JIT: Preserve vector division exceptions during VN

[dhartglassMSFT]

Value numbering for vector division must take exception behavior into account.

fixes #133844

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[Copilot]

🔵 Needs a closer look

Unsigned vector-division coverage and a nearby exception-contract comment still need updates; the project entry also needs reordering.

Pull request overview

Fixes #133844 by preserving xarch vector integer-division exception behavior during JIT value numbering.

Changes:

• Models divide-by-zero and signed overflow exceptions for vector division.
• Adds regression coverage for divide-by-zero and overflow.
• Registers the new regression test.

File summaries

File	Summary	Final review findings
src/tests/JIT/Regression/Regression_o_2.csproj	Registers the regression test.	Nit (1 vote): Move Runtime_133844 into lexicographic order between Runtime_133209 and Runtime_13417.
src/tests/JIT/Regression/JitBlue/Runtime_133844/Runtime_133844.cs	Tests preserved vector division exceptions.	None.
src/coreclr/jit/valuenum.cpp	Models vector division exceptions during value numbering.	Moderate (1 vote): Add optimized Vector<uint> divide-by-zero coverage. Nit (1 vote): Update the nearby comment so it does not incorrectly describe the vector division exception contract.

Review details

Suppressed comments (3)

src/coreclr/jit/valuenum.cpp:14336

• The comment immediately below this block (around line 14359) still says these intrinsics can only throw NullReferenceException, but this block now adds DivideByZeroException and ArithmeticException to the VN exception set for vector division. Please narrow that comment to the memory-load/store case or otherwise update it so it does not describe an incorrect exception contract.

    }

src/coreclr/jit/valuenum.cpp:14334

• The new varTypeIsSigned split leaves the unsigned vector-division path untested: this regression only uses Vector<int>, and the existing generic divide-by-zero tests cover signed types but omit uint. Since this branch intentionally excludes VNF_ArithmeticExc for Vector<uint> while still adding the divide-by-zero exception, add an optimized Vector<uint> divide-by-zero case so regressions in the unsigned behavior are caught.

        if (varTypeIsSigned(tree->GetSimdBaseType()))
        {
            ValueNumPair arithmeticExc = vnStore->VNPExcSetSingleton(
                vnStore->VNPairForFuncNoFolding(TYP_REF, VNF_ArithmeticExc, dividendVNP, divisorVNP));
            excSetPair = vnStore->VNPExcSetUnion(excSetPair, arithmeticExc);

src/tests/JIT/Regression/Regression_o_2.csproj:146

• This entry breaks the lexicographic order used by the surrounding Compile items: Runtime_133844 should be between Runtime_133209 and Runtime_13417, rather than after Runtime_41100. Please move it into that position to preserve the project's deterministic ordering.

    <Compile Include="JitBlue\Runtime_133844\Runtime_133844.cs" />

• Files reviewed: 3/3 changed files
• Comments generated: 0
• Review effort level: Lite

[Copilot]

🔵 Needs a closer look

Add unsigned-vector divide-by-zero regression coverage.

Review details

Suppressed comments (1)

src/tests/JIT/Regression_o_2/Runtime_133844.cs:29

• Please add an unsigned-vector divide-by-zero case alongside these signed tests. The production change intentionally has a separate signed/unsigned path at valuenum.cpp:14330-14335, but the regression suite only instantiates Vector<int>, so a future regression in Vector<uint> DivideByZeroException preservation would go undetected.

    [Fact]
    public static void OverflowIsPreserved()
    {
        Assert.Throws<OverflowException>(new Runtime_133844().Overflow);
    }

• Files reviewed: 2/2 changed files
• Comments generated: 0 new
• Review effort level: Lite

[EgorBo]

Can we re-use fgValueNumberAddExceptionSetForDivision / fgValueNumberDivisionExceptions used for scalar divisions?