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Simple
constraints
The simplest kind of constraint
is a string full of letters, each of which describes one kind of operand
that is permitted. The following letters are allowed.
-
m
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 off-settable. 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:
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 (either predecrement or postdecrement) is allowed.
-
-
A memory operand with autoincrement
addressing (either preincrement or postincrement) is allowed.
-
r
A register operand is allowed
provided that it is in a general register.
-
d,
a,
f,
...
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.
-
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,
J,
K,
... P
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.
-
E
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).
-
F
An immediate floating operand
(expression code const_
double) is allowed.
-
G,
H
G
and H
be defined in a machine-dependent fashion to permit immediate floating
operands in particular ranges of values.
-
s
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
load-ing 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.
-
g
Any register, memory or
immediate integer operand is allowed, except for registers that are not
general registers.
-
X
Any operand whatsoever is
allowed.
-
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 which 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.
p
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.
Q,
R,
S,
... U
Letters in the range Q
through U may be defined
in a machine-dependent fashion to stand for arbitrary operand types.
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