|GNU Compiler Collection (GCC) Internals|
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. Here is how it is done for fullword logical-or on the 68000:
(define_insn "iorsi3" [(set (match_operand:SI 0 "general_operand" "=m,d") (ior:SI (match_operand:SI 1 "general_operand" "%0,0") (match_operand:SI 2 "general_operand" "dKs,dmKs")))] ...)
The first alternative has ‘m’ (memory) for operand 0, ‘0’ for operand 1 (meaning it must match operand 0), and ‘dKs’ for operand 2. The second alternative has ‘d’ (data register) for operand 0, ‘0’ for operand 1, and ‘dmKs’ for operand 2. The ‘=’ and ‘%’ in the constraints apply to all the alternatives; their meaning is explained in the next section (see Class Preferences).
If all the operands fit any one alternative, the instruction is valid. Otherwise, for each alternative, the compiler counts how many instructions must be added to copy the operands so that that alternative applies. The alternative requiring the least copying is chosen. If two alternatives need the same amount of copying, the one that comes first is chosen. These choices can be altered with the ‘?’ and ‘!’ characters:
When an insn pattern has multiple alternatives in its constraints, often
the appearance of the assembler code is determined mostly by which
alternative was matched. When this is so, the C code for writing the
assembler code can use the variable
which_alternative, which is
the ordinal number of the alternative that was actually satisfied (0 for
the first, 1 for the second alternative, etc.). See Output Statement.