GNU Compiler Collection (GCC) Internals: Mode Iterators

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#### 17.23.1 Mode Iterators

Ports often need to define similar patterns for two or more different modes. For example:

- If a processor has hardware support for both single and double floating-point arithmetic, the `SFmode` patterns tend to be very similar to the `DFmode` ones.
- If a port uses `SImode` pointers in one configuration and `DImode` pointers in another, it will usually have very similar `SImode` and `DImode` patterns for manipulating pointers.

Mode iterators allow several patterns to be instantiated from one `.md` file template. They can be used with any type of rtx-based construct, such as a `define_insn`, `define_split`, or `define_peephole2`.