src/gpu/gl/GrGpuGL_program.cpp

/*
 * Copyright 2011 Google Inc.
 *
 * Use of this source code is governed by a BSD-style license that can be
 * found in the LICENSE file.
 */

#include "GrGpuGL.h"

#include "GrEffect.h"
#include "GrGLEffect.h"

typedef GrGLUniformManager::UniformHandle UniformHandle;
static const UniformHandle kInvalidUniformHandle = GrGLUniformManager::kInvalidUniformHandle;

#define SKIP_CACHE_CHECK    true
#define GR_UINT32_MAX   static_cast<uint32_t>(-1)

GrGpuGL::ProgramCache::ProgramCache(const GrGLContextInfo& gl)
    : fCount(0)
    , fCurrLRUStamp(0)
    , fGL(gl) {
}

void GrGpuGL::ProgramCache::abandon() {
    for (int i = 0; i < fCount; ++i) {
        GrAssert(NULL != fEntries[i].fProgram.get());
        fEntries[i].fProgram->abandon();
        fEntries[i].fProgram.reset(NULL);
    }
    fCount = 0;
}

GrGLProgram* GrGpuGL::ProgramCache::getProgram(const ProgramDesc& desc,
                                               const GrEffectStage* stages[]) {
    Entry newEntry;
    newEntry.fKey.setKeyData(desc.asKey());

    Entry* entry = fHashCache.find(newEntry.fKey);
    if (NULL == entry) {
        newEntry.fProgram.reset(GrGLProgram::Create(fGL, desc, stages));
        if (NULL == newEntry.fProgram.get()) {
            return NULL;
        }
        if (fCount < kMaxEntries) {
            entry = fEntries + fCount;
            ++fCount;
        } else {
            GrAssert(kMaxEntries == fCount);
            entry = fEntries;
            for (int i = 1; i < kMaxEntries; ++i) {
                if (fEntries[i].fLRUStamp < entry->fLRUStamp) {
                    entry = fEntries + i;
                }
            }
            fHashCache.remove(entry->fKey, entry);
        }
        *entry = newEntry;
        fHashCache.insert(entry->fKey, entry);
    }

    entry->fLRUStamp = fCurrLRUStamp;
    if (GR_UINT32_MAX == fCurrLRUStamp) {
        // wrap around! just trash our LRU, one time hit.
        for (int i = 0; i < fCount; ++i) {
            fEntries[i].fLRUStamp = 0;
        }
    }
    ++fCurrLRUStamp;
    return entry->fProgram;
}

////////////////////////////////////////////////////////////////////////////////

void GrGpuGL::abandonResources(){
    INHERITED::abandonResources();
    fProgramCache->abandon();
    fHWProgramID = 0;
}

////////////////////////////////////////////////////////////////////////////////

#define GL_CALL(X) GR_GL_CALL(this->glInterface(), X)

void GrGpuGL::flushViewMatrix(DrawType type) {
    const GrGLRenderTarget* rt = static_cast<const GrGLRenderTarget*>(this->getDrawState().getRenderTarget());
    SkISize viewportSize;
    const GrGLIRect& viewport = rt->getViewport();
    viewportSize.set(viewport.fWidth, viewport.fHeight);

    const SkMatrix& vm = this->getDrawState().getViewMatrix();

    if (kStencilPath_DrawType == type) {
        if (fHWPathMatrixState.fLastOrigin != rt->origin() ||
            fHWPathMatrixState.fViewMatrix != vm ||
            fHWPathMatrixState.fRTSize != viewportSize) {
            // rescale the coords from skia's "device" coords to GL's normalized coords,
            // and perform a y-flip if required.
            SkMatrix m;
            if (kBottomLeft_GrSurfaceOrigin == rt->origin()) {
                m.setScale(SkIntToScalar(2) / rt->width(), SkIntToScalar(-2) / rt->height());
                m.postTranslate(-SK_Scalar1, SK_Scalar1);
            } else {
                m.setScale(SkIntToScalar(2) / rt->width(), SkIntToScalar(2) / rt->height());
                m.postTranslate(-SK_Scalar1, -SK_Scalar1);
            }
            m.preConcat(vm);

