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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.