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 addm3 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.