            // GL wants a column-major 4x4.
            GrGLfloat mv[]  = {
                // col 0
                SkScalarToFloat(m[SkMatrix::kMScaleX]),
                SkScalarToFloat(m[SkMatrix::kMSkewY]),
                0,
                SkScalarToFloat(m[SkMatrix::kMPersp0]),

                // col 1
                SkScalarToFloat(m[SkMatrix::kMSkewX]),
                SkScalarToFloat(m[SkMatrix::kMScaleY]),
                0,
                SkScalarToFloat(m[SkMatrix::kMPersp1]),

                // col 2
                0, 0, 0, 0,

                // col3
                SkScalarToFloat(m[SkMatrix::kMTransX]),
                SkScalarToFloat(m[SkMatrix::kMTransY]),
                0.0f,
                SkScalarToFloat(m[SkMatrix::kMPersp2])
            };
            GL_CALL(MatrixMode(GR_GL_PROJECTION));
            GL_CALL(LoadMatrixf(mv));
            fHWPathMatrixState.fViewMatrix = vm;
            fHWPathMatrixState.fRTSize = viewportSize;
            fHWPathMatrixState.fLastOrigin = rt->origin();
        }
    } else if (fCurrentProgram->fOrigin != rt->origin() ||
               !fCurrentProgram->fViewMatrix.cheapEqualTo(vm) ||
               fCurrentProgram->fViewportSize != viewportSize) {
        SkMatrix m;
        if (kBottomLeft_GrSurfaceOrigin == rt->origin()) {
            m.setAll(
                SkIntToScalar(2) / viewportSize.fWidth, 0, -SK_Scalar1,
                0,-SkIntToScalar(2) / viewportSize.fHeight, SK_Scalar1,
            0, 0, SkMatrix::I()[8]);
        } else {
            m.setAll(
                SkIntToScalar(2) / viewportSize.fWidth, 0, -SK_Scalar1,
                0, SkIntToScalar(2) / viewportSize.fHeight,-SK_Scalar1,
            0, 0, SkMatrix::I()[8]);
        }
        m.setConcat(m, vm);

        // ES doesn't allow you to pass true to the transpose param,
        // so do our own transpose
        GrGLfloat mt[]  = {
            SkScalarToFloat(m[SkMatrix::kMScaleX]),
            SkScalarToFloat(m[SkMatrix::kMSkewY]),
            SkScalarToFloat(m[SkMatrix::kMPersp0]),
            SkScalarToFloat(m[SkMatrix::kMSkewX]),
            SkScalarToFloat(m[SkMatrix::kMScaleY]),
            SkScalarToFloat(m[SkMatrix::kMPersp1]),
            SkScalarToFloat(m[SkMatrix::kMTransX]),
            SkScalarToFloat(m[SkMatrix::kMTransY]),
            SkScalarToFloat(m[SkMatrix::kMPersp2])
        };
        fCurrentProgram->fUniformManager.setMatrix3f(
                                            fCurrentProgram->fUniformHandles.fViewMatrixUni,
                                            mt);
        fCurrentProgram->fViewMatrix = vm;
        fCurrentProgram->fViewportSize = viewportSize;
        fCurrentProgram->fOrigin = rt->origin();
    }
}

