Bug 1337085 - Add a gfxVar to control whether or not webrender is enabled. r=sotaro

MozReview-Commit-ID: 8Ebc0DbhYbZ

gfx/config/gfxVars.h

@@ -33,6 +33,7 @@ class gfxVarReceiver;
   _(PDMWMFDisableD3D11Dlls,     nsCString,        nsCString())          \
   _(PDMWMFDisableD3D9Dlls,      nsCString,        nsCString())          \
   _(DXInterop2Blocked,          bool,             false)                \
+  _(UseWebRender,               bool,             false)                \
 
   /* Add new entries above this line. */
 

gfx/thebes/gfxPlatform.cpp

@@ -689,6 +689,18 @@ gfxPlatform::Init()
 #endif
     gPlatform->InitAcceleration();
 
+#ifdef MOZ_ENABLE_WEBRENDER
+    if (XRE_IsParentProcess()) {
+      // XXX: right now this is just based on the pref. But we may want to
+      // do other runtime detection of hardware support etc.
+      // This pref defaults to true, so builds with --enable-webrender have it
+      // turned on by default, but can have it disabled via pref. The ifdef
+      // guards this entire block, so builds without --enable-webrender will
+      // always have the gfxVar default to false.
+      gfxVars::SetUseWebRender(Preferences::GetBool("gfx.webrender.enabled", true));
+    }
+#endif
+
     if (gfxConfig::IsEnabled(Feature::GPU_PROCESS)) {
       GPUProcessManager* gpu = GPUProcessManager::Get();
       gpu->LaunchGPUProcess();

widget/nsBaseWidget.cpp

@@ -64,6 +64,7 @@
 #include "mozilla/dom/ContentChild.h"
 #include "mozilla/dom/TabParent.h"
 #include "mozilla/gfx/GPUProcessManager.h"
+#include "mozilla/gfx/gfxVars.h"
 #include "mozilla/Move.h"
 #include "mozilla/Services.h"
 #include "mozilla/Sprintf.h"
@@ -1300,11 +1301,7 @@ void nsBaseWidget::CreateCompositor(int aWidth, int aHeight)
 
   CreateCompositorVsyncDispatcher();
 
-  // For now we decide whether or not to enable WR on this widget by the current
-  // value of the pref (this is the only place in the code allowed to check the
-  // value of the pref). We might want to change this eventually and drop the
-  // pref entirely.
-  bool enableWR = Preferences::GetBool("gfx.webrender.enabled", false);
+  bool enableWR = gfx::gfxVars::UseWebRender();
   bool enableAPZ = UseAPZ();
   if (enableWR && !gfxPrefs::APZAllowWithWebRender()) {
     // Disable APZ on widgets using WebRender, since it doesn't work yet. Allow