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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. Here are the letters that are allowed:

`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 resultschine 100644 0 0 3441 6255306520 14260 0ustar rootroot Using and Porting GNU Pascal - Constraints Go to the first, previous, next, last section, table of contents.

Operand Constraints

Each match_operand in an instruction pattern can specify a constraint for the type of operands allowed. 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.

Go to the first, previous, next, last section, table of contents. usr/doc/gpc/html/gpc_123.html100644 0 0 32141 6255306520 14300 0ustar rootroot Using and Porting GNU Pascal - Simple Constraints Go to the first, previous, next, last section, table of contents.

Simple Constraints

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:

`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 resultschine