Well this is exactly the point. They cannot do this the license will not permit them to alter the architecture it's all or nothing (indeed if the new ISA was too ARM like the lawyers would be sure to come knocking).
I'm sure the could reuse quite a lot of compiler technology but the entire point of the article is by doing clean slate ISA design you can do something radical and get gains. Whether this is true is unclear, but it would require some serious work on the compiler.
Producing their own conventional ISA would seem to be pointless as it wouldn't give them anything vs ARM (or indeed x86).
Your assertion that their license will not permit them to alter the architecture is wrong. This is true of the vast majority of ARM licenses, but not Apple's.
They can take the ARM ISA and extend it in any way they want, and they can take ARM cores and adjust them, or design their own-- they have already done all of this (though to a small degree, not enough to be called a "new ISA".)
> They can take the ARM ISA and extend it in any way they want, and they can take ARM cores and adjust them, or design their own-- they have already done all of this (though to a small degree, not enough to be called a "new ISA".)
What is your source for this? As far as I know ARM do not permit modification of the designs they sell or alterations to the architecture. After all allowing such things could lead to the errosion of their business (e.g. by letting apple slowly slide to a non ARM architecture).
"Companies can also obtain an ARM architectural licence for designing their own CPU cores using the ARM instruction sets. These cores must comply fully with the ARM architecture."
Given the lego-like structure of the ARM instruction set (the 32-bit variant), with zillions of extensions (Jazelle, DSP instructions, Neon, Thumb, Thumb-2, various revisions of vector floating point instructions) and explicit support for "coprocessors" (https://en.wikipedia.org/wiki/ARM_architecture#Coprocessors), I suspect (based on common sense and nothing else) that the license allows expanding the instruction set and dropping whole modules.
But as I said elsewhere: concrete proof for that is lacking.
Can you give a reference for that? As far as I know, Apple has an architectural license that allows them to ship anything that passes a test suite. Depending on the way the test suite is specified, that may not rule out shipping a superset of ARM, for example with a few extra instructions.
One that I _think_may_ be useful is one to load values from tagged pointers (caveat: I know little of CPU architecture)
I can't find one actually, so perhaps I am wrong. If it strictly based upon the test suite then maybe it's sufficiently thorough that interesting alterations are effectively impossible (e.g. it could test you get invalid instruction exceptions when you feed in unused bits of instruction encoding space, preventing you from adding any).
It has two extra integer division instructions and some extra floating point. Though the blog says that the extra floating point instructions are also present in ARMv7 (in XCode) but unused.
So perhaps they're allowed to add whatever they want? I suspect only the lawyers truely know...
What is pretty certain is they couldn't break compatability with the ARM architecture.
edit: for what possible reason can this be downvoted? It's just a datapoint.