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Here are specific details on what constraint letters you can use with asm operands. 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.
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:
Whitespace characters are ignored and can be inserted at any position except the first. This enables each alternative for different operands to be visually aligned in the machine description even if they have different number of constraints and modifiers.
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).
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.
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.
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 or const_vector) is allowed.
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.
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 number is allowed to be more than a single digit. If multiple digits are encountered consecutively, they are interpreted as a single decimal integer. There is scant chance for ambiguity, since to-date it has never been desirable that 10 be interpreted as matching either operand 1 or operand 0. Should this be desired, one can use multiple alternatives instead.
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.
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.
Other letters can be defined in machine-dependent fashion to stand for particular classes of registers or other arbitrary operand types. d, a and f are defined on the 68000/68020 to stand for data, address and floating point registers.
Sometimes a single instruction has multiple alternative sets of possible operands. For example, on the 68000, a logical-or instruction can combine register or an immediate value into memory, or it can combine any kind of operand into a register; but it cannot combine one memory location into another.
These constraints are represented as multiple alternatives. An alternative can be described by a series of letters for each operand. The overall constraint for an operand is made from the letters for this operand from the first alternative, a comma, the letters for this operand from the second alternative, a comma, and so on until the last alternative.
If all the operands fit any one alternative, the instruction is valid. Otherwise, for each alternative, the compiler counts how many instructions must be added to copy the operands so that that alternative applies. The alternative requiring the least copying is chosen. If two alternatives need the same amount of copying, the one that comes first is chosen. These choices can be altered with the ? and ! characters:
Disparage slightly the alternative that the ? appears in, as a choice when no alternative applies exactly. The compiler regards this alternative as one unit more costly for each ? that appears in it.
Disparage severely the alternative that the ! appears in. This alternative can still be used if it fits without reloading, but if reloading is needed, some other alternative will be used.
Here are constraint modifier characters.
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.
& applies only to the alternative in which it is written. In constraints with multiple alternatives, sometimes one alternative requires & while others do not. See, for example, the movdf insn of the 68000.
An input operand can be tied to an earlyclobber operand if its only use as an input occurs before the early result is written. Adding alternatives of this form often allows GCC to produce better code when only some of the inputs can be affected by the earlyclobber.
& does not obviate the need to write =.
Declares the instruction to be commutative for this operand and the following operand. This means that the compiler may interchange the two operands if that is the cheapest way to make all operands fit the constraints. GCC can only handle one commutative pair in an asm; if you use more, the compiler may fail.
Says that all following characters, up to the next comma, are to be ignored as a constraint. They are significant only for choosing register preferences.
Says that the following character should be ignored when choosing register preferences. * has no effect on the meaning of the constraint as a constraint, and no effect on reloading.
Whenever possible, you should use the general-purpose constraint letters in asm arguments, since they will convey meaning more readily to people reading your code. Failing that, use the constraint letters that usually have very similar meanings across architectures. The most commonly used constraints are m and r (for memory and general-purpose registers respectively; Section 6.38.1 Simple Constraints), and I, usually the letter indicating the most common immediate-constant format.
For each machine architecture, the config/machine/machine.h file defines additional constraints. These constraints are used by the compiler itself for instruction generation, as well as for asm statements; therefore, some of the constraints are not particularly interesting for asm. The constraints are defined through these macros:
Register class constraints (usually lower case).
Immediate constant constraints, for non-floating point constants of word size or smaller precision (usually upper case).
Immediate constant constraints, for all floating point constants and for constants of greater than word size precision (usually upper case).
Special cases of registers or memory. This macro is not required, and is only defined for some machines.
Inspecting these macro definitions in the compiler source for your machine is the best way to be certain you have the right constraints. However, here is a summary of the machine-dependent constraints available on some particular machines.
Address base register
Floating point register
MQ, CTR, or LINK register
MQ register
CTR register
LINK register
CR register (condition register) number 0
CR register (condition register)
FPMEM stack memory for FPR-GPR transfers
Signed 16-bit constant
Unsigned 16-bit constant shifted left 16 bits (use L instead for SImode constants)
Unsigned 16-bit constant
Signed 16-bit constant shifted left 16 bits
Constant larger than 31
Exact power of 2
Zero
Constant whose negation is a signed 16-bit constant
Floating point constant that can be loaded into a register with one instruction per word
Memory operand that is an offset from a register (m is preferable for asm statements)
AIX TOC entry
Constant suitable as a 64-bit mask operand
Constant suitable as a 32-bit mask operand
a, b, c, or d register for the i386. For x86-64 it is equivalent to r class. (for 8-bit instructions that do not use upper halves)
a, b, c, or d register. (for 8-bit instructions, that do use upper halves)
Legacy register--equivalent to r class in i386 mode. (for non-8-bit registers used together with 8-bit upper halves in a single instruction)
Specifies the a or d registers. This is primarily useful for 64-bit integer values (when in 32-bit mode) intended to be returned with the d register holding the most significant bits and the a register holding the least significant bits.
