Add support for sized deallocation.

This adds a new `sdallocx` function to the external API, allowing the
size to be passed by the caller.  It avoids some extra reads in the
thread cache fast path.  In the case where stats are enabled, this
avoids the work of calculating the size from the pointer.

An assertion validates the size that's passed in, so enabling debugging
will allow users of the API to debug cases where an incorrect size is
passed in.

The performance win for a contrived microbenchmark doing an allocation
and immediately freeing it is ~10%.  It may have a different impact on a
real workload.

Closes #28

src/jemalloc.c

@@ -1223,6 +1223,24 @@ ifree(void *ptr, bool try_tcache)
 	JEMALLOC_VALGRIND_FREE(ptr, rzsize);
 }
 
+JEMALLOC_INLINE_C void
+isfree(void *ptr, size_t usize, bool try_tcache)
+{
+	UNUSED size_t rzsize JEMALLOC_CC_SILENCE_INIT(0);
+
+	assert(ptr != NULL);
+	assert(malloc_initialized || IS_INITIALIZER);
+
+	if (config_prof && opt_prof)
+		prof_free(ptr, usize);
+	if (config_stats)
+		thread_allocated_tsd_get()->deallocated += usize;
+	if (config_valgrind && in_valgrind)
+		rzsize = p2rz(ptr);
+	isqalloc(ptr, usize, try_tcache);
+	JEMALLOC_VALGRIND_FREE(ptr, rzsize);
+}
+
 void *
 je_realloc(void *ptr, size_t size)
 {
@@ -1820,6 +1838,32 @@ je_dallocx(void *ptr, int flags)
 	ifree(ptr, try_tcache);
 }
 
+void
+je_sdallocx(void *ptr, size_t size, int flags)
+{
+	bool try_tcache;
+
+	assert(ptr != NULL);
+	assert(malloc_initialized || IS_INITIALIZER);
+	assert(size == isalloc(ptr, config_prof));
+
+	if ((flags & MALLOCX_LG_ALIGN_MASK) == 0)
+		size = s2u(size);
+	else
+		size = sa2u(size, MALLOCX_ALIGN_GET_SPECIFIED(flags));
+
+	if ((flags & MALLOCX_ARENA_MASK) != 0) {
+		unsigned arena_ind = MALLOCX_ARENA_GET(flags);
+		arena_chunk_t *chunk = (arena_chunk_t *)CHUNK_ADDR2BASE(ptr);
+		try_tcache = (chunk == ptr || chunk->arena !=
+		    arenas[arena_ind]);
+	} else
+		try_tcache = true;
+
+	UTRACE(ptr, 0, 0);
+	isfree(ptr, size, try_tcache);
+}
+
 size_t
 je_nallocx(size_t size, int flags)
 {