[Ffmpeg-devel] [RFC] another attempt at memalign realloc

[Reimar Döffinger]

Hello,
On Thu, Dec 07, 2006 at 01:09:51PM +0100, Reimar D?ffinger wrote:
> On Thu, Dec 07, 2006 at 12:55:47PM +0100, Michael Niedermayer wrote:
> > > Too bad I can't find any good solution for the
> > > non-memalign-hack case.
> > 
> > p= realloc(p, size);
> > if(!((long)p&15))
> >     return p;
> > p2= av_malloc(size);
> > if(p2)
> >     memcpy(p2, p, size);
> > av_free(p);
> > return p2;
> > 
> > ?
> > 
> > btw, this one with a small modification should work in the memalign-hack
> > case too i think, would simplify the code though it would be slower in the
> > memalign-hack case ...
> 
> Yes, I was thinking about that, and probably it is fine for ffmpeg, but
> it does not meet this part of realloc:
> "If realloc() fails the original block is left untouched".
> If we do it this change, at least we must free p always and say
> "If av_realloc() fails the original block is freed".
> Unless of course I miss something *g*

Actually, there is a rather obvious solution.
Disadvantage: av_realloc without MEMALIGN_HACK now _always_ has the time
and space costs of av_malloc.

Greetings,
Reimar D?ffinger
-------------- next part --------------
Index: libavutil/mem.c
===================================================================
--- libavutil/mem.c	(revision 7217)
+++ libavutil/mem.c	(working copy)
@@ -105,19 +105,40 @@
 {
 #ifdef CONFIG_MEMALIGN_HACK
     int diff;
+    char *p = ptr;
 #endif
+    char *p2;
 
     /* let's disallow possible ambiguous cases */
     if(size > (INT_MAX-16) )
         return NULL;
+    if(!size) {
+      av_free(ptr);
+      return NULL;
+    }
+    if(!ptr) return av_malloc(size);
 
 #ifdef CONFIG_MEMALIGN_HACK
-    //FIXME this isn't aligned correctly, though it probably isn't needed
-    if(!ptr) return av_malloc(size);
-    diff= ((char*)ptr)[-1];
-    return realloc(ptr - diff, size + diff) + diff;
+    diff = p[-1];
+    p2 = realloc(p - diff, size + 16);
+    p2 += diff;
+    if(!((long)p2&15))
+        return p2;
+    p = p2 - diff;
+    diff = ((-(long)p - 1)&15) + 1;
+    p += diff;
+    memmove(p, p2, size);
+    p[-1] = diff;
+    return p;
 #else
-    return realloc(ptr, size);
+    p2 = av_malloc(size);
+    if (!p2) return NULL;
+    ptr = realloc(ptr, size);
+    if(!((long)ptr&15))
+        return ptr;
+    memcpy(p2, ptr, size);
+    free(ptr);
+    return p2;
 #endif
 }