float xfermodes (burn, dodge, softlight) in Sk8f, possibly using AVX.

src/opts/SkXfermode_opts.h

Index: src/opts/SkXfermode_opts.h
diff --git a/src/opts/SkXfermode_opts.h b/src/opts/SkXfermode_opts.h
index 69f2b420f5f4e6fe2166d2175803322b622a0906..a40faec21a9289c16eeb4aca19f4875917eb767e 100644
--- a/src/opts/SkXfermode_opts.h
+++ b/src/opts/SkXfermode_opts.h
@@ -109,71 +109,76 @@ XFERMODE(Lighten) {
 }
 #undef XFERMODE
 
-// Some xfermodes use math like divide or sqrt that's best done in floats 1 pixel at a time.
-#define XFERMODE(Name) static Sk4f SK_VECTORCALL Name(Sk4f d, Sk4f s)
+// Some xfermodes use math like divide or sqrt that's best done in floats.
+// We write it generically, then call it 1 or 2 pixels at a time (T == Sk4f or Sk8f).
+#define XFERMODE(Name) struct Name { template <typename T> T operator()(const T&, const T&); }; \
+    template <typename T> T Name::operator()(const T& d, const T& s)
 
+static_assert(SK_A32_SHIFT == 24, "");
 static inline Sk4f a_rgb(const Sk4f& a, const Sk4f& rgb) {
-    static_assert(SK_A32_SHIFT == 24, "");
     return a * Sk4f(0,0,0,1) + rgb * Sk4f(1,1,1,0);
 }
-static inline Sk4f alphas(const Sk4f& f) {
-    return SkNx_dup<SK_A32_SHIFT/8>(f);
+static inline Sk8f a_rgb(const Sk8f& a, const Sk8f& rgb) {
+    // TODO: SkNx_blend<0,0,0,1,0,0,0,1>(a, rgb) to let us use _mm256_blend_ps?
+    return a * Sk8f(0,0,0,1,0,0,0,1) + rgb * Sk8f(1,1,1,0,1,1,1,0);
 }
+static inline Sk4f alphas(const Sk4f& f) { return SkNx_shuffle<3,3,3,3>        (f); }
+static inline Sk8f alphas(const Sk8f& f) { return SkNx_shuffle<3,3,3,3,7,7,7,7>(f); }

[msarett, 2015/11/09 23:25:06]

Where is shuffle defined for AVX?

Oh, it looks like it uses SkNx's default implementation with kth().  Is a
platform specific implementation a TODO?  Or can we not do better than the
default?

[mtklein, 2015/11/10 00:22:05]

Right, default implementation.  This compiles into one instruction, vpermilps
$255, %ymmN, %ymmM.  (Shuffle element 3 into each of lanes 0,1,2,3 and element
3+4=7 into 4,5,6,7).   Pretty hard to beat.

[msarett, 2015/11/10 14:54:16]

Great, cool instruction!

I'm impressed by the compiler.  Is having the shared code the benefit of
obscuring this?

[mtklein, 2015/11/11 19:27:13]

Yeah, Clang's pretty impressive.  Mostly SkNx_shuffle works the way it does in
case we find things the compiler can't quite do best.  There have been a couple
shuffles we think we can do better on NEON, for instance, though none are
currently landed.  SkNx_shuffle also gives us a point where we can hook in and
help out less smart compilers, not to name names.

 
 XFERMODE(ColorDodge) {
     auto sa = alphas(s),
          da = alphas(d),
-         isa = Sk4f(1)-sa,
-         ida = Sk4f(1)-da;
+         isa = T(1)-sa,
+         ida = T(1)-da;
 
     auto srcover = s + d*isa,
          dstover = d + s*ida,
-         otherwise = sa * Sk4f::Min(da, (d*sa)*(sa-s).approxInvert()) + s*ida + d*isa;
+         otherwise = sa * T::Min(da, (d*sa)*(sa-s).approxInvert()) + s*ida + d*isa;
 
