SSE2 implementation of RGBA box blurs. This yields ~2X perf improvement on

src/opts/SkBlurImage_opts_SSE2.h

+/*
+ * Copyright 2013 The Android Open Source Project
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#include "SkColorPriv.h"
+#include "SkBlurImage_opts.h"
+
+template <SkBlurDirection srcDirection, SkBlurDirection dstDirection>

[mtklein, 2013/11/08 02:17:59]

Okay, this breaks my brain / whole model of templates.  How does this work!?  Do
the declarations at the bottom of the .cpp force those versions to be
instantiated in SkBlurImage_opts_SSE2.o?  What happens if you leave out the
as-yet-unused YX?

[Stephen White, 2013/11/08 15:47:47]

Yes, this is explicit template instantiation. If I left out the instantiation of
YX, it would simply leave out a function for the linker to find. Think about it
in non-templated terms: it would be as if I left out a function definition from
the .cpp: if I left it in, it would be removed from the binary at link time. If
I removed the others, the link would fail on a missing function.

At any rate, now that we've removed the need for the common enum, I've removed
the explicit template instatiation as well.

[mtklein, 2013/11/08 18:42:33]

Thanks, that helps.  I guess what still surprises me is that declaring the
specializations is enough to instantiate them.

+void SkBoxBlur_SSE2(const SkPMColor* src, int srcStride, SkPMColor* dst, int kernelSize,
+                    int leftOffset, int rightOffset, int width, int height);