Itanium® Architecture for Programmers: Understanding 64-Bit Processors and EPIC Principles

5.6. DOTCLOOP: Using the Loop Count Register

Counted loops (DO in FORTRAN, for in C) are common, frequently nested, structures in application programming. Efficient handling of innermost loops is highly important for good software engineering.

The Itanium architecture provides specialized facilities to implement these loops efficiently using the application register ar.lc (the loop count register) and the branch instruction br.cloop. The ar.lc register is a required member of a set of as many as 128 application registers in addition to the general, predicate, branch, and floating-point registers already mentioned. Appendix D describes the many types of Itanium processor registers, while Appendix C lists the many varieties of branch instructions. ...