     // Order matters here, preferring d==0 over s==sa.
-    auto colors = (d == Sk4f(0)).thenElse(dstover,
-                  (s ==      sa).thenElse(srcover,
-                                          otherwise));
+    auto colors = (d ==  0).thenElse(dstover,
+                  (s == sa).thenElse(srcover,
+                                     otherwise));
     return a_rgb(srcover, colors);
 }
 XFERMODE(ColorBurn) {
     auto sa = alphas(s),
          da = alphas(d),
-         isa = Sk4f(1)-sa,
-         ida = Sk4f(1)-da;
+         isa = T(1)-sa,
+         ida = T(1)-da;
 
     auto srcover = s + d*isa,
          dstover = d + s*ida,
-         otherwise = sa*(da-Sk4f::Min(da, (da-d)*sa*s.approxInvert())) + s*ida + d*isa;
+         otherwise = sa*(da-T::Min(da, (da-d)*sa*s.approxInvert())) + s*ida + d*isa;
 
     // Order matters here, preferring d==da over s==0.
-    auto colors = (d ==      da).thenElse(dstover,
-                  (s == Sk4f(0)).thenElse(srcover,
-                                          otherwise));
+    auto colors = (d == da).thenElse(dstover,
+                  (s ==  0).thenElse(srcover,
+                                     otherwise));
     return a_rgb(srcover, colors);
 }
 XFERMODE(SoftLight) {
     auto sa = alphas(s),
          da = alphas(d),
-         isa = Sk4f(1)-sa,
-         ida = Sk4f(1)-da;
+         isa = T(1)-sa,
+         ida = T(1)-da;
 
     // Some common terms.
-    auto m  = (da > Sk4f(0)).thenElse(d / da, Sk4f(0)),
-         s2 = Sk4f(2)*s,
-         m4 = Sk4f(4)*m;
+    auto m  = (da > 0).thenElse(d / da, 0),
+         s2 = s*2,
+         m4 = m*4;
 
     // The logic forks three ways:
     //    1. dark src?
     //    2. light src, dark dst?
     //    3. light src, light dst?
-    auto darkSrc = d*(sa + (s2 - sa)*(Sk4f(1) - m)),        // Used in case 1.
-         darkDst = (m4*m4 + m4)*(m - Sk4f(1)) + Sk4f(7)*m,  // Used in case 2.
-         liteDst = m.sqrt() - m,                            // Used in case 3.
-         liteSrc = d*sa + da*(s2-sa)*(Sk4f(4)*d <= da).thenElse(darkDst, liteDst); // Case 2 or 3?
+    auto darkSrc = d*(sa + (s2 - sa)*(T(1) - m)),  // Used in case 1.
+         darkDst = (m4*m4 + m4)*(m - 1) + m*7,     // Used in case 2.
+         liteDst = m.sqrt() - m,                   // Used in case 3.
+         liteSrc = d*sa + da*(s2-sa)*(d*4 <= da).thenElse(darkDst, liteDst); // Case 2 or 3?
 
     auto alpha  = s + d*isa;
-    auto colors = s*ida + d*isa + (s2 <= sa).thenElse(darkSrc, liteSrc);           // Case 1 or 2/3?
+    auto colors = s*ida + d*isa + (s2 <= sa).thenElse(darkSrc, liteSrc);     // Case 1 or 2/3?
 
     return a_rgb(alpha, colors);
 }
@@ -240,53 +245,52 @@ private:
     typedef SkProcCoeffXfermode INHERITED;
 };
 
-class Sk4fXfermode : public SkProcCoeffXfermode {
+template <typename BlendFn>
+class FloatXfermode : public SkProcCoeffXfermode {
 public:
-    typedef Sk4f (SK_VECTORCALL *ProcF)(Sk4f, Sk4f);
-    Sk4fXfermode(const ProcCoeff& rec, SkXfermode::Mode mode, ProcF procf)
-        : INHERITED(rec, mode)
-        , fProcF(procf) {}
+    FloatXfermode(const ProcCoeff& rec, SkXfermode::Mode mode)
+        : INHERITED(rec, mode) {}
 
     void xfer32(SkPMColor dst[], const SkPMColor src[], int n, const SkAlpha aa[]) const override {

[msarett, 2015/11/09 23:25:06]

It looks like you've deleted some private helper functions and pulled them into
the implementation of this function?  Which allows you to efficiently combine
Sk8f and Sk4f?

[mtklein, 2015/11/10 00:22:05]

Yep, just moving things around.

[msarett, 2015/11/10 14:54:16]

Acknowledged.

