JIT: optimize redundant Span.Clear

[EgorBo]

void Pattern(int len)
{
    Span<byte> span = (uint)len > 1024 ? new byte[len] : stackalloc byte[len];
    span.Clear();
    Consume(span);
}

Here JIT is expected to avoid calling Clear for new byte[len] path. Also, it should avoid it at all unless SkipLocalsInit is set.

Currently, in order to avoid this perf impact, developers complicate their code.

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[Tornhoof]

Honest question, how often is span.Clear() called in such patterns? I personally haven't done that ever at all I think.

[EgorBo]

Honest question, how often is span.Clear() called in such patterns? I personally haven't done that ever at all I think.

it's from BCL, in some cases we have to call Clear() because we know that SkipLocalsInit is defined globally.