Ethan wrote:
Agner, what's your opinion on the Itanium instruction
set in isolation, assuming a compiler is written and
backwards compatibility do not matter?
The advantage of the Itanium instruction set was of course that decoding was easy. The biggest problem with the Itanium instruction set was indeed that it was almost impossible to write a good compiler for it. It is quite inflexible because the compiler always has to schedule instructions 3 at a time, whether this fits the actual amount of parallelism in the code or not. Branching is messy when all instructions are organized into triplets. The instruction size is fixed at 41 bits and 5 bits are wasted on a template. If you need more bits and make an 82 bit instruction then it has to be paired with a 41 bit instruction. |
Proposal for an ideal extensible instruction set - Agner - 2015-12-27
Itanium - Agner - 2015-12-28