-        for (int i = 0; i < n; i++) {
-            dst[i] = aa ? this->xfer32(dst[i], src[i], aa[i])
-                        : this->xfer32(dst[i], src[i]);
+        BlendFn blend;
+        while (n >= 2) {
+            auto d = Sk8f::FromBytes((const uint8_t*)dst) * (1.0f/255),

[msarett, 2015/11/09 23:25:06]

nit:  Any reason not to write 1.0f / 255.0f?

[mtklein, 2015/11/10 00:22:05]

Nope, either way works the same and gets to the same compile time constant.  I'd
write 1.0/255, but MSVC complains that 1.0/255 != 1.0f/255.  Gotta get an 'f' in
there somewhere.

+                 s = Sk8f::FromBytes((const uint8_t*)src) * (1.0f/255),
+                 b = blend(d, s);
+            if (aa) {
+                auto a255 = Sk8f(aa[0],aa[0],aa[0],aa[0], aa[1],aa[1],aa[1],aa[1]);
+                (b*a255 + d*(Sk8f(255)-a255) + 0.5).toBytes((uint8_t*)dst);
+                aa += 2;
+            } else {
+                (b * 255 + 0.5).toBytes((uint8_t*)dst);
+            }
+            dst += 2;
+            src += 2;
+            n   -= 2;
+        }
+        if (n) {
+            auto d = Sk4f::FromBytes((const uint8_t*)dst) * (1.0f/255),
+                 s = Sk4f::FromBytes((const uint8_t*)src) * (1.0f/255),
+                 b = blend(d, s);
+            if (aa) {
+                auto a255 = Sk4f(aa[0],aa[0],aa[0],aa[0]);
+                (b*a255 + d*(Sk4f(255)-a255) + 0.5).toBytes((uint8_t*)dst);
+                aa++;
+            } else {
+                (b * 255 + 0.5).toBytes((uint8_t*)dst);
+            }
         }
     }
 
     void xfer16(uint16_t dst[], const SkPMColor src[], int n, const SkAlpha aa[]) const override {
         for (int i = 0; i < n; i++) {
-            SkPMColor dst32 = SkPixel16ToPixel32(dst[i]);
-            dst32 = aa ? this->xfer32(dst32, src[i], aa[i])
-                       : this->xfer32(dst32, src[i]);
-            dst[i] = SkPixel32ToPixel16(dst32);
+            SkPMColor dst32 = SkPixel16ToPixel32(dst[i]);           // Convert dst up to 8888.
+            this->xfer32(&dst32, src+i, 1, aa ? aa+i : nullptr);    // Blend 1 pixel.
+            dst[i] = SkPixel32ToPixel16(dst32);                     // Repack dst to 565 and store.

[msarett, 2015/11/09 23:25:06]

This seems slow?  Although not different from before.

[mtklein, 2015/11/10 00:22:05]

Right.  No different from before.  Just always 1 pixel at a time.  Seems like a
fine compromise for 565.

         }
     }
 
 private:
-    static Sk4f Load(SkPMColor c) {
-        return Sk4f::FromBytes((uint8_t*)&c) * Sk4f(1.0f/255);
-    }
-    static SkPMColor Round(const Sk4f& f) {
-        SkPMColor c;
-        (f * Sk4f(255) + Sk4f(0.5f)).toBytes((uint8_t*)&c);
-        return c;
-    }
-    inline SkPMColor xfer32(SkPMColor dst, SkPMColor src) const {
-        return Round(fProcF(Load(dst), Load(src)));
-    }
-
-    inline SkPMColor xfer32(SkPMColor dst, SkPMColor src, SkAlpha aa) const {
-        Sk4f s(Load(src)),
-             d(Load(dst)),
-             b(fProcF(d,s));
-        // We do aa in full float precision before going back down to bytes, because we can!
-        Sk4f a = Sk4f(aa) * Sk4f(1.0f/255);
-        b = b*a + d*(Sk4f(1)-a);
-        return Round(b);
-    }
-
-    ProcF fProcF;
     typedef SkProcCoeffXfermode INHERITED;
 };
 
@@ -323,7 +327,7 @@ static SkXfermode* create_xfermode(const ProcCoeff& rec, SkXfermode::Mode mode)
     #undef CASE
 
 #define CASE(Mode) \
-    case SkXfermode::k##Mode##_Mode: return new Sk4fXfermode(rec, mode, &Mode)
+    case SkXfermode::k##Mode##_Mode: return new FloatXfermode<Mode>(rec, mode)
         CASE(ColorDodge);
         CASE(ColorBurn);
         CASE(SoftLight);