Using and Porting GNU CC

15.6.6: 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.