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Constraint
Modifier Characters
The following are constraint
modifier characters.
=
Means that this
operand is write-only for this instruction: the previous value is discarded
and replaced by output data.
+
Means that this
operand is both read and written by the instruction.
When the compiler fixes
up the operands to satisfy the constraints, it needs to know which operands
are inputs to the instruction and which are outputs from it. ‘=’
identifies an output; ‘+’
identifies an operand that is both input and output; all other operands
are assumed to be input only.
&
Means (in a particular
alternative) that this operand is an earlycobber operand which is
modified before the instruction is finished using the input operands. Therefore,
this operand may not lie in a register that is used as an input operand
or as part of any memory address.
‘&’
applies only to the alternative in which it is written. In constraints
with multiple alternatives, sometimes one alternative requires ‘&’
while others do not. See, for example, the ‘movdf’
insn of the 68000.
‘&’
does not obviate the need to write ‘=’.
%
Declares the instruction
to be commutative for this operand and the following operand. This means
that the compiler may interchange the two operands if that is the cheapest
way to make all operands fit the constraints.
#
Says that all
following characters, up to the next comma, are to be ignored as a constraint.
They are significant only for choosing register preferences.