[PATCH v2] Try and get overlapping cases fixed.

[Cyril Brulebois]

From: Adam Jackson <ajax at redhat.com>

The memcpy fast path implicitly assumes that the copy walks
left-to-right.  That's not something memcpy guarantees, and newer glibc
on some processors will indeed break that assumption.  Since we walk a
line at a time, check the source and destination against the width of
the blit to determine whether we can be sloppy enough to allow memcpy.
(Having done this, we can remove the check for !reverse as well.)

On an Intel Core i7-2630QM with an NVIDIA GeForce GTX 460M running in
NoAccel, the broken code and various fixes for -copywinwin{10,100,500}
gives (edited to fit in 80 columns):

1: Disable the fastpath entirely
2: Replace memcpy with memmove
3: This fix
4: The code before this fix

  1            2                 3                 4           Operation
------   ---------------   ---------------   ---------------   ------------
258000   269000 (  1.04)   544000 (  2.11)   552000 (  2.14)   Copy 10x10
 21300    23000 (  1.08)    43700 (  2.05)    47100 (  2.21)   Copy 100x100
   960      962 (  1.00)     1990 (  2.09)     1990 (  2.07)   Copy 500x500

So it's a modest performance hit, but correctness demands it, and it's
probably worth keeping the 2x speedup from having the fast path in the
first place.

Signed-off-by: Adam Jackson <ajax at redhat.com>

v2: Fix limit cases thanks to Soeren Sandmann, and apply a tiny
    optimization by Walter Harms.

Signed-off-by: Cyril Brulebois <kibi at debian.org>
---
 fb/fbblt.c |    5 ++++-
 1 files changed, 4 insertions(+), 1 deletions(-)


Tested on amd64 on top of xorg-server's server-1.10-branch.


diff --git a/fb/fbblt.c b/fb/fbblt.c
index 38271c0..b6e7785 100644
--- a/fb/fbblt.c
+++ b/fb/fbblt.c
@@ -65,6 +65,7 @@ fbBlt (FbBits   *srcLine,
     int	    n, nmiddle;
     Bool    destInvarient;
     int	    startbyte, endbyte;
+    int     careful;
     FbDeclareMergeRop ();
 
 #ifdef FB_24BIT
@@ -76,7 +77,9 @@ fbBlt (FbBits   *srcLine,
     }
 #endif
 
-    if (alu == GXcopy && pm == FB_ALLONES && !reverse &&
+    careful = (width * (bpp / 8) > abs(srcLine-dstLine)) || (bpp % 8);
+
+    if (alu == GXcopy && pm == FB_ALLONES && !careful &&
             !(srcX & 7) && !(dstX & 7) && !(width & 7)) {
         int i;
         CARD8 *src = (CARD8 *) srcLine;
-- 
1.7.5.1