The syntax for defining a mode iterator is:
(define_mode_iterator name [(mode1 "cond1") ... (moden "condn")])
This allows subsequent .md file constructs to use the mode suffix
:name. Every construct that does so will be expanded
n times, once with every use of :name replaced by
:mode1, once with every use replaced by :mode2,
and so on. In the expansion for a particular modei, every
C condition will also require that condi be true.
For example:
(define_mode_iterator P [(SI "Pmode == SImode") (DI "Pmode == DImode")])
defines a new mode suffix :P. Every construct that uses
:P will be expanded twice, once with every :P replaced
by :SI and once with every :P replaced by :DI.
The :SI version will only apply if Pmode == SImode and
the :DI version will only apply if Pmode == DImode.
As with other .md conditions, an empty string is treated
as “always true”. (mode "") can also be abbreviated
to mode. For example:
(define_mode_iterator GPR [SI (DI "TARGET_64BIT")])
means that the :DI expansion only applies if TARGET_64BIT
but that the :SI expansion has no such constraint.
Iterators are applied in the order they are defined. This can be significant if two iterators are used in a construct that requires substitutions. See Substitutions.