GNU Compiler Collection (GCC) Internals: Output Template

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16.5 Output Templates and Operand Substitution

The output template is a string which specifies how to output the assembler code for an instruction pattern. Most of the template is a fixed string which is output literally. The character ‘%’ is used to specify where to substitute an operand; it can also be used to identify places where different variants of the assembler require different syntax.

In the simplest case, a ‘%’ followed by a digit n says to output operand n at that point in the string.

‘%’ followed by a letter and a digit says to output an operand in an alternate fashion. Four letters have standard, built-in meanings described below. The machine description macro PRINT_OPERAND can define additional letters with nonstandard meanings.

‘%cdigit ’ can be used to substitute an operand that is a constant value without the syntax that normally indicates an immediate operand.

‘%ndigit ’ is like ‘%cdigit ’ except that the value of the constant is negated before printing.

‘%adigit ’ can be used to substitute an operand as if it were a memory reference, with the actual operand treated as the address. This may be useful when outputting a “load address” instruction, because often the assembler syntax for such an instruction requires you to write the operand as if it were a memory reference.

‘%ldigit ’ is used to substitute a label_ref into a jump instruction.

‘%=’ outputs a number which is unique to each instruction in the entire compilation. This is useful for making local labels to be referred to more than once in a single template that generates multiple assembler instructions.

‘%’ followed by a punctuation character specifies a substitution that does not use an operand. Only one case is standard: ‘%%’ outputs a ‘%’ into the assembler code. Other nonstandard cases can be defined in the PRINT_OPERAND macro. You must also define which punctuation characters are valid with the PRINT_OPERAND_PUNCT_VALID_P macro.

The template may generate multiple assembler instructions. Write the text for the instructions, with ‘\;’ between them.

When the RTL contains two operands which are required by constraint to match each other, the output template must refer only to the lower-numbered operand. Matching operands are not always identical, and the rest of the compiler arranges to put the proper RTL expression for printing into the lower-numbered operand.

One use of nonstandard letters or punctuation following ‘%’ is to distinguish between different assembler languages for the same machine; for example, Motorola syntax versus MIT syntax for the 68000. Motorola syntax requires periods in most opcode names, while MIT syntax does not. For example, the opcode ‘movel’ in MIT syntax is ‘move.l’ in Motorola syntax. The same file of patterns is used for both kinds of output syntax, but the character sequence ‘%.’ is used in each place where Motorola syntax wants a period. The PRINT_OPERAND macro for Motorola syntax defines the sequence to output a period; the macro for MIT syntax defines it to do nothing.

As a special case, a template consisting of the single character # instructs the compiler to first split the insn, and then output the resulting instructions separately. This helps eliminate redundancy in the output templates. If you have a define_insn that needs to emit multiple assembler instructions, and there is a matching define_split already defined, then you can simply use # as the output template instead of writing an output template that emits the multiple assembler instructions.

If the macro ASSEMBLER_DIALECT is defined, you can use construct of the form ‘{option0|option1|option2}’ in the templates. These describe multiple variants of assembler language syntax. See Instruction Output.

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