A register operand is allowed provided that it is in a general
register.
-@cindex @samp{d} in constraint
-@item @samp{d}, @samp{a}, @samp{f}, @dots{}
-Other letters can be defined in machine-dependent fashion to stand for
-particular classes of registers. @samp{d}, @samp{a} and @samp{f} are
-defined on the 68000/68020 to stand for data, address and floating
-point registers.
-
@cindex constants in constraints
@cindex @samp{i} in constraint
@item @samp{i}
the mode specified in the @code{match_operand} as the mode of the memory
reference for which the address would be valid.
+@cindex other register constraints
@cindex extensible constraints
-@cindex @samp{Q}, in constraint
-@item @samp{Q}, @samp{R}, @samp{S}, @dots{} @samp{U}
-Letters in the range @samp{Q} through @samp{U} may be defined in a
-machine-dependent fashion to stand for arbitrary operand types.
-@ifset INTERNALS
-The machine description macro @code{EXTRA_CONSTRAINT} is passed the
-operand as its first argument and the constraint letter as its
-second operand.
+@item @var{other letters}
+Other letters can be defined in machine-dependent fashion to stand for
+particular classes of registers or other arbitrary operand types.
+@samp{d}, @samp{a} and @samp{f} are defined on the 68000/68020 to stand
+for data, address and floating point registers.
-A typical use for this would be to distinguish certain types of
-memory references that affect other insn operands.
+@ifset INTERNALS
+The machine description macro @code{REG_CLASS_FROM_LETTER} has first
+cut at the otherwise unused letters. If it evaluates to @code{NO_REGS},
+then @code{EXTRA_CONSTRAINT} is evaluated.
-Do not define these constraint letters to accept register references
-(@code{reg}); the reload pass does not expect this and would not handle
-it properly.
+A typical use for @code{EXTRA_CONSTRANT} would be to distinguish certain
+types of memory references that affect other insn operands.
@end ifset
@end table
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