Add blend optimization helpers and use to convert rect draws to clears.

src/gpu/GrPaint.cpp

Index: src/gpu/GrPaint.cpp
diff --git a/src/gpu/GrPaint.cpp b/src/gpu/GrPaint.cpp
index d67f2e8707124d034003a7a170826c26e48a685a..898a32ff70f53ac2e4aaf9f9537eb214d1aebecf 100644
--- a/src/gpu/GrPaint.cpp
+++ b/src/gpu/GrPaint.cpp
@@ -8,6 +8,7 @@
 
 #include "GrPaint.h"
 
+#include "GrBlend.h"
 #include "effects/GrSimpleTextureEffect.h"
 
 void GrPaint::addColorTextureEffect(GrTexture* texture, const SkMatrix& matrix) {
@@ -33,3 +34,96 @@ void GrPaint::addCoverageTextureEffect(GrTexture* texture,
     GrEffectRef* effect = GrSimpleTextureEffect::Create(texture, matrix, params);
     this->addCoverageEffect(effect)->unref();
 }
+
+bool GrPaint::isOpaque() const {
+    return this->getOpaqueAndKnownColor(NULL, NULL);
+}
+
+bool GrPaint::isOpaqueAndConstantColor(GrColor* color) const {
+    GrColor tempColor;
+    uint32_t colorComps;
+    if (this->getOpaqueAndKnownColor(&tempColor, &colorComps)) {
+        if (kRGBA_GrColorComponentFlags == colorComps) {
+            *color = tempColor;
+            return true;
+        }
+    }
+    return false;
+}
+
+bool GrPaint::getOpaqueAndKnownColor(GrColor* solidColor,
+                                     uint32_t* solidColorKnownComponents) const {
+
+    // TODO: Share this implementation with GrDrawState
+
+    // Since fColorFilterXfermode is going away soon, we aren't attempting to handle anything but
+    // the default setting.
+    if (SkXfermode::kDst_Mode != fColorFilterXfermode) {
+        return false;
+    }
+
+    GrColor coverage = GrColorPackRGBA(fCoverage, fCoverage, fCoverage, fCoverage);
+    uint32_t coverageComps = kRGBA_GrColorComponentFlags;
+    int count = fCoverageStages.count();
+    for (int i = 0; i < count; ++i) {
+        (*fCoverageStages[i].getEffect())->getConstantColorComponents(&coverage, &coverageComps);
+    }
+    if (kRGBA_GrColorComponentFlags != coverageComps || 0xffffffff != coverage) {
+        return false;
+    }
+
+    GrColor color = fColor;
+    uint32_t colorComps = kRGBA_GrColorComponentFlags;
+    count = fColorStages.count();
+    for (int i = 0; i < count; ++i) {
+        (*fColorStages[i].getEffect())->getConstantColorComponents(&color, &colorComps);
+    }
+
+    GrAssert((NULL == solidColor) == (NULL == solidColorKnownComponents));
+
+    GrBlendCoeff srcCoeff = fSrcBlendCoeff;
+    GrBlendCoeff dstCoeff = fDstBlendCoeff;
+    GrSimplifyBlend(&srcCoeff, &dstCoeff, color, colorComps, 0, 0, 0);
+
+    bool opaque = kZero_GrBlendCoeff == dstCoeff && !GrBlendCoeffRefsDst(srcCoeff);
+    if (NULL != solidColor) {
+        if (opaque) {
+            switch (srcCoeff) {
+                case kZero_GrBlendCoeff:
+                    *solidColor = 0;
+                    *solidColorKnownComponents = kRGBA_GrColorComponentFlags;
+                    break;
+
+                case kOne_GrBlendCoeff:
+                    *solidColor = color;
+                    *solidColorKnownComponents = colorComps;
+                    break;
+
+                // The src coeff should never refer to the src and if it refers to dst then opaque
+                // should have been false.
+                case kSC_GrBlendCoeff:
+                case kISC_GrBlendCoeff:
+                case kDC_GrBlendCoeff:
+                case kIDC_GrBlendCoeff:
+                case kSA_GrBlendCoeff:
+                case kISA_GrBlendCoeff:
+                case kDA_GrBlendCoeff:
+                case kIDA_GrBlendCoeff:
+                default:
+                    GrCrash("srcCoeff should not refer to src or dst.");
+                    break;
+
+                // TODO: update this once GrPaint actually has a const color.
+                case kConstC_GrBlendCoeff:
+                case kIConstC_GrBlendCoeff:
+                case kConstA_GrBlendCoeff:
+                case kIConstA_GrBlendCoeff:
+                    *solidColorKnownComponents = 0;
+                    break;
+            }
+        } else {
+            solidColorKnownComponents = 0;
+        }
+    }
+    return opaque;
+}