///////////////////////////////////////////////////////////////////////////////

void GrGpuGL::flushColor(GrColor color) {
    const ProgramDesc& desc = fCurrentProgram->getDesc();
    const GrDrawState& drawState = this->getDrawState();

    if (drawState.getVertexLayout() & GrDrawState::kColor_VertexLayoutBit) {
        // color will be specified per-vertex as an attribute
        // invalidate the const vertex attrib color
        fHWConstAttribColor = GrColor_ILLEGAL;
    } else {
        switch (desc.fColorInput) {
            case ProgramDesc::kAttribute_ColorInput:
                if (fHWConstAttribColor != color) {
                    // OpenGL ES only supports the float varieties of glVertexAttrib
                    GrGLfloat c[4];
                    GrColorToRGBAFloat(color, c);
                    GL_CALL(VertexAttrib4fv(GrGLProgram::ColorAttributeIdx(), c));
                    fHWConstAttribColor = color;
                }
                break;
            case ProgramDesc::kUniform_ColorInput:
                if (fCurrentProgram->fColor != color) {
                    // OpenGL ES doesn't support unsigned byte varieties of glUniform
                    GrGLfloat c[4];
                    GrColorToRGBAFloat(color, c);
                    GrAssert(kInvalidUniformHandle !=  fCurrentProgram->fUniformHandles.fColorUni);
                    fCurrentProgram->fUniformManager.set4fv(
                                                        fCurrentProgram->fUniformHandles.fColorUni,
                                                        0, 1, c);
                    fCurrentProgram->fColor = color;
                }
                break;
            case ProgramDesc::kSolidWhite_ColorInput:
            case ProgramDesc::kTransBlack_ColorInput:
                break;
            default:
                GrCrash("Unknown color type.");
        }
    }
    UniformHandle filterColorUni = fCurrentProgram->fUniformHandles.fColorFilterUni;
    if (kInvalidUniformHandle != filterColorUni &&
        fCurrentProgram->fColorFilterColor != drawState.getColorFilterColor()) {
        GrGLfloat c[4];
        GrColorToRGBAFloat(drawState.getColorFilterColor(), c);
        fCurrentProgram->fUniformManager.set4fv(filterColorUni, 0, 1, c);
        fCurrentProgram->fColorFilterColor = drawState.getColorFilterColor();
    }
}

void GrGpuGL::flushCoverage(GrColor coverage) {
    const ProgramDesc& desc = fCurrentProgram->getDesc();
    // const GrDrawState& drawState = this->getDrawState();


    if (this->getDrawState().getVertexLayout() & GrDrawState::kCoverage_VertexLayoutBit) {
        // coverage will be specified per-vertex as an attribute
        // invalidate the const vertex attrib coverage
        fHWConstAttribCoverage = GrColor_ILLEGAL;
    } else {
        switch (desc.fCoverageInput) {
            case ProgramDesc::kAttribute_ColorInput:
                if (fHWConstAttribCoverage != coverage) {
                    // OpenGL ES only supports the float varieties of
                    // glVertexAttrib
                    GrGLfloat c[4];
                    GrColorToRGBAFloat(coverage, c);
                    GL_CALL(VertexAttrib4fv(GrGLProgram::CoverageAttributeIdx(),
                                            c));
                    fHWConstAttribCoverage = coverage;
                }
                break;
            case ProgramDesc::kUniform_ColorInput:
                if (fCurrentProgram->fCoverage != coverage) {
                    // OpenGL ES doesn't support unsigned byte varieties of
                    // glUniform
                    GrGLfloat c[4];
                    GrColorToRGBAFloat(coverage, c);
                    GrAssert(kInvalidUniformHandle !=
                             fCurrentProgram->fUniformHandles.fCoverageUni);
                    fCurrentProgram->fUniformManager.set4fv(
                                                    fCurrentProgram->fUniformHandles.fCoverageUni,
                                                    0, 1, c);
                    fCurrentProgram->fCoverage = coverage;
                }
                break;
            case ProgramDesc::kSolidWhite_ColorInput:
            case ProgramDesc::kTransBlack_ColorInput:
                break;
            default:
                GrCrash("Unknown coverage type.");
        }
    }
}

bool GrGpuGL::flushGraphicsState(DrawType type) {
    const GrDrawState& drawState = this->getDrawState();

