Argument unpacking may need to create references inside the array
that is being unpacked. However, it currently can only do this
if a plain variable is unpacked, not for any nested accesses,
because the value is fetched for read. Resolve this by fetching
the operands for RW.
This commit disallows the use of trailing positional arguments
after argument unpacking was used. The following calls are no
longer valid:
fn(...$array, $var);
fn(...$array1, $var, ...$array2);
However, all of the following continue to be valid:
fn($var, ...$array);
fn(...$array1, ...$array2);
fn($var, ...$array1, ...$array2);
The reason behind this change is a stack allocation issue pointed
out by Dmitry: As of PHP 5.5 the stack necessary for pushing
arguments is precomputed and preallocated, as such the individual
SEND opcodes no longer verify that there is enough stack space.
The unpacked arguments will occupy some of that preallocated
space and as such following positional arguments could write past
a stack page boundary.
An alternative resolution for this issue is to ensure that there
is enough space for the remaining arguments in the UNPACK opcode.
However making this allocation precise (rather than using a
conversative over-estimate) would require some effort. Given that
this particular aspect of the feature wasn't very popular in the
first place, it doesn't seem worth the effort.
If multiple unpacks were used (or mixed with normal arguments)
parts of the arguments could land on different stack pages. If
this occurs the arguments will now be copied to a new stack page.
The code used to do this is copied verbatim from the PHP 5.4 branch
and only modified to reduce the amount of inlined code.
