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GNU Info (gcc-295.info)Output TemplateOutput 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 an 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. Note: Instruction Output. automatically generated by info2www version 1.2.2.9 |