[corlib] Default to fast Span<T>

[marek-safar]

Part of #7249

[vargaz]

This needs a rebase.

[vargaz]

Seems to be missing Buffer:Memmove, also the definition of the non generic version of memmove doesn't seem right:

            internal static unsafe void Memmove (byte *dest, byte *src, uint len)
            {
                    Memcpy (dest, src, (int) len);
            }

            internal static void Memmove<T>(ref T destination, ref T source, nuint elementCount)
            {
                    throw new NotImplementedException ("Buffer::Memmove<T>");
            }

[marek-safar]

@vargaz correct, not sure if we should do ical or try to adopt quite ugly managed implementation from coreclr

[vargaz]

The build doesn't define BIT64, and some code like Span.Clear () depends on it being the correct value:

           SpanHelpers.ClearWithReferences(ref Unsafe.As<T, IntPtr>(ref _pointer.Value), (nuint)_length * (nuint)(Unsafe.SizeOf<T>() / sizeof(nuint)));

[vargaz]

The build seems to work now but there are a lot of class lib failures.

[akoeplinger]

we'll need to implement this, I just stubbed it out to get the branch to compile (it calls into System.Globalization.Native.so native code in corefx)

[vargaz]

The 32 bit failures are (mostly) caused by the BIT64 define when compiling corlib, some coreclr code depends on a correct type for nuint:

   public void Clear()
    {
        if (RuntimeHelpers.IsReferenceOrContainsReferences<T>())
        {
            SpanHelpers.ClearWithReferences(ref Unsafe.As<T, IntPtr>(ref _pointer.Value), (nuint)_length * (nuint)(Unsafe.SizeOf<T>() / sizeof(nuint)));
        }
        else
        {
            SpanHelpers.ClearWithoutReferences(ref Unsafe.As<T, byte>(ref _pointer.Value), (nuint)_length * (nuint)Unsafe.SizeOf<T>());
        }
    }

[marek-safar]

@monojenkins commit apidiff and csproj

[marek-safar]

@monojenkins backport 2018-06

[monojenkins]

@marek-safar backporting to 2018-06 failed, the patch results in conflicts:

Applying: Get Fast Span to compile
Using index info to reconstruct a base tree...
M	external/corefx
.git/rebase-apply/patch:151: trailing whitespace.
				// PERF: This depends on the fact that the String objects are always zero terminated 
.git/rebase-apply/patch:185: trailing whitespace.
			
.git/rebase-apply/patch:197: trailing whitespace.
				
.git/rebase-apply/patch:210: trailing whitespace.
					
.git/rebase-apply/patch:239: trailing whitespace.
			
warning: squelched 2 whitespace errors
warning: 7 lines add whitespace errors.
Falling back to patching base and 3-way merge...
warning: Failed to merge submodule external/corefx (not checked out)
Removing mcs/class/corlib/corefx/MemoryMarshal.cs
Removing mcs/class/corlib/corefx/MemoryExtensions.cs
Auto-merging external/corefx
CONFLICT (submodule): Merge conflict in external/corefx
error: Failed to merge in the changes.
Patch failed at 0001 Get Fast Span to compile

Please backport manually!