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#### 17.23.4 Subst Iterators

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.