Floating point register
First (top of stack) floating point register
Second floating point register
a register
b register
c register
Specifies constant that can be easily constructed in SSE register without loading it from memory.
d register
di register
si register
xmm SSE register
MMX register
Constant in range 0 to 31 (for 32-bit shifts)
Constant in range 0 to 63 (for 64-bit shifts)
0xff
0xffff
0, 1, 2, or 3 (shifts for lea instruction)
Constant in range 0 to 255 (for out instruction)
Constant in range 0 to 0xffffffff or symbolic reference known to fit specified range. (for using immediates in zero extending 32-bit to 64-bit x86-64 instructions)
Constant in range −2147483648 to 2147483647 or symbolic reference known to fit specified range. (for using immediates in 64-bit x86-64 instructions)
Standard 80387 floating point constant
General register r0 to r3 for addl instruction
Branch register
Predicate register (c as in "conditional")
Application register residing in M-unit
Application register residing in I-unit
Floating-point register
Memory operand. Remember that m allows postincrement and postdecrement which require printing with %Pn on IA-64. Use S to disallow postincrement and postdecrement.
Floating-point constant 0.0 or 1.0
14-bit signed integer constant
22-bit signed integer constant
8-bit signed integer constant for logical instructions
8-bit adjusted signed integer constant for compare pseudo-ops
6-bit unsigned integer constant for shift counts
9-bit signed integer constant for load and store postincrements
The constant zero
0 or -1 for dep instruction
Non-volatile memory for floating-point loads and stores
Integer constant in the range 1 to 4 for shladd instruction
Memory operand except postincrement and postdecrement
DP or IP registers (general address)
IP register
IPL register
IPH register
DP register
DPH register
DPL register
SP register
DP or SP registers (offsettable address)
Non-pointer registers (not SP, DP, IP)
Non-SP registers (everything except SP)
Indirect thru IP - Avoid this except for QImode, since we can't access extra bytes
Indirect thru SP or DP with short displacement (0..127)
Data-section immediate value
Integers from −255 to −1
Integers from 0 to 7--valid bit number in a register
Integers from 0 to 127--valid displacement for addressing mode
Integers from 1 to 127
Integer −1
Integer 1
Zero
Integers from 0 to 255
Floating-point register on the SPARC-V8 architecture and lower floating-point register on the SPARC-V9 architecture.
Floating-point register. It is equivalent to f on the SPARC-V8 architecture and contains both lower and upper floating-point registers on the SPARC-V9 architecture.
Floating-point condition code register.
Lower floating-point register. It is only valid on the SPARC-V9 architecture when the Visual Instruction Set is available.
Floating-point register. It is only valid on the SPARC-V9 architecture when the Visual Instruction Set is available.
64-bit global or out register for the SPARC-V8+ architecture.
Signed 13-bit constant
Zero
32-bit constant with the low 12 bits clear (a constant that can be loaded with the sethi instruction)
A constant in the range supported by movcc instructions
A constant in the range supported by movrcc instructions
Same as K, except that it verifies that bits that are not in the lower 32-bit range are all zero. Must be used instead of K for modes wider than SImode
The constant 4096
Floating-point zero
Signed 13-bit constant, sign-extended to 32 or 64 bits
Floating-point constant whose integral representation can be moved into an integer register using a single sethi instruction
Floating-point constant whose integral representation can be moved into an integer register using a single mov instruction
Floating-point constant whose integral representation can be moved into an integer register using a high/lo_sum instruction sequence
Memory address aligned to an 8-byte boundary
Even register
Memory address for e constraint registers.
Address register (general purpose register except r0)
Data register (arbitrary general purpose register)
Floating-point register
Unsigned 8-bit constant (0-255)
Unsigned 12-bit constant (0-4095)
Signed 16-bit constant (−32768-32767)
Unsigned 16-bit constant (0-65535)
Memory reference without index register
Symbolic constant suitable for use with the larl instruction