Sometimes a single instruction has multiple alternative sets of possible operands. For example, on the 68000, a logical-or instruction can combine register or an immediate value into memory, or it can combine any kind of operand into a register; but it cannot combine one memory location into another.
These constraints are represented as multiple alternatives. An alternative can be described by a series of letters for each operand. The overall constraint for an operand is made from the letters for this operand from the first alternative, a comma, the letters for this operand from the second alternative, a comma, and so on until the last alternative. All operands for a single instruction must have the same number of alternatives.
So the first alternative for the 68000's logical-or could be written as
"+m" (output) : "ir" (input)
. The second could be
"+r" (output): "irm" (input)
. However, the fact that two memory locations cannot be used in a single instruction prevents simply using
"+rm" (output) : "irm" (input)
. Using multi-alternatives, this might be written as
"+m,r" (output) : "ir,irm" (input)
. This describes all the available alternatives to the compiler, allowing it to choose the most efficient one for the current conditions.
There is no way within the template to determine which alternative was chosen. However you may be able to wrap your
statements with builtins such as
to achieve the desired results.