GL: yet another workaround for SwiftShader, now with XFB buffer bindings.

doc/changelog.dox

@@ -60,6 +60,10 @@ See also:
     disabling @gl_extension{ANGLE,instanced_arrays} and
     @gl_extension{OES,texture_3D} extensions on SwiftShader due to missing
     suffixed entry points. See @ref opengl-workarounds for more information.
+-   New @cpp "swiftshader-broken-xfb-buffer-binding-target" @ce workaround
+    that's fixing misbehaving buffers when using
+    @ref GL::Buffer::TargetHint::TransformFeedback. See @ref opengl-workarounds
+    for more information.
 -   New `--magnum-gpu-validation` @ref GL-Context-command-line "command-line option"
     and a corresponding environment variable to conveniently enable
     @gl_extension{KHR,debug} debug output

src/Magnum/GL/Buffer.cpp

@@ -157,11 +157,17 @@ void Buffer::copy(Buffer& read, Buffer& write, const GLintptr readOffset, const
 }
 #endif
 
-Buffer::Buffer(const TargetHint targetHint): _targetHint{targetHint}, _flags{ObjectFlag::DeleteOnDestruction} {
+Buffer::Buffer(const TargetHint targetHint): _flags{ObjectFlag::DeleteOnDestruction} {
-    (this->*Context::current().state().buffer->createImplementation)();
+    Implementation::BufferState& state = *Context::current().state().buffer;
+    (this->*state.createImplementation)();
+    (this->*state.setTargetHintImplementation)(targetHint);
     CORRADE_INTERNAL_ASSERT(_id != Implementation::State::DisengagedBinding);
 }
 
+Buffer::Buffer(GLuint id, TargetHint targetHint, ObjectFlags flags) noexcept: _id{id}, _flags{flags} {
+    (this->*Context::current().state().buffer->setTargetHintImplementation)(targetHint);
+}
+
 void Buffer::createImplementationDefault() {
     glGenBuffers(1, &_id);
 }
@@ -186,6 +192,23 @@ Buffer::~Buffer() {
     glDeleteBuffers(1, &_id);
 }
 
+Buffer& Buffer::setTargetHint(TargetHint hint) {
+    (this->*Context::current().state().buffer->setTargetHintImplementation)(hint);
+    return *this;
+}
+
+void Buffer::setTargetHintImplementationDefault(const TargetHint hint) {
+    _targetHint = hint;
+}
+
+#if defined(MAGNUM_TARGET_GLES) && !defined(MAGNUM_TARGET_GLES2)
+void Buffer::setTargetHintImplementationSwiftShader(const TargetHint hint) {
+    /* See the "swiftshader-broken-xfb-buffer-binding-target" workaround for
+       details */
+    _targetHint = hint == TargetHint::TransformFeedback ? TargetHint::Array : hint;
+}
+#endif
+
 inline void Buffer::createIfNotAlready() {
     if(_flags & ObjectFlag::Created) return;
 

src/Magnum/GL/Buffer.h

@@ -878,10 +878,7 @@ class MAGNUM_GL_EXPORT Buffer: public AbstractObject {
          * to @fn_gl{BindBuffer}. Default target hint is @ref TargetHint::Array.
          * @see @ref setData(), @ref setSubData()
          */
-        Buffer& setTargetHint(TargetHint hint) {
+        Buffer& setTargetHint(TargetHint hint);
-            _targetHint = hint;
-            return *this;
-        }
 
         #ifndef MAGNUM_TARGET_GLES2
         /**
@@ -1251,13 +1248,18 @@ class MAGNUM_GL_EXPORT Buffer: public AbstractObject {
         #endif
         #endif
 
-        explicit Buffer(GLuint id, TargetHint targetHint, ObjectFlags flags) noexcept: _id{id}, _targetHint{targetHint}, _flags{flags} {}
+        explicit Buffer(GLuint id, TargetHint targetHint, ObjectFlags flags) noexcept;
 
         void MAGNUM_GL_LOCAL createImplementationDefault();
         #ifndef MAGNUM_TARGET_GLES
         void MAGNUM_GL_LOCAL createImplementationDSA();
         #endif
 
+        void MAGNUM_GL_LOCAL setTargetHintImplementationDefault(TargetHint hint);
+        #if defined(MAGNUM_TARGET_GLES) && !defined(MAGNUM_TARGET_GLES2)
+        void MAGNUM_GL_LOCAL setTargetHintImplementationSwiftShader(TargetHint hint);
+        #endif
+
         void MAGNUM_GL_LOCAL createIfNotAlready();
 
         #ifndef MAGNUM_TARGET_WEBGL

src/Magnum/GL/Implementation/BufferState.cpp

@@ -182,6 +182,17 @@ BufferState::BufferState(Context& context, std::vector<std::string>& extensions)
     }
     #endif
 
+    #ifdef MAGNUM_TARGET_GLES
+    if((context.detectedDriver() & Context::DetectedDriver::SwiftShader) &&
+      !context.isDriverWorkaroundDisabled("swiftshader-broken-xfb-buffer-binding-target"))
+    {
+        setTargetHintImplementation = &Buffer::setTargetHintImplementationSwiftShader;
+    } else
+    #endif
+    {
+        setTargetHintImplementation = &Buffer::setTargetHintImplementationDefault;
+    }
+
     #ifdef MAGNUM_TARGET_GLES
     static_cast<void>(context);
     static_cast<void>(extensions);

src/Magnum/GL/Implementation/BufferState.h

@@ -54,6 +54,7 @@ struct BufferState {
     void(*copyImplementation)(Buffer&, Buffer&, GLintptr, GLintptr, GLsizeiptr);
     #endif
     void(Buffer::*createImplementation)();
+    void(Buffer::*setTargetHintImplementation)(Buffer::TargetHint);
     void(Buffer::*getParameterImplementation)(GLenum, GLint*);
     #ifndef MAGNUM_TARGET_GLES2
     void(Buffer::*getSubDataImplementation)(GLintptr, GLsizeiptr, GLvoid*);

src/Magnum/GL/Implementation/driverSpecific.cpp

@@ -160,6 +160,16 @@ namespace {
         "swiftshader-no-es2-oes-texture-3d-entrypoints",
         #endif
 
+        #if defined(MAGNUM_TARGET_GLES) && !defined(MAGNUM_TARGET_GLES2) && !defined(MAGNUM_TARGET_WEBGL)
+        /* SwiftShader 4.1.0 has special handling for binding buffers to the
+           transform feedback target, requiring an XFB object to be active when
+           a buffer is bound to GL_TRANSFORM_FEEDBACK_BUFFER and ignoring the
+           glBindBuffer() call otherwise. No other driver does that. As a
+           workaround, setting Buffer::TargetHint::TransformFeedback will make
+           it use Buffer::TargetHint::Array instead, as that works okay. */
+        "swiftshader-broken-xfb-buffer-binding-target",
+        #endif
+
         #ifndef MAGNUM_TARGET_GLES
         /* Even with the DSA variant, where GL_IMPLEMENTATION_COLOR_READ_* is
            passed to glGetNamedFramebufferParameter(), Mesa complains that the