Fix RangeOps::And incorrect range analysis for bitwise AND

[Copilot]

Description

RangeOps::And was producing incorrect range estimates when one operand had an unknown or wide range with a negative lower bound. This caused the JIT to make incorrect optimization decisions.

Repro:

public static sbyte s_4 = 1;
public static void Main()
{
    // Debug: condition is false (correct)
    // Release: condition was true (bug)
    if (35009 > (1264240267 & (sbyte)(-s_4)))
    {
        System.Console.WriteLine(1UL);
    }
}

[1264240267..1264240267] & [-128..127] = [0..127] which is incorrect

Changes

• Range::IsSingleConstValue — New helper to check if both limits are equal constants
• RangeOps::And — Rewritten to be more conservative:
  • Both operands single constants → return actual AND result
  • One operand is a non-negative single constant → return [0, constant]
  • Otherwise → return unknown (instead of potentially incorrect bounds)
• RangeOps::UnsignedMod — Simplified to use IsSingleConstValue helper
• Regression test — Runtime_123583.cs

Original prompt

File a PR to fix #123583 bug.

This RangeOps::And implementation should fix it - use it:

static Range And(const Range& r1, const Range& r2)
{
    // Conservatively expect at least one operand to be a single-value constant.
    // The math gets too complicated otherwise (and rarely useful in practice).
    int r1ConstVal, r2ConstVal;
    bool r1IsConstVal = r1.IsSingleConstValue(&r1ConstVal);
    bool r2IsConstVal = r2.IsSingleConstValue(&r2ConstVal);

    // Both ranges are single constant values.
    // Example: [7..7] & [3..3] = [3..3]
    if (r1IsConstVal && r2IsConstVal)
    {
        return Range(Limit(Limit::keConstant, r1ConstVal & r2ConstVal));
    }

    // One of the ranges is a single constant value.
    // Example: [unknown..unknown] & [3..3] = [0..3]
    if (r1IsConstVal && (r1ConstVal >= 0))
    {
        return Range(Limit(Limit::keConstant, 0), Limit(Limit::keConstant, r1ConstVal));
    }
    if (r2IsConstVal && (r2ConstVal >= 0))
    {
        return Range(Limit(Limit::keConstant, 0), Limit(Limit::keConstant, r2ConstVal));
    }

    // Otherwise, we cannot determine the result range.
    return Range(Limit(Limit::keUnknown));
}

implement Range::IsSingleConstValue.

Also, add a regression test for this change, use src/tests/JIT/Regression/JitBlue/Runtime_122288/Runtime_122288.cs as a reference, but name it Runtime_123583 and use the repro from the issue description.

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[EgorBo]

@copilot add a comment on top of IsSingleConstValue:

// Check if the range represents a single constant value. Example: [7..7]

Also, use IsSingleConstValue in RangeOps::UnsignedMod (to simplify impl)

[Copilot]

@copilot add a comment on top of IsSingleConstValue:

// Check if the range represents a single constant value. Example: [7..7]

...

Done in c11cc3a. Added the comment and simplified UnsignedMod to use IsSingleConstValue.

[Copilot]

Pull request overview

This PR fixes a critical bug in the JIT compiler's range analysis for bitwise AND operations. The bug caused incorrect optimization decisions when one operand had a wide range with a negative lower bound, leading to wrong program behavior in release builds.

Changes:

• Fixed RangeOps::And to be more conservative and correct in its range analysis
• Added Range::IsSingleConstValue helper method to check if a range represents a single constant
• Simplified UnsignedMod to use the new helper
• Added regression test to prevent future regressions

Reviewed changes

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

File	Description
src/coreclr/jit/rangecheck.h	Added IsSingleConstValue helper; rewrote And to fix incorrect range calculation; simplified UnsignedMod; improved const-correctness
src/tests/JIT/Regression/JitBlue/Runtime_123583/Runtime_123583.cs	Added regression test to verify the fix prevents incorrect optimization of bitwise AND with mixed-sign ranges

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