.Length - cns]"

[EgorBo]

Motivation: #84210 (comment)

voidTest1(int[]arr){if(arr.Length==0||arr[arr.Length-1]==0)Console.WriteLine();}voidTest2(int[]arr){if(arr.Length<10)return;if(arr[^10]==0)Console.WriteLine();}

Previous codegen:

; Method Test1(int[]):thissubrsp,40moveax, dword ptr [rdx+08H]testeax,eaxje SHORT G_M32023_IG04leaecx,[rax-01H]cmpecx,eaxjae SHORT G_M32023_IG06moveax,ecxcmp dword ptr [rdx+4*rax+10H],0jne SHORT G_M32023_IG05G_M32023_IG04:addrsp,40 tail.jmp[System.Console:WriteLine()]G_M32023_IG05:addrsp,40retG_M32023_IG06:call CORINFO_HELP_RNGCHKFAILint3; Total bytes of code: 48; Method Test2(int[]):thissubrsp,40moveax, dword ptr [rdx+08H]cmpeax,10jge SHORT G_M54292_IG04addrsp,40retG_M54292_IG04:leaecx,[rax-0AH]cmpecx,eaxjae SHORT G_M54292_IG07moveax,ecxcmp dword ptr [rdx+4*rax+10H],0jne SHORT G_M54292_IG06addrsp,40 tail.jmp[System.Console:WriteLine()]G_M54292_IG06:addrsp,40retG_M54292_IG07:call CORINFO_HELP_RNGCHKFAILint3; Total bytes of code: 54

New codegen:

; Method Test1(int[]):thismoveax, dword ptr [rdx+08H]testeax,eaxje SHORT G_M32023_IG04deceaxcmp dword ptr [rdx+4*rax+10H],0jne SHORT G_M32023_IG05G_M32023_IG04: tail.jmp[System.Console:WriteLine()]G_M32023_IG05:ret; Total bytes of code: 23; Method Test2(int[]):thismoveax, dword ptr [rdx+08H]cmpeax,10jge SHORT G_M54292_IG04retG_M54292_IG04:addeax,-10cmp dword ptr [rdx+4*rax+10H],0jne SHORT G_M54292_IG06 tail.jmp[System.Console:WriteLine()]G_M54292_IG06:ret; Total bytes of code: 26

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Issue Details

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voidTest(int[]arr){if(arr.Length==0||arr[arr.Length-1]==0){Console.WriteLine();}}

Was:

; Method Prog:Test(int[]):thissubrsp,40moveax, dword ptr [rdx+08H]testeax,eaxje SHORT G_M2580_IG04leaecx,[rax-01H]cmpecx,eaxjae SHORT G_M2580_IG07moveax,ecxcmp dword ptr [rdx+4*rax+10H],0jne SHORT G_M2580_IG05G_M2580_IG04:call[System.Console:WriteLine()]G_M2580_IG05:nopaddrsp,40retG_M2580_IG07:call CORINFO_HELP_RNGCHKFAILint3; Total bytes of code: 45

New:

; Method Prog:Test(int[]):thissubrsp,40moveax, dword ptr [rdx+08H]testeax,eaxje SHORT G_M2580_IG04deceaxcmp dword ptr [rdx+4*rax+10H],0jne SHORT G_M2580_IG05G_M2580_IG04:call[System.Console:WriteLine()]G_M2580_IG05:nopaddrsp,40ret; Total bytes of code: 32

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

[EgorBo]

@AndyAyersMS@jakobbotsch @dotnet/jit-contrib PTAL

[EgorBo]

Diffs, no TP impact.

[AndyAyersMS]

LGTM.