SPONSORED LINKS
Here is some
example code with the mysterious values explained:
volatile inline long
change_V0(void)
{
long retval;
asm(
"move $2, $6;"
"move %0, $6;"
: "=r"(retval) ::
"$2");
return(retval);
}
There are 4 parts
to an asm(); function, each separated by a colon. The first is the assembly
instructions. The second defines the return values, in this case, retval. You
must put an "=r" in, the equals sign defines a return value, the 'r'
a register value. The third (absent from this example, hence 2 colons together)
defines the variables passed to the asm, in order (just use a comma as a
delimiter). The fourth is the clobber list and defines the registers which are
to be protected, ie their value restored when execution passes back to the C
program. Here is another, bigger example :
#define f16Mul(__x,__y) ({register
INT32 __z; asm ( \
"mfhi
$12;" \
"mflo
$13;" \
"mult
%1,%2;" \
"mfhi
$14;" \
"mflo
$15;" \
"sll
$14,$14,16;" \
"srl
$15,$15,16;" \
"or %0,$14,$15;" \
"mthi $12;" \
"mtlo $13;" \
": "=d" (__z):
"d" (__x), "d" (__y) : "t4", "t5",
"t6", "t7"); __z;})
Here, there are
parameters passed in, __x and __y. To refer to the parameters you do it in
numerical order starting at %0 ie %0 is __z, %1 is __x and %2 is __y. The
notation used in passing parameters (r and d) is as follows (but note I've only
ever used "d" and "r"):
|
`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 (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' may 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 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.
|
`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 in the RTL insn. For
example, an add insn has two input operands and one output operand in the RTL,
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.
For operands to match in a particular case usually means that they are
identical-looking RTL expressions. But in a few special cases specific kinds of
dissimilarity are allowed. For example, *x as an input operand will match *x++
as an output operand. For proper results in such cases, the output template
should always use the output-operand's number when printing the operand.
|
`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. The machine description macro
EXTRA_CONSTRAINT is passed the operand as its first argument and the
constraint letter as its second operand. A typical use for this would be to
distinguish certain types of memory references that affect other insn
operands. Do not define these constraint letters to accept register references
(reg); the reload pass does not expect this and would not handle it properly. |