realloc: Return unchanged if request is within usable size

If there is enough space in the chunk to satisfy the new size, return
the old pointer as is, thus avoiding any locks or reallocations.  The
only real place this has a benefit is in large chunks that tend to get
satisfied with mmap, since there is a large enough spare size (up to a
page) for it to matter.  For allocations on heap, the extra size is
typically barely a few bytes (up to 15) and it's unlikely that it would
make much difference in performance.

Also added a smoke test to ensure that the old pointer is returned
unchanged if the new size to realloc is within usable size of the old
pointer.

Signed-off-by: Siddhesh Poyarekar <siddhesh@sourceware.org>
Reviewed-by: DJ Delorie <dj@redhat.com>

malloc/malloc.c

@@ -1100,6 +1100,8 @@ static void munmap_chunk(mchunkptr p);
 static mchunkptr mremap_chunk(mchunkptr p, size_t new_size);
 #endif
 
+static size_t musable (void *mem);
+
 /* ------------------ MMAP support ------------------  */
 
 
@@ -3396,6 +3398,14 @@ __libc_realloc (void *oldmem, size_t bytes)
   if (__glibc_unlikely (mtag_enabled))
     *(volatile char*) oldmem;
 
+  /* Return the chunk as is whenever possible, i.e. there's enough usable space
+     but not so much that we end up fragmenting the block.  We use the trim
+     threshold as the heuristic to decide the latter.  */
+  size_t usable = musable (oldmem);
+  if (bytes <= usable
+      && (unsigned long) (usable - bytes) <= mp_.trim_threshold)
+    return oldmem;
+
   /* chunk corresponding to oldmem */
   const mchunkptr oldp = mem2chunk (oldmem);
   /* its size */

malloc/tst-realloc.c

@@ -17,6 +17,7 @@
 
 #include <errno.h>
 #include <malloc.h>
+#include <stdint.h>
 #include <stdio.h>
 #include <string.h>
 #include <libc-diag.h>
@@ -142,6 +143,28 @@ do_test (void)
 
   free (p);
 
+  /* Smoke test to make sure that allocations do not move if they have enough
+     space to expand in the chunk.  */
+  for (size_t sz = 3; sz < 256 * 1024; sz += 2048)
+    {
+      p = realloc (NULL, sz);
+      if (p == NULL)
+	FAIL_EXIT1 ("realloc (NULL, %zu) returned NULL.", sz);
+      size_t newsz = malloc_usable_size (p);
+      printf ("size: %zu, usable size: %zu, extra: %zu\n",
+	      sz, newsz, newsz - sz);
+      uintptr_t oldp = (uintptr_t) p;
+      void *new_p = realloc (p, newsz);
+      if ((uintptr_t) new_p != oldp)
+	FAIL_EXIT1 ("Expanding (%zu bytes) to usable size (%zu) moved block",
+		    sz, newsz);
+      free (new_p);
+
+      /* We encountered a large enough extra size at least once.  */
+      if (newsz - sz > 1024)
+	break;
+    }
+
   return 0;
 }