Add FileCheck test for elided bounds checks

[Copilot]

Fixes #123583

Description

Adds a FileCheck test that verifies codegen doesn't contain bounds checks (CORINFO_HELP_RNGCHKFAIL) for patterns where the JIT should prove access is in-bounds.

Test Methods

• ComplexBinaryOperators - Masking/shifting restricts index to 0-63 for 65-element span
• LastCharCheck - String length comparison before indexed access
• CountDigits - ulong.Log2() result (0-63) indexes 64-element span
• AndByConst - Bitwise AND with constant
• AndByLength - Bitwise AND with (span.Length - 1) pattern

Files Added

• src/tests/JIT/opt/RangeChecks/ElidedBoundsChecks.cs - Test methods with X64-NOT and ARM64-NOT FileCheck annotations
• src/tests/JIT/opt/RangeChecks/ElidedBoundsChecks.csproj - Project with HasDisasmCheck=true

[MethodImpl(MethodImplOptions.NoInlining)]
static byte AndByLength(int i)
{
    // X64-NOT: CORINFO_HELP_RNGCHKFAIL
    // ARM64-NOT: CORINFO_HELP_RNGCHKFAIL
    ReadOnlySpan<byte> span = new byte[] { 1, 2, 3, 4 };
    return span[i & (span.Length - 1)];
}

Original prompt

Add a test that uses FileCheck to varify that codegen doesn't contain bounds checks at all

the test should be located in src\tests\JIT\opt\RangeChecks folder

Check on x64 and arm64, here are the asm examples how bounds checks look like:

; x64:
call     CORINFO_HELP_RNGCHKFAIL
; arm64:
bl      CORINFO_HELP_RNGCHKFAIL

Use .cs and .csproj in C:\prj\runtime-main2\src\tests\JIT\opt\Subtract\ as a reference on how to create FileCheck tests (HasDisasmCheck)

Here are the functions I want to make sure they don't have bounds checks under optimizations:

public class ElidedBoundsChecks
{
    [MethodImpl(MethodImplOptions.NoInlining)]
    static int ComplexBinaryOperators(byte inData)
    {
        ReadOnlySpan<byte> base64 = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/="u8;
        return base64[((inData & 0x03) << 4) | ((inData & 0xf0) >> 4)];
    }

    [MethodImpl(MethodImplOptions.NoInlining)]
    static bool LastCharCheck(string prefix, string path)
    {
        if (prefix.Length < path.Length)
            return (path[prefix.Length] == '/');
        return false;
    }

    [MethodImpl(MethodImplOptions.NoInlining)]
    static nint CountDigits(ulong value)
    {
        ReadOnlySpan<byte> log2ToPow10 =
        [
            1,  1,  1,  2,  2,  2,  3,  3,  3,  4,  4,  4,  4,  5,  5,  5,
            6,  6,  6,  7,  7,  7,  7,  8,  8,  8,  9,  9,  9,  10, 10, 10,
            10, 11, 11, 11, 12, 12, 12, 13, 13, 13, 13, 14, 14, 14, 15, 15,
            15, 16, 16, 16, 16, 17, 17, 17, 18, 18, 18, 19, 19, 19, 19, 20
        ];
        return log2ToPow10[(int)ulong.Log2(value)];
    }

    [MethodImpl(MethodImplOptions.NoInlining)]
    static byte AndByConst(int i)
    {
        ReadOnlySpan<byte> span = new byte[] { 1, 2, 3, 4 };
        return span[i & 2];
    }

    [MethodImpl(MethodImplOptions.NoInlining)]
    static byte AndByLength(int i)
    {
        ReadOnlySpan<byte> span = new byte[] { 1, 2, 3, 4 };
        return span[i & (span.Length - 1)];
    }
}

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[Copilot]

Pull request overview

Adds a JIT FileCheck-based regression test to ensure bounds checks are fully elided (no CORINFO_HELP_RNGCHKFAIL) for several common “proven in-range” indexing patterns on x64 and arm64.

Changes:

• Add new ElidedBoundsChecks test methods that exercise range-analysis patterns where the JIT should prove indices are in-bounds.
• Add FileCheck X64-NOT / ARM64-NOT annotations to ensure no CORINFO_HELP_RNGCHKFAIL appears in generated code.
• Add a dedicated test project with HasDisasmCheck=true and environment settings to force optimized JIT compilation.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/tests/JIT/opt/RangeChecks/ElidedBoundsChecks.cs	Introduces FileCheck-annotated methods and an entrypoint that exercises them.
src/tests/JIT/opt/RangeChecks/ElidedBoundsChecks.csproj	Enables disasm-based checking (HasDisasmCheck) and configures environment for stable optimized codegen.

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[EgorBo]

@dotnet/jit-contrib PTAL a FileCheck test. I just wanted to make sure none of my BCE related tests silently break these cases (will probably add various loop-related tests at some point)

[EgorBo]

/ba-g deadletter