gcc.info: No Constraints

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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.