Don't ref/unref the static src-over xp

src/gpu/effects/GrPorterDuffXferProcessor.cpp

 /*
  * Copyright 2014 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "effects/GrPorterDuffXferProcessor.h"
 
 #include "GrBlend.h"
@@ skipping 832 matching lines @@
     }
     BlendFormula blendFormula = static_cast<const PorterDuffXferProcessor*>(xp)->getBlendFormula();
     *outPrimary = blendFormula.fPrimaryOutputType;
     *outSecondary = blendFormula.fSecondaryOutputType;
 }
 
 
 ////////////////////////////////////////////////////////////////////////////////////////////////
 // SrcOver Global functions
 ////////////////////////////////////////////////////////////////////////////////////////////////
+const GrXferProcessor& GrPorterDuffXPFactory::SimpleSrcOverXP() {
+    static BlendFormula gSrcOverBlendFormula = COEFF_FORMULA(kOne_GrBlendCoeff,
+                                                             kISA_GrBlendCoeff);
+    static PorterDuffXferProcessor gSrcOverXP(gSrcOverBlendFormula);
+    return gSrcOverXP;
+}
 
 GrXferProcessor* GrPorterDuffXPFactory::CreateSrcOverXferProcessor(
         const GrCaps& caps,
         const GrPipelineOptimizations& optimizations,
         bool hasMixedSamples,
         const GrXferProcessor::DstTexture* dstTexture) {
     if (!optimizations.fCoveragePOI.isFourChannelOutput() &&
         !(optimizations.fCoveragePOI.isSolidWhite() &&
           !hasMixedSamples &&
           optimizations.fColorPOI.isOpaque())) {
-        static BlendFormula gSrcOverBlendFormula = COEFF_FORMULA(kOne_GrBlendCoeff,
-                                                                 kISA_GrBlendCoeff);
-        static PorterDuffXferProcessor gSrcOverXP(gSrcOverBlendFormula);
-        SkASSERT(!dstTexture || !dstTexture->texture());
-        gSrcOverXP.ref();
-        return &gSrcOverXP;
+        // We return nullptr here, which our caller interprets as meaning "use SimpleSrcOverXP".
+        // We don't simply return the address of that XP here because our caller would have to unref
+        // it and since it is a global object and GrProgramElement's ref-cnting system is not thread
+        // safe.
+        return nullptr;
     }
 
     BlendFormula blendFormula;
     if (optimizations.fCoveragePOI.isFourChannelOutput()) {
         if (kRGBA_GrColorComponentFlags == optimizations.fColorPOI.validFlags() &&
             !caps.shaderCaps()->dualSourceBlendingSupport() &&
             !caps.shaderCaps()->dstReadInShaderSupport()) {
             // If we don't have dual source blending or in shader dst reads, we fall
             // back to this trick for rendering SrcOver LCD text instead of doing a
             // dst copy.
@@ skipping 32 matching lines @@
         }
         return get_lcd_blend_formula(optimizations.fCoveragePOI, 
                                      SkXfermode::kSrcOver_Mode).hasSecondaryOutput();
     }
     // We fallback on the shader XP when the blend formula would use dual source blending but we
     // don't have support for it.
     return get_blend_formula(optimizations.fColorPOI, optimizations.fCoveragePOI,
                              hasMixedSamples, SkXfermode::kSrcOver_Mode).hasSecondaryOutput();
 }