Remove bounds checks for 'i + cns < len' pattern

[EgorBo]

Closes #121262

public static bool Test(ReadOnlySpan<char> span)
{
    for (int i = 0; i < span.Length; i++)
    {
        if (span[i] == '%' && (uint)(i + 2) < (uint)span.Length)
        {
            if (span[i + 1] == 'F' && // Bounds check
                span[i + 2] == 'F')
            {
                return true;
            }
        }
    }
    return false;
}

New codegen:

; Method Prog:Test(System.ReadOnlySpan`1[char]):bool (FullOpts)
       mov      rax, bword ptr [rcx]
       mov      ecx, dword ptr [rcx+0x08]
       xor      edx, edx
       test     ecx, ecx
       je       SHORT G_M967_IG06
       align    [0 bytes for IG03]
G_M967_IG03:
       cmp      word  ptr [rax+2*rdx], 37
       jne      SHORT G_M967_IG05
       lea      r8d, [rdx+0x02]
       cmp      r8d, ecx
       jae      SHORT G_M967_IG05
       lea      r10d, [rdx+0x01]
       cmp      word  ptr [rax+2*r10], 70
       jne      SHORT G_M967_IG05
       cmp      word  ptr [rax+2*r8], 70
       je       SHORT G_M967_IG08
G_M967_IG05:
       inc      edx
       cmp      edx, ecx
       jl       SHORT G_M967_IG03
G_M967_IG06:
       xor      eax, eax
       ret      
G_M967_IG08:
       mov      eax, 1
       ret      
; Total bytes of code: 63

Diffs

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[Copilot]

Pull request overview

This PR updates CoreCLR JIT range-check analysis to recognize unsigned compare patterns like (uint)(i + cns) < (uint)len and use them to tighten inferred ranges, enabling elimination of redundant bounds checks for nearby indices (e.g., i+1, i+2) in common Span/array loops.

Changes:

• Extend RangeCheck::MergeEdgeAssertions to derive a range for a base value number (normalLclVN) from an assertion on an offset form (normalLclVN + cns).
• Prefer deductions that relate the base induction variable to the “preferred” checked bound VN (typically the length VN).

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated no new comments.

[EgorBo]

PTAL @jakobbotsch @dotnet/jit-contrib

[EgorBo]

/ba-g deadletter + infra issues + filed #124279