dotnet/roslyn

Int64 indices are silently truncated when used to access MD array elements

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#30,648 opened on Oct 22, 2018

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Area-CompilersBughelp wanted

Description

The following code should produce an exception (IndexOutOfRangeException but OverflowException would do too I guess) but it does not:

        static void Main() => Test(new int[42, 42], 41, 41);
        [MethodImpl(MethodImplOptions.NoInlining)]
        static void Test(int[,] a, long x, long y) => a[x | (0x12345678L << 32), y] = 42;

The generated IL code is:

    ldarg.0
    ldarg.1
    ldc.i8     0x1234567800000000
    or
    conv.ovf.i
    ldarg.2
    conv.ovf.i
    ldc.i4.s   42
    call instance void int32[0..., 0...]::Set(int32,int32,int32)

The spec appears to be silent about this case. From the generated code it looks like the intention was to handle this case by using a checked cast, the same is done with SZ arrays.

There must have been some confusion with ldelem. That one accepts both int32 and native int but MD array accessors support only int32 indices and passing a native int argument in this case would result in silent truncation. That conv.ovf.i should really be conv.ovf.i4.

Reproduced with VS 15.8.7 but I'd guess that the problem is much older.

Contributor guide

Tech stack
csharp
Domain
backend
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
5
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
over 1 week
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
C# and .NET arraysIL and conv.ovf instructionsRoslyn source code structureCompiler debugging skills
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
15
Research direction
Investigate the IL emission for multidimensional array access in the Roslyn compiler. Look at the code that generates the `conv.ovf.i` instruction and change it to `conv.ovf.i4` for MD array indices. The relevant code is likely in the emit phase. Also verify that the same issue doesn't exist for other array access methods.