* Itanium has register rotations, so that you can modulo-schedule a loop.
* Itanium has so many registers that a context switch is going to involve spilling several KB of memory.
* The main registers have "Not-a-Thing" values to be able to handle things like speculative loads that would have trapped. Handling this for register spills (or context switches!) appears to be "fun."
* It's a bi-endian architecture.
* The way you pack instructions in the EPIC encoding is... fun.
* The rules of how you can execute instructions mean that you kind of have branch delay slots, but not really.
* There are four floating-point environments because why not.
* Also, Itanium is predicated.
* The hints, oh god the hints. It feels like every time someone came up with an idea for a hint that might be useful to the processor, it was thrown in there. How is a compiler supposed to be able to generate all of these hints?
* It's an architecture that's complicated enough that you need to handwrite assembly to get good performance, but the assembly has enough arcane rules that handwriting assembly is unnecessarily difficult.
The fact that Itanium had register windows was such a strange choice. I thought they had been shown to not be worthwhile like branch delay slots in MIPS. Basically hangovers from the early constraints of hardware implementations.
* Itanium has register windows.
* Itanium has register rotations, so that you can modulo-schedule a loop.
* Itanium has so many registers that a context switch is going to involve spilling several KB of memory.
* The main registers have "Not-a-Thing" values to be able to handle things like speculative loads that would have trapped. Handling this for register spills (or context switches!) appears to be "fun."
* It's a bi-endian architecture.
* The way you pack instructions in the EPIC encoding is... fun.
* The rules of how you can execute instructions mean that you kind of have branch delay slots, but not really.
* There are four floating-point environments because why not.
* Also, Itanium is predicated.
* The hints, oh god the hints. It feels like every time someone came up with an idea for a hint that might be useful to the processor, it was thrown in there. How is a compiler supposed to be able to generate all of these hints?
* It's an architecture that's complicated enough that you need to handwrite assembly to get good performance, but the assembly has enough arcane rules that handwriting assembly is unnecessarily difficult.