(gcc.info.gz) No Constraints

 
 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.

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(gcc.info.gz) Machine Constraints

(gcc.info.gz) Constraints

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