This is the same as slice::first(), except it returns the first element
by value.
The implementation can avoid checking the list length since empty lists
already have a special representation.
Now that we have a value list of the arguments, we can get rid of:
- The first_arg and last_arg members in EbbData,
- The next member in the ValueData::Arg variant.
This is the first step of the value list refactoring which will replace
linked lists of values with value lists.
- Keep a ValueList in the EbbData struct containing all the EBB
arguments.
- Change dfg.ebb_args() to return a slice instead of an iterator.
This leaves us in a temporary hybrid state where we maintain both a
linked list and a ValueList vector of the EBB arguments.
Any code that needs to manipulate a variable argument list on an
instruction will need to remove the instruction's value list first,
change the list, and then put it back on the instruction. This is
required to avoid fighting the borrow checker over mutable locks on the
DataFlowGraph and its value list pool.
Add a generated InstructionData::take_value_list() method which lifts
out and existing value list and returns it, levaing an empty list in its
place, like Option::take() does it.
Add a generated InstructionData::put_value_list() which puts it back,
verifying that no existing value list is overwritten.
Add a new kind of instruction format that keeps all of its value
arguments in a value list. These value lists are all allocated out of
the dfg.value_lists memory pool.
Instruction formats with the value_list property set store *all* of
their value arguments in a single value list. There is no distinction
between fixed arguments and variable arguments.
Change the Call instruction format to use the value list representation
for its arguments.
This change is only the beginning. The intent is to eliminate the
boxed_storage instruction formats completely. Value lists use less
memory, and when the transition is complete, InstructionData will have a
trivial Drop implementation.
