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##### 17.23.1.2 Substitution in Mode Iterators

If an `.md` file construct uses mode iterators, each version of the construct will often need slightly different strings or modes. For example:

- When a `define_expand` defines several `add` m `3` patterns (see Standard Names), each expander will need to use the appropriate mode name for m.
- When a `define_insn` defines several instruction patterns, each instruction will often use a different assembler mnemonic.
- When a `define_insn` requires operands with different modes, using an iterator for one of the operand modes usually requires a specific mode for the other operand(s).

GCC supports such variations through a system of “mode attributes”. There are two standard attributes: `mode`, which is the name of the mode in lower case, and `MODE`, which is the same thing in upper case. You can define other attributes using:

         (define_mode_attr name [(mode1 "value1") ... (moden "valuen")])

where name is the name of the attribute and valuei is the value associated with modei.

When GCC replaces some :iterator with :mode, it will scan each string and mode in the pattern for sequences of the form `<`iterator`:`attr`>`, where attr is the name of a mode attribute. If the attribute is defined for mode, the whole `<...>` sequence will be replaced by the appropriate attribute value.

For example, suppose an `.md` file has:

         (define_mode_iterator P [(SI "Pmode == SImode") (DI "Pmode == DImode")])
         (define_mode_attr load [(SI "lw") (DI "ld")])

If one of the patterns that uses `:P` contains the string `"<P:load>\t%0,%1"`, the `SI` version of that pattern will use `"lw\t%0,%1"` and the `DI` version will use `"ld\t%0,%1"`.

Here is an example of using an attribute for a mode:

         (define_mode_iterator LONG [SI DI])
         (define_mode_attr SHORT [(SI "HI") (DI "SI")])
         (define_insn ...
           (sign_extend:LONG (match_operand:<LONG:SHORT> ...)) ...)

The iterator`:` prefix may be omitted, in which case the substitution will be attempted for every iterator expansion.