Operand Constraints
Each match operand in an instruction pattern can specify a
constraint for the type of operands allowed. 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. Here will be
decribed only these operand constraints which are valid for Motorola 68000
processor, and which are valid with the built-in assembler, i.e. which
are applicable with the TIGCC.
Simple Constraints
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:
- 'm'
A memory operand is allowed, with any kind of address that the machine
supports in general.
- 'o'
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 which 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 which 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).
- 'V'
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 is allowed.
- '>'
A memory operand with autoincrement addressing is allowed.
- 'r'
A register operand is allowed provided that it is in a general
register.
- 'd'
Data register is allowed. This is Motorola-specific constraint.
- 'd'
Address register is allowed. This is Motorola-specific constraint.
- 'f'
68881 floating-point register is allowed, if available (of course, it is not
available on the TI-89 and TI-92 Plus). This is Motorola-specific constraint.
- 'i'
An immediate integer operand (one with constant value) is allowed.
This includes symbolic constants whose values will be known only at
assembly time.
- 'n'
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'.
- 'I'
Integer in the range 1 to 8 is allowed. This is Motorola-specific constraint, and this is
for example the range permitted as a shift count in the shift instructions.
- 'J'
16 bit signed number is allowed. This is Motorola-specific constraint.
- 'K'
Signed number whose magnitude is greater than 0x80 is allowed. This is Motorola-specific constraint.
- 'L'
Integer in the range -8 to -1 is allowed. This is Motorola-specific constraint.
- 'M'
Signed number whose magnitude is greater than 0x100. This is Motorola-specific constraint.
- 's'
An immediate integer operand whose value is not an explicit integer is
allowed.
This might appear strange; if an instruction 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. GNU team 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.
- 'g'
Any register, memory or immediate integer operand is allowed, except for
registers that are not general registers.
- 'X'
Any operand whatsoever is allowed, even if it does not satisfy
general_operand
. This is normally used in the constraint of
a match_scratch
when certain alternatives will not actually
require a scratch register.
- '0', '1', '2', ... '9'
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
considered separate. 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.
- 'p'
An operand that is a valid memory address is allowed.
In order to have valid assembler code, each operand must satisfy
its constraint. But a failure to do so does not prevent the pattern
from applying to an instruction pattern. Instead, it directs the compiler to modify
the code so that the constraint will be satisfied. Usually this is
done by copying an operand into a register.
Here are constraint modifier characters, which are meaningful with the TIGCC.
- '='
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.
'&' does not obviate the need to write '='.