    // GrGpu::setupClipAndFlushState should have already checked this
    // and bailed if not true.
    GrAssert(NULL != drawState.getRenderTarget());

    if (kStencilPath_DrawType != type) {
        this->flushMiscFixedFunctionState();

        GrBlendCoeff srcCoeff;
        GrBlendCoeff dstCoeff;
        BlendOptFlags blendOpts = this->getBlendOpts(false, &srcCoeff, &dstCoeff);
        if (kSkipDraw_BlendOptFlag & blendOpts) {
            return false;
        }

        const GrEffectStage* stages[GrDrawState::kNumStages];
        for (int i = 0; i < GrDrawState::kNumStages; ++i) {
            stages[i] = drawState.isStageEnabled(i) ? &drawState.getStage(i) : NULL;
        }
        GrGLProgram::Desc desc;
        this->buildProgram(kDrawPoints_DrawType == type, blendOpts, dstCoeff, &desc);

        fCurrentProgram.reset(fProgramCache->getProgram(desc, stages));
        if (NULL == fCurrentProgram.get()) {
            GrAssert(!"Failed to create program!");
            return false;
        }
        fCurrentProgram.get()->ref();

        if (fHWProgramID != fCurrentProgram->fProgramID) {
            GL_CALL(UseProgram(fCurrentProgram->fProgramID));
            fHWProgramID = fCurrentProgram->fProgramID;
        }
        fCurrentProgram->overrideBlend(&srcCoeff, &dstCoeff);
        this->flushBlend(kDrawLines_DrawType == type, srcCoeff, dstCoeff);

        GrColor color;
        GrColor coverage;
        if (blendOpts & kEmitTransBlack_BlendOptFlag) {
            color = 0;
            coverage = 0;
        } else if (blendOpts & kEmitCoverage_BlendOptFlag) {
            color = 0xffffffff;
            coverage = drawState.getCoverage();
        } else {
            color = drawState.getColor();
            coverage = drawState.getCoverage();
        }
        this->flushColor(color);
        this->flushCoverage(coverage);

        fCurrentProgram->setData(this);
    }
    this->flushStencil(type);
    this->flushViewMatrix(type);
    this->flushScissor();
    this->flushAAState(type);

    GrIRect* devRect = NULL;
    GrIRect devClipBounds;
    if (drawState.isClipState()) {
        this->getClip()->getConservativeBounds(drawState.getRenderTarget(), &devClipBounds);
        devRect = &devClipBounds;
    }
    // This must come after textures are flushed because a texture may need
    // to be msaa-resolved (which will modify bound FBO state).
    this->flushRenderTarget(devRect);

    return true;
}

void GrGpuGL::setupGeometry(const DrawInfo& info, int* startIndexOffset) {

    int newColorOffset;
    int newCoverageOffset;
    int newTexCoordOffsets[GrDrawState::kMaxTexCoords];
    int newEdgeOffset;

    GrVertexLayout currLayout = this->getDrawState().getVertexLayout();

    GrGLsizei newStride = GrDrawState::VertexSizeAndOffsetsByIdx(currLayout,
                                                                 newTexCoordOffsets,
                                                                 &newColorOffset,
                                                                 &newCoverageOffset,
                                                                 &newEdgeOffset);
    int oldColorOffset;
    int oldCoverageOffset;
    int oldTexCoordOffsets[GrDrawState::kMaxTexCoords];
    int oldEdgeOffset;

    GrGLsizei oldStride = GrDrawState::VertexSizeAndOffsetsByIdx(fHWGeometryState.fVertexLayout,
                                                                 oldTexCoordOffsets,
                                                                 &oldColorOffset,
                                                                 &oldCoverageOffset,
                                                                 &oldEdgeOffset);

    int extraVertexOffset;
    this->setBuffers(info.isIndexed(), &extraVertexOffset, startIndexOffset);

    size_t vertexOffset = (info.startVertex() + extraVertexOffset) * newStride;

