Create empty dynamic memory arrays more efficiently.

docs/assembly.rst

@@ -647,6 +647,11 @@ Solidity manages memory in a very simple way: There is a "free memory pointer"
 at position ``0x40`` in memory. If you want to allocate memory, just use the memory
 from that point on and update the pointer accordingly.
 
+The first 64 bytes of memory can be used as "scratch space" for short-term
+allocation. The 32 bytes after the free memory pointer (i.e. starting at ``0x60``)
+is meant to be zero permanently and is used as the initial value for
+empty dynamic memory arrays.
+
 Elements in memory arrays in Solidity always occupy multiples of 32 bytes (yes, this is
 even true for ``byte[]``, but not for ``bytes`` and ``string``). Multi-dimensional memory
 arrays are pointers to memory arrays. The length of a dynamic array is stored at the

docs/miscellaneous.rst

@@ -64,12 +64,15 @@ The position of ``data[4][9].b`` is at ``keccak256(uint256(9) . keccak256(uint25
 Layout in Memory
 ****************
 
-Solidity reserves three 256-bit slots:
+Solidity reserves four 32 byte slots:
 
--  0 - 64: scratch space for hashing methods
-- 64 - 96: currently allocated memory size (aka. free memory pointer)
+- ``0x00`` - ``0x3f``: scratch space for hashing methods
+- ``0x40`` - ``0x5f``: currently allocated memory size (aka. free memory pointer)
+- ``0x60`` - ``0x7f``: zero slot
 
-Scratch space can be used between statements (ie. within inline assembly).
+Scratch space can be used between statements (ie. within inline assembly). The zero slot
+is used as initial value for dynamic memory arrays and should never be written to
+(the free memory pointer points to ``0x80`` initially).
 
 Solidity always places new objects at the free memory pointer and memory is never freed (this might change in the future).