The CHERI capability model: Revisiting RISC in an age of risk (Light Blue Touchpaper)

Posted Jul 4, 2014 11:37 UTC (Fri) by dps (guest, #5725)
In reply to: The CHERI capability model: Revisiting RISC in an age of risk (Light Blue Touchpaper) by pjdc
Parent article: The CHERI capability model: Revisiting RISC in an age of risk (Light Blue Touchpaper)

The last time I heard about it IA-64 was new design that could be very fast if you could exploit the large number of registers and explicitly parallel instructions. Unfortunately the wished for compilers that could do this proved impossible to write.

Compilers for Itanium

Posted Jul 6, 2014 14:45 UTC (Sun) by CChittleborough (subscriber, #60775) [Link] (1 responses)

Yep. The distinctive feature of the Itanium architecture is Explicitly Parallel Instruction Computing (EPIC) which expects the compiler to generate and annotate code for predicated execution and other fancy things. In theory, EPIC allows faster execution than CPUs which have to recognise and exploit implicit parallelism to get the best use of their out-of-order and/or multiple-issue execution. In practice, even Intel could not produce compilers that made good enough use of EPIC.

On the other hand, the Itanium architecture does have excellent, compiler-friendly support for loop unrolling. Since a lot of demand for supercomputer time is for Fortran programs with deeply nested DO loops and run times measured in days, Itanium-based supercomputers were quite popular for a while.

(BTW, am I being excessively cynical when I wonder if an unwritten design goal for the Itanium architecture was to given Intel a natural monopoly by being too complex for anyone to clone?)

Compilers for Itanium

Posted Jul 8, 2014 12:46 UTC (Tue) by james (subscriber, #1325) [Link]

I think your cynicism is misplaced. The preferred route was allegedly patents held by a joint Intel-HP company, which would presumably not have been covered by any cross-licensing deals Intel had.

Making a processor too complex for anyone else to clone means that it's too complex for you to make a compatible processor, when the time comes to replace the initial implementation.

(Also, Itanium was notably better at floating point performance than the integer workloads it mostly ended up running. The Fortran programs you mention sound like the sort of programs where compilers might be able to extract parallelism, either into EPIC or SSE models. But I'm very sure other subscribers know more about that than I do!)