Not Using Constraints
---------------------
Some machines are so clean that operand constraints are not
required. For example, on the Vax, an operand valid in one context is
valid in any other context. On such a machine, every operand
constraint would be `g', excepting only operands of "load address"
instructions which are written as if they referred to a memory
location's contents but actual refer to its address. They would have
constraint `p'.
For such machines, instead of writing `g' and `p' for all the
constraints, you can choose to write a description with empty
constraints. Then you write `""' for the constraint in every
`match_operand'. Address operands are identified by writing an
`address' expression around the `match_operand', not by their
constraints.
When the machine description has just empty constraints, certain
parts of compilation are skipped, making the compiler faster. However,
few machines actually do not need constraints; all machine descriptions
now in existence use constraints.