Here are specific details on what constraint letters you can use with asm operands. Constraints can say whether an operand may be in a register, and which kinds of register; whether the operand can be a memory reference, and which kinds of address; whether the operand may be an immediate constant, and which possible values it may have. Constraints can also require two operands to match.
The simplest kind of constraint is a string full of letters, each of which describes one kind of operand that is permitted. Here are the letters that are allowed:
A memory operand is allowed, with any kind of address that the machine supports in general.
A memory operand is allowed, but only if the address is offsettable. This means that adding a small integer (actually, the width in bytes of the operand, as determined by its machine mode) may be added to the address and the result is also a valid memory address.
For example, an address that is constant is offsettable; so is an address that is the sum of a register and a constant (as long as a slightly larger constant is also within the range of address-offsets supported by the machine); but an autoincrement or autodecrement address is not offsettable. More complicated indirect/indexed addresses may or may not be offsettable depending on the other addressing modes that the machine supports.
Note that in an output operand that can be matched by another operand, the constraint letter "o" is valid only when accompanied by both "<" (if the target machine has predecrement addressing) and ">" (if the target machine has preincrement addressing).
A memory operand that is not offsettable. In other words, anything that would fit the "m" constraint but not the "o" constraint.
A memory operand with autodecrement addressing (either predecrement or postdecrement) is allowed.
A memory operand with autoincrement addressing (either preincrement or postincrement) is allowed.
A register operand is allowed provided that it is in a general register.
Other letters can be defined in machine-dependent fashion to stand for particular classes of registers. "d", "a" and "f" are defined on the 68000/68020 to stand for data, address and floating-point registers.
An immediate integer operand (one with constant value) is allowed. This includes symbolic constants whose values will be known only at assembly time.
An immediate integer operand with a known numeric value is allowed. Many systems cannot support assembly-time constants for operands less than a word wide. Constraints for these operands should use "n" rather than "i".
Other letters in the range "I" through "P" may be defined in a machine-dependent fashion to permit immediate integer operands with explicit integer values in specified ranges. For example, on the 68000, "I" is defined to stand for the range of values 1 to 8. This is the range permitted as a shift count in the shift instructions.
An immediate floating operand (expression code const_double) is allowed, but only if the target floating point format is the same as that of the host machine (on which the compiler is running).
An immediate floating operand (expression code const_double) is allowed.
"G" and "H" may be defined in a machine-dependent fashion to permit immediate floating operands in particular ranges of values.
An immediate integer operand whose value is not an explicit integer is allowed.
This might appear strange; if an insn allows a constant operand with a value not known at compile time, it certainly must allow any known value. So why use "s" instead of "i"? Sometimes it allows better code to be generated.
For example, on the 68000 in a fullword instruction it is possible to use an immediate operand; but if the immediate value is between -128 and 127, better code results from loading the value into a register and using the register. This is because the load into the register can be done with a "moveq" instruction. We arrange for this to happen by defining the letter "K" to mean "any integer outside the range -128 to 127", and then specifying "Ks" in the operand constraints.
Any register, memory or immediate integer operand is allowed, except for registers that are not general registers.
Any operand whatsoever is allowed.
An operand that matches the specified operand number is allowed. If a digit is used together with letters within the same alternative, the digit should come last.
This is called a matching constraint and what it really means is that the assembler has only a single operand that fills two roles that asm distinguishes. For example, an add instruction uses two input operands and an output operand, but on most CISC machines an add instruction really has only two operands, one of them an input-output operand:
addl #35,r12
Matching constraints are used in these circumstances. More precisely, the two operands that match must include one input-only operand and one output-only operand. Moreover, the digit must be a smaller number than the number of the operand that uses it in the constraint.
An operand that is a valid memory address is allowed. This is for "load address" and "push address" instructions.
"p" in the constraint must be accompanied by address_operand as the predicate in the match_operand. This predicate interprets the mode specified in the match_operand as the mode of the memory reference for which the address would be valid.
Letters in the range "Q" through "U" may be defined in a machine-dependent fashion to stand for arbitrary operand types.
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.
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:
Disparage slightly the alternative that the "?" appears in, as a choice when no alternative applies exactly. The compiler regards this alternative as one unit more costly for each "?" that appears in it.
Disparage severely the alternative that the "!" appears in. This alternative can still be used if it fits without reloading, but if reloading is needed, some other alternative will be used.
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.
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.
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.