    // all the Pointers must be set if any of these are true
    bool allOffsetsChange =  fHWGeometryState.fArrayPtrsDirty ||
                             vertexOffset != fHWGeometryState.fVertexOffset ||
                             newStride != oldStride;

    if (allOffsetsChange) {
        int idx = GrGLProgram::PositionAttributeIdx();
        GL_CALL(VertexAttribPointer(idx, 2, GR_GL_FLOAT, false, newStride, (GrGLvoid*)vertexOffset));
        fHWGeometryState.fVertexOffset = vertexOffset;
    }

    for (int t = 0; t < GrDrawState::kMaxTexCoords; ++t) {
        if (newTexCoordOffsets[t] > 0) {
            GrGLvoid* texCoordOffset = (GrGLvoid*)(vertexOffset + newTexCoordOffsets[t]);
            int idx = GrGLProgram::TexCoordAttributeIdx(t);
            if (oldTexCoordOffsets[t] <= 0) {
                GL_CALL(EnableVertexAttribArray(idx));
                GL_CALL(VertexAttribPointer(idx, 2, GR_GL_FLOAT, false, newStride, texCoordOffset));
            } else if (allOffsetsChange || newTexCoordOffsets[t] != oldTexCoordOffsets[t]) {
                GL_CALL(VertexAttribPointer(idx, 2, GR_GL_FLOAT, false, newStride, texCoordOffset));
            }
        } else if (oldTexCoordOffsets[t] > 0) {
            GL_CALL(DisableVertexAttribArray(GrGLProgram::TexCoordAttributeIdx(t)));
        }
    }

    if (newColorOffset > 0) {
        GrGLvoid* colorOffset = (int8_t*)(vertexOffset + newColorOffset);
        int idx = GrGLProgram::ColorAttributeIdx();
        if (oldColorOffset <= 0) {
            GL_CALL(EnableVertexAttribArray(idx));
            GL_CALL(VertexAttribPointer(idx, 4, GR_GL_UNSIGNED_BYTE, true, newStride, colorOffset));
        } else if (allOffsetsChange || newColorOffset != oldColorOffset) {
            GL_CALL(VertexAttribPointer(idx, 4, GR_GL_UNSIGNED_BYTE, true, newStride, colorOffset));
        }
    } else if (oldColorOffset > 0) {
        GL_CALL(DisableVertexAttribArray(GrGLProgram::ColorAttributeIdx()));
    }

    if (newCoverageOffset > 0) {
        GrGLvoid* coverageOffset = (int8_t*)(vertexOffset + newCoverageOffset);
        int idx = GrGLProgram::CoverageAttributeIdx();
        if (oldCoverageOffset <= 0) {
            GL_CALL(EnableVertexAttribArray(idx));
            GL_CALL(VertexAttribPointer(idx, 4, GR_GL_UNSIGNED_BYTE,
                                        true, newStride, coverageOffset));
        } else if (allOffsetsChange || newCoverageOffset != oldCoverageOffset) {
            GL_CALL(VertexAttribPointer(idx, 4, GR_GL_UNSIGNED_BYTE,
                                        true, newStride, coverageOffset));
        }
    } else if (oldCoverageOffset > 0) {
        GL_CALL(DisableVertexAttribArray(GrGLProgram::CoverageAttributeIdx()));
    }

    if (newEdgeOffset > 0) {
        GrGLvoid* edgeOffset = (int8_t*)(vertexOffset + newEdgeOffset);
        int idx = GrGLProgram::EdgeAttributeIdx();
        if (oldEdgeOffset <= 0) {
            GL_CALL(EnableVertexAttribArray(idx));
            GL_CALL(VertexAttribPointer(idx, 4, GR_GL_FLOAT, false, newStride, edgeOffset));
        } else if (allOffsetsChange || newEdgeOffset != oldEdgeOffset) {
            GL_CALL(VertexAttribPointer(idx, 4, GR_GL_FLOAT, false, newStride, edgeOffset));
        }
    } else if (oldEdgeOffset > 0) {
        GL_CALL(DisableVertexAttribArray(GrGLProgram::EdgeAttributeIdx()));
    }

