Fix EXT_draw_buffers detection on some GL ES 3 contexts

gpu/command_buffer/service/feature_info.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "gpu/command_buffer/service/feature_info.h"
 
 #include <stddef.h>
 
 #include <set>
 #include <vector>
@@ skipping 1134 matching lines @@
       (extensions.Contains("GL_ANGLE_instanced_arrays") ||
        (extensions.Contains("GL_ARB_instanced_arrays") &&
         extensions.Contains("GL_ARB_draw_instanced")) ||
        gl_version_info_->is_es3 ||
        gl_version_info_->is_desktop_core_profile)) {
     AddExtensionString("GL_ANGLE_instanced_arrays");
     feature_flags_.angle_instanced_arrays = true;
     validators_.vertex_attribute.AddValue(GL_VERTEX_ATTRIB_ARRAY_DIVISOR_ANGLE);
   }
 
-  bool vendor_agnostic_draw_buffers =
-      extensions.Contains("GL_ARB_draw_buffers") ||
-      extensions.Contains("GL_EXT_draw_buffers");
-  if (!workarounds_.disable_ext_draw_buffers &&
-      (vendor_agnostic_draw_buffers ||
-       (extensions.Contains("GL_NV_draw_buffers") &&
-        gl_version_info_->is_es3) ||
-       gl_version_info_->is_desktop_core_profile)) {
+  bool support_draw_buffers = extensions.Contains("GL_ARB_draw_buffers") ||
+                              extensions.Contains("GL_EXT_draw_buffers") ||
+                              gl_version_info_->is_es3 ||

[Zhenyao Mo, 2017/02/15 18:42:37]

nit: probably put the last two conditions first as they are cheaper and will be
hit more often.

+                              gl_version_info_->is_desktop_core_profile;
+  bool have_draw_buffers = !workarounds_.disable_ext_draw_buffers &&
+                           IsWebGL1OrES2Context() && support_draw_buffers;
+  if (have_draw_buffers) {
     AddExtensionString("GL_EXT_draw_buffers");
     feature_flags_.ext_draw_buffers = true;
+  }
 
-    // This flag is set to enable emulation of EXT_draw_buffers when we're
-    // running on GLES 3.0+, NV_draw_buffers extension is supported and
-    // glDrawBuffers from GLES 3.0 core has been bound. It toggles using the
-    // NV_draw_buffers extension directive instead of EXT_draw_buffers extension
-    // directive in ESSL 100 shaders translated by ANGLE, enabling them to write
-    // into multiple gl_FragData values, which is not by default possible in
-    // ESSL 100 with core GLES 3.0. For more information, see the
-    // NV_draw_buffers specification.
-    feature_flags_.nv_draw_buffers = !vendor_agnostic_draw_buffers;
-
+  if (IsWebGL2OrES3Context() || have_draw_buffers) {
     GLint max_color_attachments = 0;
     glGetIntegerv(GL_MAX_COLOR_ATTACHMENTS_EXT, &max_color_attachments);
     for (GLenum i = GL_COLOR_ATTACHMENT1_EXT;
          i < static_cast<GLenum>(GL_COLOR_ATTACHMENT0 + max_color_attachments);
          ++i) {
       validators_.attachment.AddValue(i);
       validators_.attachment_query.AddValue(i);
     }
     static_assert(GL_COLOR_ATTACHMENT0_EXT == GL_COLOR_ATTACHMENT0,
                   "GL_COLOR_ATTACHMENT0_EXT should equal GL_COLOR_ATTACHMENT0");
@@ skipping 351 matching lines @@
   if (pos == std::string::npos) {
     extensions_ += (extensions_.empty() ? "" : " ") + str;
   }
 }
 
 FeatureInfo::~FeatureInfo() {
 }
 
 }  // namespace gles2
 }  // namespace gpu