Subst iterators are special type of iterators with the following restrictions: they could not be declared explicitly, they always have only two values, and they do not have explicit dedicated name. Subst-iterators are triggered only when corresponding subst-attribute is used in RTL-pattern.
Subst iterators transform templates in the following way: the templates
are duplicated, the subst-attributes in these templates are replaced
with the corresponding values, and a new attribute is implicitly added
to the given define_insn/define_expand. The name of the
added attribute matches the name of define_subst. Such
attributes are declared implicitly, and it is not allowed to have a
define_attr named as a define_subst.
Each subst iterator is linked to a define_subst. It is declared
implicitly by the first appearance of the corresponding
define_subst_attr, and it is not allowed to define it explicitly.
Declarations of subst-attributes have the following syntax:
(define_subst_attr "name"
"subst-name"
"no-subst-value"
"subst-applied-value")
name is a string with which the given subst-attribute could be referred to.
subst-name shows which define_subst should be applied to an
RTL-template if the given subst-attribute is present in the
RTL-template.
no-subst-value is a value with which subst-attribute would be replaced in the first copy of the original RTL-template.
subst-applied-value is a value with which subst-attribute would be replaced in the second copy of the original RTL-template.