    fHWGeometryState.fVertexLayout = currLayout;
    fHWGeometryState.fArrayPtrsDirty = false;
}

void GrGpuGL::buildProgram(bool isPoints,
                           BlendOptFlags blendOpts,
                           GrBlendCoeff dstCoeff,
                           ProgramDesc* desc) {
    const GrDrawState& drawState = this->getDrawState();

    // This should already have been caught
    GrAssert(!(kSkipDraw_BlendOptFlag & blendOpts));

    bool skipCoverage = SkToBool(blendOpts & kEmitTransBlack_BlendOptFlag);

    bool skipColor = SkToBool(blendOpts & (kEmitTransBlack_BlendOptFlag |
                                           kEmitCoverage_BlendOptFlag));

    // The descriptor is used as a cache key. Thus when a field of the
    // descriptor will not affect program generation (because of the vertex
    // layout in use or other descriptor field settings) it should be set
    // to a canonical value to avoid duplicate programs with different keys.

    // Must initialize all fields or cache will have false negatives!
    desc->fVertexLayout = this->getDrawState().getVertexLayout();

    desc->fEmitsPointSize = isPoints;

    bool requiresAttributeColors = !skipColor &&
                                   SkToBool(desc->fVertexLayout & GrDrawState::kColor_VertexLayoutBit);
    bool requiresAttributeCoverage = !skipCoverage &&
                                     SkToBool(desc->fVertexLayout & GrDrawState::kCoverage_VertexLayoutBit);

    // fColorInput/fCoverageInput records how colors are specified for the.
    // program. So we strip the bits from the layout to avoid false negatives
    // when searching for an existing program in the cache.
    desc->fVertexLayout &= ~(GrDrawState::kColor_VertexLayoutBit | GrDrawState::kCoverage_VertexLayoutBit);

    desc->fColorFilterXfermode = skipColor ?
                                SkXfermode::kDst_Mode :
                                drawState.getColorFilterMode();

    // no reason to do edge aa or look at per-vertex coverage if coverage is
    // ignored
    if (skipCoverage) {
        desc->fVertexLayout &= ~(GrDrawState::kEdge_VertexLayoutBit | GrDrawState::kCoverage_VertexLayoutBit);
    }

    bool colorIsTransBlack = SkToBool(blendOpts & kEmitTransBlack_BlendOptFlag);
    bool colorIsSolidWhite = (blendOpts & kEmitCoverage_BlendOptFlag) ||
                             (!requiresAttributeColors && 0xffffffff == drawState.getColor());
    if (colorIsTransBlack) {
        desc->fColorInput = ProgramDesc::kTransBlack_ColorInput;
    } else if (colorIsSolidWhite) {
        desc->fColorInput = ProgramDesc::kSolidWhite_ColorInput;
    } else if (GR_GL_NO_CONSTANT_ATTRIBUTES && !requiresAttributeColors) {
        desc->fColorInput = ProgramDesc::kUniform_ColorInput;
    } else {
        desc->fColorInput = ProgramDesc::kAttribute_ColorInput;
    }

    bool covIsSolidWhite = !requiresAttributeCoverage && 0xffffffff == drawState.getCoverage();

    if (skipCoverage) {
        desc->fCoverageInput = ProgramDesc::kTransBlack_ColorInput;
    } else if (covIsSolidWhite) {
        desc->fCoverageInput = ProgramDesc::kSolidWhite_ColorInput;
    } else if (GR_GL_NO_CONSTANT_ATTRIBUTES && !requiresAttributeCoverage) {
        desc->fCoverageInput = ProgramDesc::kUniform_ColorInput;
    } else {
        desc->fCoverageInput = ProgramDesc::kAttribute_ColorInput;
    }

