(gcc.info.gz) Modifiers

 
 5.38.3 Constraint Modifier Characters
 -------------------------------------
 
 Here 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.
 
      If you specify `=' or `+' in a constraint, you put it in the first
      character of the constraint string.
 
 `&'
      Means (in a particular alternative) that this operand is an
      "earlyclobber" 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.
 
      An input operand can be tied to an earlyclobber operand if its only
      use as an input occurs before the early result is written.  Adding
      alternatives of this form often allows GCC to produce better code
      when only some of the inputs can be affected by the earlyclobber.
      See, for example, the `mulsi3' insn of the ARM.
 
      `&' 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.  GCC can only handle one commutative pair in
      an asm; if you use more, the compiler may fail.  Note that you
      need not use the modifier if the two alternatives are strictly
      identical; this would only waste time in the reload pass.  The
      modifier is not operational after register allocation, so the
      result of `define_peephole2' and `define_split's performed after
      reload cannot rely on `%' to make the intended insn match.
 
 `#'
      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.
 
 `*'
      Says that the following character should be ignored when choosing
      register preferences.  `*' has no effect on the meaning of the
      constraint as a constraint, and no effect on reloading.
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