Inline Assembler Macros 

Here is some example code with the mysterious values explained:


volatile inline long change_V0(void)


long retval;


"move $2, $6;"

"move %0, $6;"

: "=r"(retval) :: "$2");




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"):



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 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.


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.


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'. `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.



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', `H'

`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, 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.



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.