    int lastEnabledStage = -1;

    if (!skipCoverage && (desc->fVertexLayout &GrDrawState::kEdge_VertexLayoutBit)) {
        desc->fVertexEdgeType = drawState.getVertexEdgeType();
        desc->fDiscardIfOutsideEdge = drawState.getStencil().doesWrite();
    } else {
        // Use canonical values when edge-aa is not enabled to avoid program cache misses.
        desc->fVertexEdgeType = GrDrawState::kHairLine_EdgeType;
        desc->fDiscardIfOutsideEdge = false;
    }

    for (int s = 0; s < GrDrawState::kNumStages; ++s) {

        bool skip = s < drawState.getFirstCoverageStage() ? skipColor : skipCoverage;
        if (!skip && drawState.isStageEnabled(s)) {
            lastEnabledStage = s;
            const GrEffectRef& effect = *drawState.getStage(s).getEffect();
            const GrBackendEffectFactory& factory = effect->getFactory();
            desc->fEffectKeys[s] = factory.glEffectKey(drawState.getStage(s), this->glCaps());
        } else {
            desc->fEffectKeys[s] = 0;
        }
    }

    desc->fDualSrcOutput = ProgramDesc::kNone_DualSrcOutput;

    // Currently the experimental GS will only work with triangle prims (and it doesn't do anything
    // other than pass through values from the VS to the FS anyway).
#if 0 && GR_GL_EXPERIMENTAL_GS
    desc->fExperimentalGS = this->getCaps().fGeometryShaderSupport;
#endif

    // We want to avoid generating programs with different "first cov stage" values when they would
    // compute the same result. We set field in the desc to kNumStages when either there are no
    // coverage stages or the distinction between coverage and color is immaterial.
    int firstCoverageStage = GrDrawState::kNumStages;
    desc->fFirstCoverageStage = GrDrawState::kNumStages;
    bool hasCoverage = drawState.getFirstCoverageStage() <= lastEnabledStage;
    if (hasCoverage) {
        firstCoverageStage = drawState.getFirstCoverageStage();
    }

    // other coverage inputs
    if (!hasCoverage) {
        hasCoverage = requiresAttributeCoverage ||
                      (desc->fVertexLayout & GrDrawState::kEdge_VertexLayoutBit);
    }

    if (hasCoverage) {
        // color filter is applied between color/coverage computation
        if (SkXfermode::kDst_Mode != desc->fColorFilterXfermode) {
            desc->fFirstCoverageStage = firstCoverageStage;
        }

        if (this->getCaps().dualSourceBlendingSupport() &&
            !(blendOpts & (kEmitCoverage_BlendOptFlag | kCoverageAsAlpha_BlendOptFlag))) {
            if (kZero_GrBlendCoeff == dstCoeff) {
                // write the coverage value to second color
                desc->fDualSrcOutput =  ProgramDesc::kCoverage_DualSrcOutput;
                desc->fFirstCoverageStage = firstCoverageStage;
            } else if (kSA_GrBlendCoeff == dstCoeff) {
                // SA dst coeff becomes 1-(1-SA)*coverage when dst is partially covered.
                desc->fDualSrcOutput = ProgramDesc::kCoverageISA_DualSrcOutput;
                desc->fFirstCoverageStage = firstCoverageStage;
            } else if (kSC_GrBlendCoeff == dstCoeff) {
                // SA dst coeff becomes 1-(1-SA)*coverage when dst is partially covered.
                desc->fDualSrcOutput = ProgramDesc::kCoverageISC_DualSrcOutput;
                desc->fFirstCoverageStage = firstCoverageStage;
            }
        }
    }
}