[LLVMdev] mips16

[Reed Kotler]

I'm working on the mips16.

Mips16 is a mode of the Mips32 (or Mips64) processor. For the most part, 
it is a compressed form of the MIPS32 instruction set, though not all 
instructions are supported. Most of the same opcodes and formats are 
present though sometimes with some restriction.  (The micro mips 
architecture is a true 16 bit compressed form of MIps32 though also with 
some limitations regarding immediate sizes and such).

Mips16 has a mapping of it's registers to those of the MIPS32.

Through all of it's instructions, it has direct access to registers  
2,3,4,5,6,7, 16,17 though the encoding for 16,17 are 0,1.

It's possible to access all other registers only through a move instruction.

I'm looking for guidance from those that have done more ports 
(especially ARM) as to how to organize the register sets and such.

It seems like it would be simplest to just create a separate register 
set for MIPS16 but that would
be a hack because those registers are in fact MIPS32 registers, even if 
they may be encoded
differently at times.

For optimization, it's possible to change modes and do some computation 
and then change back, so there are some advantages to not treating the 
mips16 as if it were a different processor.

For example, mips16 has no floating point so in a region of code with 
floating point you could switch to mips32 and then come back to mips16. 
Otherwise you will be making a procedure call for each
floating point operation.

I'm not intending to do such optimizations in this first port but 
clearly it will be less of a hack in the future if the register sets 
reflect reality.

If you are reading assembly code, it's often hard to tell you are 
looking at Mips16. They look the same as mips32.

I'm trying to avoid duplicating any more of the mips32 than in is 
necessary. In principal mips16 is just an alternate encoding of Mips32.

Thoughts?

Tia.

Reed