Testing Span memory alignment

[zmj]

I have a method that takes a Span of bytes, and need to operate on the data as ulongs. Currently, I'm copying the input to a 64-bit aligned buffer. What's the best way to test if the input buffer is already aligned, in order to use MemoryMarshal.Cast and skip the copy?

[gfoidl]

Be aware of potential GC that moves the data, so alignment may not hold when not being pinned.

---

Get the address of data, then do a modulo against the desired alignment. E.g.

static unsafe bool IsAligned(ReadOnlySpan<byte> span, nuint alignmentInBytes)
{
    // Note: GC can happen anytime, it's not deterministic from user PoV, so the address may change anytime too as there's no indication that data is pinned in memory
    ref byte r = ref MemoryMarshal.GetReference(span);
    void* ptr  = Unsafe.AsPointer(ref r);

    return (nuint)ptr % alignmentInBytes == 0;
}

or saver when data is already pinned (fixed-keyword):

static unsafe bool IsAligned(void* ptr, nuint alignmentInBytes)
{
    return (nuint)ptr % alignmentInBytes == 0;
}

[deeprobin]

In memory tests I have always entered a NoGC region so that the test does not become flaky.