Faster 4f gradient premul path

Similar to https://codereview.chromium.org/2409583003/, perform the
premul in 4f.  It turns out it's even faster to avoid the 255 load
multiplication in this case.

Also includes some template plumbing because DstTraits<>::load now needs
to be premul-aware (previously it wasn't).

R=reed@google.com
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2416233002

Review-Url: https://codereview.chromium.org/2416233002

src/effects/gradients/Sk4fGradientBase.cpp

@@ -326,7 +326,7 @@ GradientShaderBase4fContext::shadeSpanInternal(int x, int y,
                                                int count) const {
     static const int kBufSize = 128;
     SkScalar ts[kBufSize];
-    TSampler<dstType, tileMode> sampler(*this);
+    TSampler<dstType, premul, tileMode> sampler(*this);
 
     SkASSERT(count > 0);
     do {
@@ -341,7 +341,7 @@ GradientShaderBase4fContext::shadeSpanInternal(int x, int y,
     } while (count > 0);
 }
 
-template<DstType dstType, SkShader::TileMode tileMode>
+template<DstType dstType, ApplyPremul premul, SkShader::TileMode tileMode>
 class SkGradientShaderBase::GradientShaderBase4fContext::TSampler {
 public:
     TSampler(const GradientShaderBase4fContext& ctx)
@@ -424,8 +424,8 @@ private:
     }
 
     void loadIntervalData(const Interval* i) {
-        fCc = DstTraits<dstType>::load(i->fC0);
+        fCc = DstTraits<dstType, premul>::load(i->fC0);
-        fDc = DstTraits<dstType>::load(i->fDc);
+        fDc = DstTraits<dstType, premul>::load(i->fDc);
     }
 
     const Interval* fFirstInterval;

src/effects/gradients/Sk4fGradientBase.h

@@ -60,7 +60,7 @@ private:
     void addMirrorIntervals(const SkGradientShaderBase&,
                             const Sk4f& componentScale, bool reverse);
 
-    template<DstType, SkShader::TileMode tileMode>
+    template<DstType, ApplyPremul, SkShader::TileMode tileMode>
     class TSampler;
 
     template <DstType dstType, ApplyPremul premul>

src/effects/gradients/Sk4fGradientPriv.h

@@ -29,17 +29,6 @@ enum class DstType {
     F32,  // Linear float.  Used for shaders only.
 };
 
-template <ApplyPremul premul>
-inline SkPMColor trunc_from_4f_255(const Sk4f& c) {
-    SkPMColor pmc;
-    SkNx_cast<uint8_t>(c).store(&pmc);
-    if (premul == ApplyPremul::True) {
-        pmc = SkPreMultiplyARGB(SkGetPackedA32(pmc), SkGetPackedR32(pmc),
-                                SkGetPackedG32(pmc), SkGetPackedB32(pmc));
-    }
-    return pmc;
-}
-
 template <ApplyPremul>
 struct PremulTraits;
 
@@ -69,24 +58,34 @@ struct PremulTraits<ApplyPremul::True> {
 //
 //   - store4x()    Store 4 Sk4f values to dest (opportunistic optimization).
 //
-template <DstType, ApplyPremul premul = ApplyPremul::False>
+template <DstType, ApplyPremul premul>
 struct DstTraits;
 
 template <ApplyPremul premul>
 struct DstTraits<DstType::L32, premul> {
+    using PM   = PremulTraits<premul>;
     using Type = SkPMColor;
 
-    // For L32, we prescale the values by 255 to save a per-pixel multiplication.
+    // For L32, prescaling by 255 saves a per-pixel multiplication when premul is not needed.
     static Sk4f load(const SkPM4f& c) {
-        return c.to4f_pmorder() * Sk4f(255);
+        return premul == ApplyPremul::False
+            ? c.to4f_pmorder() * Sk4f(255)
+            : c.to4f_pmorder();
     }
 
     static void store(const Sk4f& c, Type* dst) {
-        *dst = trunc_from_4f_255<premul>(c);
+        if (premul == ApplyPremul::False) {
+            // c is prescaled by 255, just store.
+            SkNx_cast<uint8_t>(c).store(dst);
+        } else {
+            *dst = Sk4f_toL32(PM::apply(c));
+        }
     }
 
     static void store(const Sk4f& c, Type* dst, int n) {
-        sk_memset32(dst, trunc_from_4f_255<premul>(c), n);
+        Type pmc;
+        store(c, &pmc);
+        sk_memset32(dst, pmc, n);
     }
 
     static void store4x(const Sk4f& c0, const Sk4f& c1,

src/effects/gradients/Sk4fLinearGradient.cpp

@@ -240,7 +240,7 @@ LinearGradient4fContext::shadeSpanInternal(int x, int y,
                   &pt);
     const SkScalar fx = pinFx<tileMode>(pt.x());
     const SkScalar dx = fDstToPos.getScaleX();
-    LinearIntervalProcessor<dstType, tileMode> proc(fIntervals.begin(),
+    LinearIntervalProcessor<dstType, premul, tileMode> proc(fIntervals.begin(),
                                                             fIntervals.end() - 1,
                                                             this->findInterval(fx),
                                                             fx,
@@ -274,7 +274,7 @@ LinearGradient4fContext::shadeSpanInternal(int x, int y,
     }
 }
 
-template<DstType dstType, SkShader::TileMode tileMode>
+template<DstType dstType, ApplyPremul premul, SkShader::TileMode tileMode>
 class SkLinearGradient::
 LinearGradient4fContext::LinearIntervalProcessor {
 public:
@@ -322,8 +322,8 @@ public:
 
 private:
     void compute_interval_props(SkScalar t) {
-        const Sk4f dC = DstTraits<dstType>::load(fInterval->fDc);
+        const Sk4f dC = DstTraits<dstType, premul>::load(fInterval->fDc);
-        fCc           = DstTraits<dstType>::load(fInterval->fC0);
+        fCc           = DstTraits<dstType, premul>::load(fInterval->fC0);
         fCc           = fCc + dC * Sk4f(t);
         fDcDx         = dC * fDx;
         fZeroRamp     = fIsVertical || fInterval->isZeroRamp();

src/effects/gradients/Sk4fLinearGradient.h

@@ -27,7 +27,7 @@ protected:
 private:
     using INHERITED = GradientShaderBase4fContext;
 
-    template<DstType, TileMode>
+    template<DstType, ApplyPremul, TileMode>
     class LinearIntervalProcessor;
 
     template <DstType dstType, ApplyPremul premul>