From: H. Peter Anvin <hpa@zytor.com>
Date: Fri, 22 Feb 2008 01:59:56 +0000 (-0800)
Subject: realloc(): better implementation allowing in-place growth
X-Git-Tag: syslinux-3.62-pre13~1
X-Git-Url: http://review.tizen.org/git/?a=commitdiff_plain;h=ce6265e0c4577aa7871e337cec549b0d844706ce;p=profile%2Fivi%2Fsyslinux.git

realloc(): better implementation allowing in-place growth

Change the realloc() implementation to allow in-place growth.  This is
an important step in handling files without knowing their sizes a
priori.
---

diff --git a/com32/lib/realloc.c b/com32/lib/realloc.c
index cfbdc76..802d973 100644
--- a/com32/lib/realloc.c
+++ b/com32/lib/realloc.c
@@ -4,16 +4,15 @@
 
 #include <stdlib.h>
 #include <string.h>
+#include <minmax.h>
 
 #include "malloc.h"
 
-/* FIXME: This is cheesy, it should be fixed later */
-
 void *realloc(void *ptr, size_t size)
 {
-  struct free_arena_header *ah;
+  struct free_arena_header *ah, *nah;
   void *newptr;
-  size_t oldsize;
+  size_t newsize, oldsize, xsize;
 
   if ( !ptr )
     return malloc(size);
@@ -23,26 +22,65 @@ void *realloc(void *ptr, size_t size)
     return NULL;
   }
 
-  /* Add the obligatory arena header, and round up */
-  size = (size+2*sizeof(struct arena_header)-1) & ARENA_SIZE_MASK;
-
   ah = (struct free_arena_header *)
     ((struct arena_header *)ptr - 1);
 
-  if ( ah->a.size >= size && size >= (ah->a.size >> 2) ) {
-    /* This field is a good size already. */
+  /* Actual size of the old block */
+  oldsize = ah->a.size;
+
+  /* Add the obligatory arena header, and round up */
+  newsize = (size+2*sizeof(struct arena_header)-1) & ARENA_SIZE_MASK;
+
+  if ( oldsize >= newsize && newsize >= (oldsize >> 2) ) {
+    /* This allocation is close enough already. */
     return ptr;
   } else {
-    /* Make me a new block.  This is kind of bogus; we should
-       be checking the adjacent blocks to see if we can do an
-       in-place adjustment... fix that later. */
+    xsize = oldsize;
 
-    oldsize = ah->a.size - sizeof(struct arena_header);
+    nah = ah->a.next;
+    if ((char *)nah == (char *)ah + ah->a.size &&
+	nah->a.type == ARENA_TYPE_FREE &&
+	oldsize + nah->a.size >= newsize) {
+      /* Merge in subsequent free block */
+      ah->a.next = nah->a.next;
+      ah->a.next->a.prev = ah;
+      nah->next_free->prev_free = nah->prev_free;
+      nah->prev_free->next_free = nah->next_free;
+      xsize = (ah->a.size += nah->a.size);
+    }
 
-    newptr = malloc(size);
-    memcpy(newptr, ptr, (size < oldsize) ? size : oldsize);
-    free(ptr);
+    if (xsize >= newsize) {
+      /* We can reallocate in place */
+      if (xsize >= newsize + 2*sizeof(struct arena_header)) {
+	/* Residual free block at end */
+	nah = (struct free_arena_header *)((char *)ah + newsize);
+	nah->a.type = ARENA_TYPE_FREE;
+	nah->a.size = xsize - newsize;
+	ah->a.size = newsize;
+
+	/* Insert into block list */
+	nah->a.next = ah->a.next;
+	ah->a.next = nah;
+	nah->a.next->a.prev = nah;
+	nah->a.prev = ah;
 
-    return newptr;
+	/* Insert into free list */
+	nah->next_free = __malloc_head.next_free;
+	nah->prev_free = &__malloc_head;
+	__malloc_head.next_free = nah;
+	nah->next_free->prev_free = nah;
+      }
+      /* otherwise, use up the whole block */
+      return ptr;
+    } else {
+      /* Last resort: need to allocate a new block and copy */
+      oldsize -= sizeof(struct arena_header);
+      newptr = malloc(size);
+      if (newptr) {
+	memcpy(newptr, ptr, min(size,oldsize));
+	free(ptr);
+      }
+      return newptr;
+    }
   }
 }