HW subregs in machine description

[Dimitar Dimitrov <dimitar at dinux dot eu>]

Hello,

I'm a newbie working on a GCC port [1] for PRU [2]. In order to achieve ABI compatibility with the proprietary TI toolchain, I need my Machine Description to support HW register subfields as indipendent first-class registers. I could not find a relevant example in the GCC source. Looks like other ports declare subregisters to be the same entities as the corresponding main register (e.g. %al , %ah and %ax are treated by the GCC register allocator as one and same i386 register).

So is it possible to describe subregs (independent fields) of 32-bit HW registers in GCC?

Currently I'm attempting to describe the  8-bit PRU subregisters as the "real" target register set, and then work on defining 16-bit and 32-bit ALU operations. But I'm not sure if that would be efficient for a 32-bit PRU target, or feasible at all.

To give an example, each 32-bit PRU register can hold either:
  - one 32-bit value (e.g. r10)
  - four independent 8-bit values (e.g. r10.b0, r10.b1, r10.b2, r10.b3).
  - two independent 16-bit values (e.g. r10.w0, r10.w2).
  - a mixture of the above (e.g. r10.w0, r10.b2, r10.b3).

And here is an example of zero-extending one 8-bit and one 16-bit value, performing a 32-bit addition, and storing the result  into another 8-bit subregister:
   add r10.b3, r10.b2, r10.w0

Thanks,
Dimitar

[1] https://github.com/dinuxbg/gnupru/tree/master/patches/gcc
[2] http://elinux.org/images/d/da/Am335xPruReferenceGuide.pdf , section 5.3.2.5.2 "Registers"