SkPx: new approach to fixed-point SIMD

SkPx is like Sk4px, except each platform implementation of SkPx can declare
a different sweet spot of N pixels, with extra loads and stores to handle the
ragged edge of 0<n<N pixels.

In this case, _sse's sweet spot remains 4 pixels.   _neon jumps up to 8 so
we can now use NEON's transposing loads and stores, and _none is just 1.
This makes operations involving alpha considerably more efficient on NEON,
as alpha is its own distinct 8x8 bit plane that's easy to toss around.

This incorporates a few other improvements I've been wanting:
  - no requirement that we're dealing with SkPMColor.  SkColor works too.
  - no anonymous namespace hack to differentiate implementations.

Codegen and perf look good on Clang/x86-64 and GCC/ARMv7.
The NEON code looks very similar to the old NEON code, as intended.
No .skp or GM diffs on my laptop.  Don't expect any.

I intend this to replace Sk4px.  Plan after landing:
  - port SkXfermode_opts.h
  - port Color32 in SkBlitRow_D32.cpp (and move to SkBlitRow_opts.h like other
    SkOpts code)
  - delete all Sk4px-related code
  - clean up evolutionary dead ends in SkNx (Sk16b, Sk16h, Sk4i, Sk4d, etc.)
    leaving Sk2f, Sk4f (and Sk2s, Sk4s).
  - find a machine with AVX2 to work on, write SkPx_avx2.h handling 8 pixels
    at a time.

In the end we'll have Sk4f for float pixels, SkPx for fixed-point pixels.

BUG=skia:4117

Review URL: https://codereview.chromium.org/1317233005

1 files changed, 89 insertions, 0 deletions

diff --git a/src/core/SkPx.h b/src/core/SkPx.h
new file mode 100644
index 0000000000..a4b1978e63
--- /dev/null
+++ b/src/core/SkPx.h
@@ -0,0 +1,89 @@
+/*
+ * Copyright 2015 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#ifndef SkPx_DEFINED
+#define SkPx_DEFINED
+
+#include "SkTypes.h"
+#include "SkColorPriv.h"
+
+// We'll include one of src/opts/SkPx_{sse,neon,none}.h to define a type SkPx.
+//
+// SkPx represents up to SkPx::N 8888 pixels.  It's agnostic to whether these
+// are SkColors or SkPMColors; it only assumes that alpha is the high byte.
+static_assert(SK_A32_SHIFT == 24, "For both SkColor and SkPMColor, alpha is always the high byte.");
+//
+// SkPx::Alpha represents up to SkPx::N 8-bit values, usually coverage or alpha.
+// SkPx::Wide represents up to SkPx::N pixels with 16 bits per component.
+//
+// SkPx supports the following methods:
+//    static SkPx Dup(uint32_t);
+//    static SkPx LoadN(const uint32_t*);
+//    static SkPx Load(int n, const uint32_t*);  // where 0<n<SkPx::N
+//    void storeN(uint32_t*) const;
+//    void store(int n, uint32_t*) const;        // where 0<n<SkPx::N
+//
+//    Alpha alpha() const;    // argb -> a
+//    Wide widenLo() const;   // argb -> 0a0r0g0b
+//    Wide widenHi() const;   // argb -> a0r0g0b0
+//    Wide widenLoHi() const; // argb -> aarrggbb
+//
+//    SkPx    operator+(const SkPx&) const;
+//    SkPx    operator-(const SkPx&) const;
+//    SkPx saturatedAdd(const SkPx&) const;
+//
+//    Wide operator*(const Alpha&) const;  // argb * A -> (a*A)(r*A)(g*A)(b*A)
+//
+//    // Fast approximate (px*a+127)/255.
+//    // Never off by more than 1, and always correct when px or a is 0 or 255.
+//    // We use the approximation (px*a+px)/256.
+//    SkPx approxMulDiv255(const Alpha&) const;
+//
+//    SkPx addAlpha(const Alpha&) const;  // argb + A -> (a+A)rgb
+//
+// SkPx::Alpha supports the following methods:
+//    static Alpha Dup(uint8_t);
+//    static Alpha LoadN(const uint8_t*);
+//    static Alpha Load(int n, const uint8_t*);  // where 0<n<SkPx::N
+//
+//    Alpha inv() const;  // a -> 255-a
+//
+// SkPx::Wide supports the following methods:
+//    Wide operator+(const Wide&);
+//    Wide operator-(const Wide&);
+//    Wide operator<<(int bits);
+//    Wide operator>>(int bits);
+//
+//    // Return the high byte of each component of (*this + o.widenLo()).
+//    SkPx addNarrowHi(const SkPx& o);
+//
+// Methods left unwritten, but certainly to come:
+//    SkPx SkPx::operator<(const SkPx&) const;
+//    SkPx SkPx::thenElse(const SkPx& then, const SkPx& else) const;
+//    Wide Wide::operator<(const Wide&) const;
+//    Wide Wide::thenElse(const Wide& then, const Wide& else) const;
+//
+//    SkPx Wide::div255() const;  // Rounds, think (*this + 127) / 255.
+//
+//  The different implementations of SkPx have complete freedom to choose
+//  SkPx::N and how they represent SkPx, SkPx::Alpha, and SkPx::Wide.
+//
+//  All observable math must remain identical.
+
+#if defined(SKNX_NO_SIMD)
+    #include "../opts/SkPx_none.h"
+#else
+    #if SK_CPU_SSE_LEVEL >= SK_CPU_SSE_LEVEL_SSE2
+        #include "../opts/SkPx_sse.h"
+    #elif defined(SK_ARM_HAS_NEON)
+        #include "../opts/SkPx_neon.h"
+    #else
+        #include "../opts/SkPx_none.h"
+    #endif
+#endif
+
+#endif//SkPx_DEFINED