[PATCH] composite: Don't bother copying the pixmap for ForgetGravity windows

[Jasper St. Pierre]

If a window has ForgetGravity in its bitGravity, that very likely
means it will repaint on the ConfigureNotify / Expose event, and thus
we don't have to copy the old pixmap into the new pixmap, we can simply
leave it blank.

Preventing this copy is super simple to do and a big win on small
devices where these blits can be expensive.

A better approach would be to actually obey BitGravity correctly rather
than assume NorthWestGravity always, but this is a big speedup for the
common case.
---
 composite/compalloc.c | 94 ++++++++++++++++++++++++++-------------------------
 1 file changed, 48 insertions(+), 46 deletions(-)

diff --git a/composite/compalloc.c b/composite/compalloc.c
index 8daded0..70c83f0 100644
--- a/composite/compalloc.c
+++ b/composite/compalloc.c
@@ -542,54 +542,56 @@ compNewPixmap(WindowPtr pWin, int x, int y, int w, int h)
     pPixmap->screen_x = x;
     pPixmap->screen_y = y;
 
-    if (pParent->drawable.depth == pWin->drawable.depth) {
-        GCPtr pGC = GetScratchGC(pWin->drawable.depth, pScreen);
-
-        if (pGC) {
-            ChangeGCVal val;
-
-            val.val = IncludeInferiors;
-            ChangeGC(NullClient, pGC, GCSubwindowMode, &val);
-            ValidateGC(&pPixmap->drawable, pGC);
-            (*pGC->ops->CopyArea) (&pParent->drawable,
-                                   &pPixmap->drawable,
-                                   pGC,
-                                   x - pParent->drawable.x,
-                                   y - pParent->drawable.y, w, h, 0, 0);
-            FreeScratchGC(pGC);
+    if (pWin->bitGravity != ForgetGravity) {
+        if (pParent->drawable.depth == pWin->drawable.depth) {
+            GCPtr pGC = GetScratchGC(pWin->drawable.depth, pScreen);
+
+            if (pGC) {
+                ChangeGCVal val;
+
+                val.val = IncludeInferiors;
+                ChangeGC(NullClient, pGC, GCSubwindowMode, &val);
+                ValidateGC(&pPixmap->drawable, pGC);
+                (*pGC->ops->CopyArea) (&pParent->drawable,
+                                       &pPixmap->drawable,
+                                       pGC,
+                                       x - pParent->drawable.x,
+                                       y - pParent->drawable.y, w, h, 0, 0);
+                FreeScratchGC(pGC);
+            }
         }
-    }
-    else {
-        PictFormatPtr pSrcFormat = PictureWindowFormat(pParent);
-        PictFormatPtr pDstFormat = PictureWindowFormat(pWin);
-        XID inferiors = IncludeInferiors;
-        int error;
-
-        PicturePtr pSrcPicture = CreatePicture(None,
-                                               &pParent->drawable,
-                                               pSrcFormat,
-                                               CPSubwindowMode,
-                                               &inferiors,
-                                               serverClient, &error);
-
-        PicturePtr pDstPicture = CreatePicture(None,
-                                               &pPixmap->drawable,
-                                               pDstFormat,
-                                               0, 0,
-                                               serverClient, &error);
-
-        if (pSrcPicture && pDstPicture) {
-            CompositePicture(PictOpSrc,
-                             pSrcPicture,
-                             NULL,
-                             pDstPicture,
-                             x - pParent->drawable.x,
-                             y - pParent->drawable.y, 0, 0, 0, 0, w, h);
+        else {
+            PictFormatPtr pSrcFormat = PictureWindowFormat(pParent);
+            PictFormatPtr pDstFormat = PictureWindowFormat(pWin);
+            XID inferiors = IncludeInferiors;
+            int error;
+
+            PicturePtr pSrcPicture = CreatePicture(None,
+                                                   &pParent->drawable,
+                                                   pSrcFormat,
+                                                   CPSubwindowMode,
+                                                   &inferiors,
+                                                   serverClient, &error);
+
+            PicturePtr pDstPicture = CreatePicture(None,
+                                                   &pPixmap->drawable,
+                                                   pDstFormat,
+                                                   0, 0,
+                                                   serverClient, &error);
+
+            if (pSrcPicture && pDstPicture) {
+                CompositePicture(PictOpSrc,
+                                 pSrcPicture,
+                                 NULL,
+                                 pDstPicture,
+                                 x - pParent->drawable.x,
+                                 y - pParent->drawable.y, 0, 0, 0, 0, w, h);
+            }
+            if (pSrcPicture)
+                FreePicture(pSrcPicture, 0);
+            if (pDstPicture)
+                FreePicture(pDstPicture, 0);
         }
-        if (pSrcPicture)
-            FreePicture(pSrcPicture, 0);
-        if (pDstPicture)
-            FreePicture(pDstPicture, 0);
     }
     return pPixmap;
 }
-- 
2.1.0