All current Macs include a T2 chip, which is a variant of the A10 chip that handles various tasks like controlling the SSD NAND, TouchID, Webcam DSP, various security tasks and more.
The scenario you mention — a upgraded "T3" chip based on a newer architecture that would act as a coprocessor used to execute ARM code natively on x86 machines — seems possible, but I don't know how likely it is.
Yeah, but what would be rationale? They want to avoid x86 as a main CPU, so either you'd get an "x86 coprocessor to run Photoshop" (let's go with the PS example here).
Or you'd have to have fat binaries to have x86/ARM execution, assuming the T3 chip would get the chance to run programs. Now either program would have to be pinned to an x86 or ARM core at their start (maybe some applications can set preference, like having PS be always pinned to x86 cores) or have the magical ability to migrate threads/processes from one arch to another, on the fly, while keeping the state consistent... I don't think such a thing has ever even been dreamed of.
I don't think there's a chance to have ARM/x86 coexist as "main CPUs" in the same computer without it being extremely expensive, and even defeating the purpose of having a custom-made CPU to begin with.
An x86 coprocessor is not that outlandish. Sun offered this with some of their SPARC workstations multiple decades ago, IIRC.
Doing so definitely would be counterproductive for Apple in the short-term, but at the same time might be a reasonable long-term play to get people exposed to and programming against the ARM processor while still being able to use the x86 processor for tasks that haven't yet been ported. Eventually the x86 processor would get sunsetted (or perhaps relegated to an add-on card or somesuch).
Either if it's for performance, battery life or cost reasons, it wouldn't really make sense:
a) performance wise, they move would be driven by having a better performing A chip
b) if they aimed at a 15W part battery life would suffer. 6W parts don't deliver good performance.
c) for cost, they'd have to buy the intel processor, and the infrastructure to support it (socket, chipset, heatsink, etc)
Specially for (c), I don't think either Intel would accept selling chips as co-processors (it'd be like admitting their processors aren't good enough to be main processors), nor Apple would put itlsef in a position to adjust the internals of their computers just to acomodate something which they are trying to get away from.
Apple probably doesn't need the integrated GPU, so an AMD-based coprocessor could trim that off for additional power savings (making room in the power budget to re-add hyperthreading or additional cores and/or to bump up the base or burst clock speeds).
> for cost, they'd have to buy the intel processor
Or AMD.
> and the infrastructure to support it (socket, chipset, heatsink, etc)
Laptops (at least the ones as thin as Macbooks) haven't used discrete "sockets"... ever, I'm pretty sure. The vast majority of the time the CPU is soldered directly to the motherboard, and indeed that seems to be the case for the above-linked APU. The heatsink is something that's already needed anyway, and these APUs don't typically need much of it. The chipset's definitely a valid point, but a lot of it can be shaved off by virtue of it being a coprocessor.
Most of it must be ARM compatible already, for the iPad version.
Also Photoshop was first released in 1987 and has been through all the same CPU transitions as Apple (m68k/ppc/...) so presumably some architecture-independence is baked in at some level.
